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Characterization of humic and fulvic acids extracted from surface horizons of contiguous Alfisols and Mollisols of southwestern Ohio and their influence on mineral weathering
Novak, Jeffrey Michael, Ph.D.
The Ohio State University, 1989
Copyright ©1989 by Novak, Jeffrey Michael. All rights reserved.
UMI 300 N. Zeeb Rd. Ann Arbor, MI 48106
CHARACTERIZATION OF HUMIC AND FULVIC ACIDS EXTRACTED FROM SURFACE HORIZONS OF CONTIGUOUS ALFISOLS AND MOLLISOLS OF SOUTHWESTERN, OHIO AND THEIR INFLUENCE ON MINERAL WEATHERING
DISSERTATION
Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University
By
Jeffrey M. Novak B.S., M.S.
*****
The Ohio State University
1989
Reading Committee: Approved By
Dr. Neil E. Smeck
Dr. Frank L. Himes
Dr. Sam Traina Advisor Department of Agronomy Dr. Gunter Faure Copyright by Jeffrey M, Novak 1989 DEDICATION
Laura and Andrew ACKNOWLEDGEMENTS
I wish to express my appreciation to my advisor, Dr.
Neil E. Smeck, for his advice, ideas, and short-term financial support through this investigation. Apprecia tion is also given to Dr. Jerry M. Bigham for his invaluable guidance in soil mineralogical investigations as well as other aspects of my research. I would also like to thank Dr. Frank Himes for his valuable assistance and guidance during the organic matter extraction and purification phase of this project. I am also grateful to
Dr. Sam Traina and Dr. Gunter Faure for their suggestions and service on committees.
I would like to thank Mrs. Sue Shipatilo for the NMR analysis. I would like to especially thank Mr. Lee
Burras, Mr. Sandy Jones, and Dr. Billy Jaynes for their excellent advice and participation in soil sample analyses.
I would also like to thank the Department of
Agronomy, The Ohio State University for long term financial support.
To my wife, Laura, for valuable assistance in preparation of the manuscript. VITA
August 26, 1958 Born - Elizabeth, New Jersey
1980 B.S., Delaware Valley College Doylestown, Pennsylvania
1980 - 1983 Graduate Assistant Department of Agronomy, Iowa State University, Ames, Iowa
1983 M.S., Iowa State University, Ames, Iowa
1983 - 1989 Graduate Student Department of Agronomy, Ohio State University, Columbus, Ohio
PUBLICATIONS
Novak, J. M., and A. M. Blackmer. 1983. 15-N Tracer studies of reactions of nitric oxide during denitrification in soils. Agron. Abstracts p. 159.
Novak, J. M., and N. E. Smeck. 1987. Characterization of humic and fulvic acids extracted from surface horizons of contiguous Alfisols and Mollisols from south western, Ohio. Agron. Abstracts p. 189.
FIELDS OF STUDY
Major Field: Soil Morphology, Mineralogy, and Genesis
Studies in Soil Science. Professors N. E. Smeck, J. M. Bigham, G. F. Hall
Studies in Humic Chemistry. Professors F. L. Himes, S. J. Traina.
Studies in Geology and Mineralogy. Professors R. T. Tettenhorst, G. Faure, G. D. McKenzie
iv TABLE OF CONTENTS
Page
DEDICATION ...... ii
ACKNOWLEDGEMENTS ...... iii
VITA ...... iv
LIST OF TABLES ...... x
LIST OF FIGURES...... xiv
INTRODUCTION ...... 1
CHAPTER
I. METHODS AND MATERIALS ...... 3 1.1 Site and Pedon Selection...... 3 1.2 Morphological Description and Sample. . . . 5 Collection 1.3 Soil Characterization ...... 5 1.3.1 Sample Preparation...... 5 1.3.2 Coarse Fragment Content ...... 6 1.3.3 Particle Size Analysis...... 6 1.3.4 p H ...... 7 1.3.5 Organic Carbon...... 7 1.3.6 Carbonate Content ...... 8 1.3.7 Extractable Acidity ...... 8 1.3.8 Extractable Cations ...... 9 1.3.9 Duplicate and Data Expression .... 9 1.4 Clay Mineralogy ...... 11 1.4.1 Fractionation of Sand, Silt and . . . 11 Clay 1.4.2 Fine and Coarse Clay Fractionation. . 12 1.4.3 X-Ray Diffraction Analysis...... 13
v TABLE OF CONTENTS (Continued)
Page
II. DESCRIPTION OF THE STUDY AREA AND SOIL...... 16 CHARACTERISTICS 2.1 Location and Physiography ...... 16 2 . 2 Bedrock G e o l o g y ...... 19 2.3 Glacial Geology ...... 20 2.4 Climate and Vegetation...... 21 2.5 Soil Characteristics...... 22 2.5.1 Morphological Description ...... 22 2.5.2 Particle Size Analysis...... 24 2.5.3 Chemical Properties ...... 3 0 2.5.4 Mineralogy...... 31 A. Clay Mineralogy of the Fine .... 31 Clay Fraction B. Clay Mineralogy of the Coarse . . . 45 Clay Fraction C. Clay Mineralogy of the Total. . . . 59 Clay Fraction
III. SOIL ORGANIC MATTER CHARACTERIZATION...... 67 3.1 Introduction...... 67 3.2 Literature Review ...... 68 3.2.1 Biochemistry of Humus Formation . . . 68 3.2.2 Extraction, Fractionation,...... 72 Purification, and Distribution of Soil Humic and Fulvic Acids 3.2.3 Chemical Characterization of Humic. . 80 and Fulvic Acids 3.3 Methods of Soil Organic Matter Extraction . 101 3.3.1 Soil P r e t r e a t m e n t ...... 103 3.3.2 Alkaline Extraction ...... 103 3.3.3 Separation of Humic and Fulvic Acids. 104 3.3.4 Purification of Humic Acids ...... 105 3.3.5 Purification of Fulvic Acids...... 106 3.3.6 Purification of H u m i n ...... 108 3.4 Methods of Humic and Fulvic Acid ...... 109 Characterization 3.4.1 Functional Group and E4/E6 Ratio. . . 109 Analyses 3.4.2 Elemental Analyses...... 110 3.4.3 Infrared Spectrometry ...... 110 3.4.4 13C CPMAS NMR Spectroscopy...... Ill 3.5 Results and Discussion...... 112 3.5.1 Extraction of Humic and Fulvic Acids. 112 3.5.2 Humic and Fulvic A c i d ...... 119 Characterization A. Elemental and Atomic Ratio .... 119 Analyses
vi TABLE OF CONTENTS (Continued)
Page
B. E4/E6 Absorption Ratio ...... 124 C. Functional Group Analyses...... 12 6 D. Infrared Absorption Spectra. . . . 129 1. Humic Acids ...... 129 2. Fulvic Acids...... 134 E. 13C CPMAS Spectra...... 138 1. Humic Acids ...... 140 a. Aliphatic Region ...... 140 b. Oxygen Alkyl Region...... 148 c. Aromatic.Region...... 150 d. Carboxyl Region...... 152 e. Carbonyl.Region...... 154 2. Fulvic Acids...... 155 a. Aliphatic Region ...... 155 b. Oxygen Alkyl Region...... 159 c. Aromatic Region...... 160 d. Carboxyl Region...... 162 e. Carbonyl Region...... 162 3.6 Summary ...... 163
IV. HUMIC SUBSTANCES IN WHOLE SOILS ...... 165 4.1 Introduction...... 165 4.2 Literature Review ...... 165 4.2.1 13C NMR Analysis of Unfractionated. . 165 Soils 4.2.2 13C NMR Analysis of Fractionated. . . 166 Soils 4 . 3 Materials and Methods ...... 168 4.3.1 Sonification of Whole Soils ...... 168 4.3.2 Collection of Organic Carbon... 169 Enriched Soil 4.3.3 13C CPMAS NMR Analysis o f ..... 170 Organic Carbon Enriched Soil 4.4 Results and Discussion...... 171 4.4.1 Particle Size Analysis of Fractions . 171 Collected by Sonification and Sedimentation 4.4.2 Percent Organic Carbon in Collected . 172 Fractions 4.4.3 13C NMR Spectra of Fine Material. . . 173 from Whole Soils 4.5 Summary ...... 176
vii TABLE OF CONTENTS (Continued)
Page
V. INFLUENCE OF SORBED HUMIC SUBSTANCES ON THE . . 178 DECOMPOSITION OF TOTAL CLAY FRACTION BY SULFURIC ACIDS 5.1 Introduction...... 178 5.2 Literature Review ...... 179 5.2.1 Weathering of Minerals in Soils . . . 179 A. Decomposition of Primary Minerals. 180 B. Decomposition of Secondary .... 187 Minerals C. Decompostion of Soil Material. . . 193 5.2.2 Sorption of Humic Substances by . . . 194 Minerals A. Sorption by Nonexpanding Minerals. 195 B. Sorption by Expanding Minerals . . 197 5.2.3 Sorption Mechanisms of Humic...... 202 Substances by Minerals A. Van der Waals Bonds...... 2 02 B. Electrostatic Bonding...... 203 C. Hydrogen Bonding ...... 203 D. Ligand Exchange...... 204 5.2.4 Summary ...... 204 5.3 Material and Methods...... 205 5.3.1 Separation of Total Clay Fraction . . 205 A. Organic Matter Removal ...... 2 06 B. Total Clay Fractionation ... 206 1. Oxidized Clays...... 206 2. Untreated Total Clays ...... 208 5.3.2 Sorption of Humic Substances by . . . 208 Oxidized Clay Fraction A. Preliminary Sorption Studies . . . 209 1. Effect of pH and Saturating . . 2 09 Cation 2. Effect of Clay:HA or FA Ratio . 210 B. Sorption of Humic and Fulvic . . . 211 Acids by Clays Used in Dissolution Studies 5.3.3 Dissolved Organic Carbon...... 212 5.3.4 Dissolution of Clays in Sulfuric. . . 213 Acids A. Dissolution of Clay in 0.1N H2SO4. 213 B. Dissolution of Clay in 0.005N. . . 215 H 2S04 5.3.5 Total Chemical Analysis ...... 217 5.4 Results and Discussion...... 218 5.4.1 Comparison of Sorption Amounts. . . . 219 Determined by Xertex and Dry Combustion Methods 5.4.2 Sorption of Humic Substances...... 223
viii TABLE OF CONTENTS (Continued)
Page
5.4.3 Clay Dissolution in Dilute...... 228 Sulfuric Acid A. Clay Dissolution in 0.1N H2SO4 . . 23 0 B. Clay Dissolution in 0.005N H2SO4 . 240 5.5 Summary ...... 248
VI. SUMMARY AND CONCLUSIONS ...... 251
APPENDICES
A. Pedon Description ...... 259 B. Characterization D a t a ...... 2 65 C. Data from Dissolution Study ...... 27 0
LIST OF REFERENCES ...... 278
ix LIST OF TABLES
Table Page
1. Instrument parameters used for atomic absorption (AAS) and flame emission (FES) spectrophotometry during the extractable cation procedure ...... 10
2. Classification of the soils studied...... 23
3. Fine to total clay ratio of select horizons from the Dana, Dana Variant, Xenia, and Rossmoyne pedons...... 28
4. Semi-quantitative estimates of clay mineralogy for the fine clay fraction from selected horizons of WA-69, 72, 70, and 72 (Dana, Dana Variant, Xenia, and R o s s m o y n e ) ...... 44
5. Semi-quantitative estimates of clay mineralogy for the coarse clay fraction from selected horizons of WA-69, 72, 70, and 71 (Dana, Dana Variant, Xenia, and Rossmoyne) ...... 58
6. Semi-quantitative estimates of clay mineralogy for the total clay fraction from selected horizons of WA-69 and 70 (Dana and Xenia) ...... 62
7. Semi-quantitative estimates of clay mineralogy for the total clay fraction from selected horizons of WA-71 and 72 (Rossmoyne and Dana Variant) ...... 63
8. Elemental analysis of humic substances (Schnitzer, 1975)...... 85
x LIST OF TABLES (Continued)
Table Page
9. Chemical shifts and structural assignments in the CPMAS 13C NMR spectrum of humic and fulvic acids ...... 98
10. Organic carbon content of soil and humin . . 113
11. Yield of purified humic and fulvic acids extracted by alkaline solution ...... 113
12. The average humic to fulvic acid (HA/FA) ratio and yield of humic substances expressed as a percentage of total organic carbon (TOC) content ...... 114
13. Percentage yield of purified humic substances expressed as a percentage of alkali-extractable organic carbon...... 114
14. Ash content in unpurified (unpur.) and purified (pur.) humic and fulvic acids. . . . u s
15. Elemental composition (%) and atomic ratios for alkaline extracted humic acid samples . . 120
16. Elemental composition (%) and atomic ratios of alkaline extracted fulvic acid samples . . 122
17. Mean E4/E6 absorbance ratios for humic and fulvic acids...... 125
18. Functional group analyses for all humic acid samples...... 127
19. Main infrared (IR) absorption bands for humic and fulvic acids (after Stevenson, 1982). . . 130
20. Distribution of C in alkaline extracted humic acid samples as determined by 13C CPMAS NMR analysis...... 14 6
21. Distribution of C in alkaline extracted fulvic acid samples as determined by 13C CPMAS NMR analysis...... 158
xi LIST OF TABLES (Continued)
Table Page
22. Yield and particle size analysis of the fine fraction obtained by sonification and sedimentation...... 171
23. Total C contents in fine and coarse fractions and in whole soils ...... 172
24. Sorption of humic acids (HA) and fulvic acids (FA) by clays as determined by C measured by Xertex C analysis of solutions and dry combustion analysis of clays.... 220
25. Regression coefficient of determinations of of sorption vs umol of Ca + Mg ions in washing solutions for several samples. . . . 222
26. Sorption of Carlisle humic (HA) and fulvic (FA) acids by oxidized Na- or Ca-saturated Xenia Bt2 horizon clay fraction as a function of p H ...... 224
27. Sorption of humic (HA) and fulvic (FA) acids by oxidized Ca-saturated Xenia Bt2 horizon clay as a function of the clay:HA or FA r a t i o s ...... 226
28. Sorption of humic (HA) and fulvic acids (FA) oxidized Ca-saturated Xenia Bt2 horizon clay fractions used in the 0.1 and 0.005N H2SO4 dissolution studies...... 228
29. Cations released expressed as mole ratios percentages of total cation released and mMol/100 g clay after incubation in 0.1N H 2S04 ...... 236
30. Percentage of Xenia clay fraction dissolved after 1320 hrs incubated in 0.1N H2SO4 . . . 238
31. Cations released expressed as mole ratios, percentages of total cations released, and mMol/100 g clay after incubation in 0.005N H2 SO 4 . . : ...... 244
32. Weight percentage of clay dissolved after 1176 hrs incubation in 0.005N H2SO4 ...... 246
33. Characterization data for Dana Taxadjunct silt loam (WA-69)...... 266
xii LIST OF TABLES (Continued)
Table Page
34. Characterization data for Xenia silt loam (WA-70)...... 267
35. Characterization data for Rossmoyne silt loam (WA-71) ...... 268
36. Characterization data for Dana Variant silt loam (WA-72) ...... 269
37. Total mMol Mg/kg clay released from Xenia Bt2 horizon clay fraction after incubation in 0.IN H2SO4 at various incubation hrs .... 271
38. Total mMol K/kg clay released from Xenia Bt2 horizon clay fraction after incubation in 0.IN H2SO4 at various incubation hrs .... 272
39. Total mMol Al/kg clay released from Xenia Bt2 horizon clay fraction after incubation in 0.1N H2SO4 at various incubation hrs .... 273
40. Total mMol Si/kg clay released from Xenia Bt2 horizon clay fraction after incubation in 0.IN H2SO4 at various incubation hrs .... 274
41. Total mMol Fe/kg clay released from Xenia Bt2 horizon clay fraction after incubation in 0.IN H2SO4 at various incubation hrs .... 275
42. Total mMol Mg/kg clay released from Xenia Bt2 horizon clay fraction after incubation in 0.005N H2SO4 at various incubation hrs . . . 276
43. Total mMol K/kg clay released from Xenia Bt2 horizon clay fraction after incubation in 0.005N H2SO4 at various incubation hrs . . . 277 LIST OF FIGURES
Figure Page
1. Location of Warren County, study sites, and glacial deposits in Ohio...... 17
2. Parent material, glacial boundry, and site selection in Warren County, Ohio...... 18
3. Fine and total clay distribution for the Xenia and Dana p e d o n s ...... 2 6
4. Fine and total clay distribution for the Rossmoyne and Dana Variant pedons .... 27
5. X-ray diffractograms of the fine clay fraction from selected horizons of WA-69 & 70, Mg-25°C t r e a t m e n t ...... 32
6. X-ray diffractograms of the fine clay fraction from selected horizons of WA-69 & 70, Mg-Glycerol (Mg-Gly) treatment...... 3 3
7. X-ray diffractograms of the fine clay fraction from selected horizons of WA-69 & 70, K-25°C treatment...... 34
8. X-ray diffractograms of the fine clay fraction from selected horizons of WA-69 & 70, K-350°C tr e a t m e n t ...... 35
9. X-ray diffractograms of the fine clay fraction from selected horizons of WA-69 & 70, K—530°C treatment ...... 36
10. X-ray diffractograms of the fine clay fraction from selected horizons of WA-71 & 72, Mg-25°C treatment ...... 37
xiv LIST OF FIGURES (continued)
Figure Page
11. X-ray diffractograms of the fine clay fraction from selected horizons of WA-71 & 72, Mg-Glycerol (Mg-Gly) treatment...... 38
12. X-ray diffractograms of the fine clay fraction from selected horizons of WA-71 & 72, K—25°C treatment...... 39
13. X-ray diffractograms of the fine clay fraction from selected horizons of WA-71 & 72, K—350°C treatment ...... 40
14. X-ray diffractograms of the fine clay fraction from selected horizons of WA-71, K-530°C treatment...... 41
15. X-ray diffractograms of the fine clay fraction from selected horizons of WA-72, K-530°C treatment...... 42
16. X-ray diffractograms of the coarse clay fraction from selected horizons of WA-69 & 70, Mg-25°C t r e a t m e n t ...... 46
17. X-ray diffractograms of the coarse clay fraction from selected horizons of WA-69 & 70, Mg-Glycerol (Mg-Gly) treatment...... 4 7
18. X-ray diffractograms of the coarse clay fraction from selected horizons of WA-69 & 70, K-25°C treatment...... 48
19. X-ray diffractograms of the coarse clay fraction from selected horizons of WA-69 & 70, K—350°C treatment ...... 49
20. X-ray diffractograms of the coarse clay fraction from selected horizons of WA-69 & 70, K—530°C treatment ...... 50
xv LIST OF FIGURES (Continued)
Figure Page
21. X-ray diffractograms of the coarse clay fraction from selected horizons of WA-71 & 72, Mg-25°C treatment ...... 51
22. X-ray diffractograms of the coarse clay fraction from selected horizons of WA-71 & 72, Mg-Glycerol (Mg-Gly) treatment...... 52
23. X-ray diffractograms of the coarse clay fraction from selected horizons of WA-71 & 72, K-25°C treatment...... 53
24. X-ray diffractograms of the coarse clay fraction from selected horizons of WA-71, K-350°C treatment...... 54
25. X-ray diffractograms of the coarse clay fraction from selected horizons of WA-72, K-350°C treatment...... 55
26. X-ray diffractograms of the coarse clay fraction from selected horizons of WA-71 & 72, K-530°C treatment ...... 56
27. X-ray diffractograms of the total clay fraction from (1) Dana Ap, (2) Dana Bt2, (3) Xenia Ap, and (4) Xenia Bt2 horizon, Mg-Glycerol (Mg-Gly) treatment. 60
28. X-ray diffractograms of the total clay fraction from (1) Dana Variant Ap, (2) Dana Variant Btl, (3) Rossmoyne Apl, and (4) Rossmoyne Bt2 horizon, Mg-Glycerol (Mg-Gly) treatment. 61
29. Mechanism for the formation of soil humic substances (after Stevenson, 1982)...... 70
30. Typical fractionation scheme for humus. . . . 77
31. IR spectral pattern (4000-900 cm-1) for (1) Dana Ap B1 (2) Dana Ap B2, (3) Xenia Ap Bl, and (4) Xenia Ap B2, humic acids s a m p l e s ...... 131
xvi LIST OF FIGURES (Continued)
Figure Page
32. IR spectral pattern (4000-900 cm-1) for (1) Dana Variant Ap Bl, (2) Dana Variant Ap B2, (3) Dana Variant A Bl, (4) Dana Variant A B2, (5) Rossmoyne Apl Bl, and (6) Rossmoyne Apl B2, humic acids samples . . 132
33. IR spectral pattern (4000-900 cm'1) for (1) Dana Ap Bl, (2) Dana Ap B2, (3) Xenia Ap Bl, and (4) Xenia Ap B2, fulvic acids s a m p l e s ...... 13 5
34. IR spectral pattern (4000-900 cm-1) for (1) Dana Variant Ap Bl, (2) Dana Variant Ap B2, (3) Dana Variant A Bl, (4) Dana Variant A B2, (5) Rossmoyne Apl Bl, and (6) Rossmoyne Apl B2, fulvic acids samples. . 13 6
35. Representative 13C CPMAS NMR spectra of humic acid showing chemical shifts and divisions into regions for intensity analysis...... 139
36. 13C CPMAS NMR spectra of Bl and B2 humic acids from Dana Ap (WA-69) pedon...... 141
37. 13C CPMAS NMR spectra of Bl and B2 humic acids from Xenia Ap (WA-70) pe d o n ...... 142
38. 13C CPMAS NMR spectra of Bl and B2 humic acids from Dana Varient Ap (WA-72) pedon. . . 14 3
39. 13C CPMAS NMR spectra of Bl and B2 humic acids from Dana Varient A (WA-72) pedon . . . 144
40. 13C CPMAS NMR spectra of Bl and B2 humic acids from Rossmoyne Apl (WA-71) pedon. . . . 145
41. 13C CPMAS NMR spectra of (1) Xenia (WA-70 Ap Bl and (2) Dana (WA-69) Ap B2 fulvic acids. . 156
42. 13C CPMAS NMR spectra of (1) Rossmoyne (WA-71) Apl Bl and (2) Dana Variant (WA-72) Ap B2 fulvic acid ...... 157
xvii LIST OF FIGURES (Continued)
Figure Page
43. 13C CPMAS NMR spectra of fine material collected from sonified whole Dana Variant Ap Xenia Ap, and Dana Ap s o i l s ...... 174
44. Regression plot of sorption vs yMol of Ca + Mg ions in washing solutions for the clay + Alfisol HA sample ...... 222
45. Plot of total mMol Mg/kg clay released from Xenia Bt2 horizon clay fraction after incubation in 0.1N H2SO4 at various incubation hrs ...... 231
46. Plot of total mMol K/kg clay released from Xenia Bt2 horizon clay fraction after incubation in 0.1N H2SO4 at various incubation hrs ...... 232
47. Plot of total mMol Al/kg clay released from Xenia Bt2 horizon clay fraction after incubation in 0.IN H2SO4 at various incubation hrs ...... 23 3
48. Plot of total mMol Si/kg clay released from Xenia Bt2 horizon clay fraction after incubation in 0.1N H2SO4 at various incubation hrs ...... 234
49. Plot of total mMol Fe/kg clay released from Xenia Bt2 horizon clay fraction after incubation in 0.1N H2SO4 at various incubation hrs ...... 235
50. pH of dialyzate liquid for each treatment from 0.IN H2SO4 dissolution study ...... 239
51. Plot of total mMol Mg/kg clay released from Xenia Bt2 horizon clay fraction after incubation in 0.005N H2SO4 at various incubation hrs ...... 24 2
52. Plot of total mMol K/kg clay released from Xenia Bt2 horizon clay fraction after incubation in 0.005N H2SO4 at various incubation hrs ...... 24 3
53. pH of dialyzate liquid for each treatment from 0.005N H2SO4 dissolution study ...... 24 7
xviii INTRODUCTION
In southwestern Ohio, moderately well-drained
Mollisols and Alfisols occur in close proximity in many landscapes. Although, these soils are visually quite distinct in the landscape, they share similar parent materials and landscape positions. They differ primarily in the content of organic matter in the surface horizon with Mollisols having the higher amount. Although data currently available document that the organic matter contents are different, the primary question posed in this investigation concerns possible chemical and structural differences of organic matter in contiguous Alfisols and
Mollisols.
The chemical and structural characteristics of organic matter in soils is of particular interest to pedologists because humic components have been implicated in the weathering of minerals and the migration of metals
(Stevenson, 1982). It is hypothesized that functional groups present in humic and fulvic acid fractions play a role in regulating the degree or extent of mineral weathering. Thus a second aspect of this study probes the relationship between organic matter chemistry and the
1 degree of mineral weathering expressed in contiguous
Alfisols and Mollisols.
Historically, soil organic matter fractions have been studied following extraction with NaOH and purification with mineral acids and exchange resins. This method has been criticized because of incomplete extraction and chemical modification of humic components during extraction. Consequently there have been attempts to characterize whole soil organic matter. Such techniques were employed in this investigation.
The objective of this study was to characterize organic matter of adjacent Mollisols and Alfisols, and to evaluate its role in mineral weathering. The study has been divided into three sections: (1) the characterization of NaOH extracted humic and fulvic acids, (2) the characterization of concentrated soil organic matter, and
(3) an evaluation of the role of humic and fulvic acids in the weathering of soil clays. CHAPTER I
MATERIALS AND METHODS
1.1 Site and Pedon Selection
The main objective during site selection was to locate soils exhibiting a wide range in organic matter contents but with as many soil forming factors being held constant as possible. Southwestern Ohio offered the best possibili ties because (1) the entire landscape is mantled with a
Wisconsin age loess blanket and (2) both well and moderate ly well-drained Mollisols (dark-colored soils with high organic matter content) and Alfisols (light-colored soils with low organic matter content) occur in this region. It was necessary to concentrate our search on well or moder ately well-drained sites because all poorly-drained soils tend to be dark-colored and have high organic matter con tents and it was desirable to maintain constant drainage classes among all soils selected for study. It was also desirable to locate the pedons as close together as possible to ensure consistency of soil forming factors. A search was initiated in Warren County to locate adjacent
Alfisols and Mollisols. The Warren County Soil Survey
Report was used to locate potential sites for field visits.
A suitable site containing Alfisols and Mollisols in close
3 proximity was located in Warren County on the Shaker School
Campus in Otterbein, Ohio. In future discussions, this will be referred to as the Otterbein site.
Whereas, the Otterbein site is underlain by
Wisconsinan glacial till, the terminal Wisconsin glacial boundary occurs 7.3 km to the southeast. Southeast of the
Wisconsin terminal moraine, the soils generally have a lower organic matter content and are generally believed to be more intensely weathered although most of the soil sola have formed in the mantle of Wisconsinan loess just like the Otterbein site. Thus, in order to extend the range of organic matter contents and degree of weathering in the soils selected for study, a decision was made to select an additional moderately well-drained pedon for study just southeast of the Wisconsinan terminal moraine. While searching for a suitable pedon, a site was located which not only contained a suitable Alfisol but also offered a contiguous Mollisol. Although moderately well-drained
Mollisols are rare and certainly unexpected on the
Illinoian surface, a decision was made to select and sample a Mollisol-Alfisol pair of pedons at each site. The second site on the Illinoian surface southeast of the Wisconsinan boundary will be referred to as the Burton site. The two sample sites are 22.5 km apart.
At the Otterbein site, two pedons, a Mollisol (WA-69,
Dana) and an Alfisol (WA-70, Xenia) were sampled. These pedons were approximately 194 m apart and occurred in contiguous polypedons. Likewise at the Burton site, two pedons, a Mollisol (WA-72, Dana Variant) and an Alfisol
(WA-71, Rossmoyne) were sampled. The pedons at the Burton site were 190 m apart and occurred in contiguous poly pedons .
1.2 Morphological Description and Sample Collection
A pit approximately 1 to 2 m deep was dug with a backhoe for each pedon during the spring of 1985. A detailed description of each soil was made using the termi nology and horizon nomenclature of Chapter 4 of the new
Soil Survey Manual (Soil Survey Staff, 1981) . Detailed profile descriptions and characterization data for all pedons are given in Appendix A & B, respectively. Bulk samples weighing approximately 12 and 5 kg were collected from surface and subsurface horizons, respectively. Bucket auger samples were collected from the Rossmoyne pedon to a depth of 3.3 m. Soil monoliths were also taken of each p e d o n .
1.3 Soil Characterization
1.3.1 Sample Preparation
Bulk soil samples were dried in a forced-draft oven at
60°C. The samples were then weighed and crushed using either a wooden mallet, a rolling pin, or two electrically driven wooden rollers. Large rock fragments were removed by hand prior to grinding. The material which passed a
2 mm (NO. 10) brass sieve was placed into quart cartons and excess soil was stored in plastic bags.
1.3.2 Coarse Fragment Content
The material greater than 2 mm was placed back into the sample bag and was weighed. The coarse fragments
(> 2 mm) content was calculated based on total sample dry weight.
1.3.3 Particle Size Analysis
The particle size distribution for each soil horizon was determined using a modification of the pipet method of
Kilmer and Alexander (1949) and the Soil Survey Staff
(1972). A 10.00 g sample was dispersed with 5 ml of dis persing agent containing 35.70 g of sodium hexameta- phosphate and 7.94 g sodium carbonate per liter of solution. After adding the dispersing agent and 250 ml of deionized water, the samples were shaken for 12 hrs with an
Eberbach Model 6000 shaker adjusted to give 120 excursions per minute. Additional water was added to the samples to a total weight of 410 g. The samples were stirred for 1 min with an electric stirrer. Equation 3-2 of Jackson (1975) was utilized to calculate the required times in which,
6.84 ml aliquots of < 20, < 5, and < 2 m were taken at depths of 8, 5, and 5 cm, respectively. These aliquots were oven dried for 16 hrs at 105°C and then weighed. The greater than 50 ym fraction (total sand) was washed through
a 300 mesh sieve, then oven dried. The total sand was
dried, sieved through a series of sieves having openings of
1000, 500, 250, 100, 74, and 50 ym. The less than 0.2 ym
(fine clay) fraction was determined by pipetting a 25 ml-
aliquot of the dispersed soil solution into a 100-ml
centrifuge tube. An IEC Model K centrifuge with No. 240 head was used to centrifuge the tubes at 1055 X g. The time required for centrifugation was calculated using
Equation 3-7 of Jackson (1975), assuming a particle density
of 2.50 g/cm3. At a depth of 2 cm, a 6.84 ml aliquot was pipetted, oven dried for at least 16 hrs at 105°C, and weighed.
1.3.4 p H
The pH was measured for all horizons with a Beckman
Expandomatic Model SS-2 pH meter in a soil-water and a
soil-0.01M CaCl2 suspension of 1:1 and 1:2, respectively.
All suspensions were allowed to equilibrate for 1 hr with
frequent stirring, before pH readings were taken.
1.3.5 Organic Carbon
The organic carbon content of noncalcareous samples was determined by dry combustion at 950°C according to the method of Nelson and Sommers (1982). The following modifi cations were employed: (1) 2.00 g samples of 2 mm sieved soil were used, (2) 0.25 g MnC>2 was added to the soil in the combustion boat to remove sulfur and halogen gasses,
(3) CuO was used in the combustion tube as a catalyst, (4)
a sulfuric acid tower, was placed at the combustion tube
outlet, to act as a gas scrubber, (5) powerded Zn tubes were placed behind the sulfuric acid tower to ensure complete oxidation of CO, and (6) blank boats were not used because of negligible gas detection during operation.
1.3.6 Carbonate Content
The gasometric method of Dreimanis (1962) employing a
Chittick apparatus was used to determine the calcite, dolomite, and calcium carbonate equivalent. Depending on the carbonate content, either 3.4, 1.7, or 0.85 g samples were used which had been ground for 10 min with an auto matic mortar and pestle. Twenty ml of 6N HC1 containing
0.6 g of ferrous chloride was added to each sample, and CO2 was measured after 30 sec for calcite and 30 min for dolomite. The percent calcite and dolomite were calculated from regression equations developed from the empirical graphs of Dreimanis (1962, Figure 5) after volume correc tions were made for temperature and pressure.
1.3.7 Extractable Acidity
Extractable acidity was determined using the barium chloride-triethanolamine method suggested by Peech et al.
(1947) and Method 6H1 of the Soil Survey Staff (1972). As recommended by Peech et al. (1947), 10.00 g of soil were leached for 3 0 min with 50 ml of a pH 8.2 buffered solution which consisted of 0.5N barium chloride and 0.2N tri- ethanolamine. Next the soil was leached with 100 ml of
0.5N BaCl2 replacement solution. The leachate was titrated with 0.15N HCl using bromocresol green and a mixed indica tor containing methyl red and methylene blue (Soil Survey
Staff, 1972). An equal volume of buffer and replacement solution was titrated as a blank.
1.3.8 Extractable Cations
Fifty ml of 1.ON ammonium acetate adjusted to pH 7.0 was used to extract cations from 2.5 g of soil. A Concept
Engineering Model 24 Mechanical Vacuum Extractor, as described by Holmgren et al. (1977) was adjusted for an extraction time of 8-10 hr. One gram of Schleicher and
Schuell No.289 Ash-Free Analytical Filter Pulp was used to hold the soil sample in the extraction syringe. A Varian
Techtron AA-6 Atomic Absorption Spectrometer was used to determine Ca, Mg, K, and Na in the leachate. Instrument settings are given in Table 1.
1.3.9 Duplicate and Data Expression
All of the laboratory soil characterization analyses except coarse fragment content were preformed in duplicate or until duplication tolerances (usually + 2%) were met.
All data were expressed on an oven-dry basis. Table l. Instrument parameters used for atomic absorption (AAS) and flame emission (FES) spectrophotometry during the extractable cation procedure.
Element
Parameter Ca Mg K Na
Method AASAASFES FES
Wave1ength, nm 422.6 285.3 766.0 589.0
Fuel Acetylene Acetylene Acetylene Acetylene
Oxidant Nitrous oxide Nitrous oxide Air Air O H
Slit width, nm 0.1 • 0.2 0.1
Lamp current, mA 9 5 ——
Standard range, yg/ml 0-10 0-5 0-40 0-20
Dilution factor^ 25 or 10 25 or 10 1 1
+A11 standards and dilutions had a matrix of IN ammonium acetate. 11
1.4 Clav Mineralogy
1.4.1 Fractionation of Sand. Silt and Clav
The less than 2 mm fraction from selected horizons of
WA-69, 70, 71, and 72 were fractionated into sand (2000-
50 ym), silt (50-2 ym), and clay (<2 ym) for further mineralogical analysis. Forty gram soil samples were placed into 1000 ml glass beakers, and were then trans ferred into a 60°C water bath. To each, approximately
20 ml of IN NaOAc was added, while stirring. The soils were incubated in the bath until CO2 effervescence had ceased (Jackson, 1975). The samples were then treated with
30% H2O2 to remove organic matter (Jackson, 1975). All pretreated samples were quantitatively transferred to 250 ml centrifuge bottlee and centrifuged for 5 min at 490 X g. The supernatant was decanted, and 100 ml of IN NaCl was added. The tubes were shaken, centrifuged, decanted, and washed twice with 100 ml portions of deionized water.
Thirty ml of 0.5N Na2C03 (a dispersing agent) was added to each centrifuge tube. The samples were then mechanically dispersed with a ultrasonic probe (Branson Sonifer Cell
Disruptor Model W 185). The sand fraction and any remain ing coarse organic matter were removed by wet sieving through a 300 mesh sieve.
The remaining silt and clay fractions were transferred to a automatic fractionator developed by Rutledge et al.
(1967). The sedimentation time was calculated for the less than 2 pm fraction for a 10 cm settling depth using
Equation 3-2 of Jackson (1975). After the sedimentation
time had elapsed, the suspended clays were siphoned into a
plastic bucket. A total of 15 sedimentation cycles were
required to achieve nearly complete separation of the clay
and silt fractions. The clay fraction was concentrated using MgCl2 as a flocculant. The silt and sand fractions were transferred to 250 ml beakers and were dried at 80°C and weighed. The clays were Mg-saturated using MgCl2 and were then dispersed and freeze-dried.
1.4.2 Fine and Coarse Clav Fractionation
Two grams of total clay from the surface and argillic horizon were further fractionated into coarse (2-0.2 pm) and fine (< 0.2 ym) separates by centrifugation. Equation
3-7 of Jackson (1975) was used to calculate a centrifuga tion time of 48 min at 1055 X g to achieve a 0.2 ym separa tion. Distances R and S were measured to be 19.5 and 10.5 cm, respectively. An average temperature of 29°C and a specific gravity of 2.50 g/cc for the clay were assumed.
Following each centrifugation the fine clay (the top 9 cm of suspension) was decanted and flocculated with MgCl2 •
Deionized water was added to the residual clay in the
250 ml centrifuge bottle to maintain the suspension height at 10 cm and was redispersed. In most cases, complete 13
fractionation was achieved before 10 cycles of centrifuga
tion and decantation was completed.
All fine and coarse clays were then Mg-saturated with
MgCl2, sonified, and freeze-dried. Samples were later
stored in glass vials.
1.4.3 X-Rav Diffraction Analysis
Fine and coarse clays were Mg- and K-saturated and X-
rayed using parallel orientation mounts (Jackson, 1975).
Potassium-saturated clays were obtained by weighing 150 mg
of freeze-dried, Mg-saturated clay into a 12 ml curved-
plastic centrifuge tube. Ten ml of IN KCl were added, and
the sample was thoroughly shaken on a vortex mixer. The
sample was then centrifuged for 5 min using a Precision
Vari-Hi-Speed Centricone table-top centrifuge. The super
natant was decanted, and the salt washings were repeated
two times followed by a 10 ml deionized water wash. The
sample was then washed sequentially with 5 ml portions of deionized water until the clay remained suspended. The
suspension was brought to a 5 ml total volume with deion
ized water, transferred to a 50 ml plastic beaker, and dispersed for about 30 sec with an ultrasonic probe. The
K-saturated clays were then transferred to a glass vial.
One hundred-fifty mg of Mg-saturated clay was also weighed into a 50 ml plastic beaker and 5 ml of deionized water was added. The sample was sonified and stored in a glass vial. One ml of the Mg-saturated clay was pipetted into the 12 ml plastic centrifuge tube and centrifuged.
The supernatant was decanted, and 1 ml of 1% glycerol was added. The tube was thoroughly mixed on a vortex mixer.
One ml (30 mg of clay) of K- and Mg- and Mg-Glycerol clays were pipetted onto 27 x 46 mm petrographic slides and allowed to air dry. The K-slides for both fine and coarse clay were X-rayed at room temperature, heated for 4 hrs at
350°C, X-rayed, heated for 4 hrs at 530°C, and X-rayed again. The Mg- and Mg-Glycerol slides were X-rayed at room temperature. Slides were stored in a desiccator over P2O5 when not being analyzed.
X-ray diffraction (XRD) analyses were conducted using a Phillips Electronics PW 1316/90 Wide Range Goniometer,
XRG 3100 X-ray Generator, and DMS-41 Data Measuring System.
A Theta Compensating Slit and AMR Focusing Monochrometer were used to maintain a constant area of irradiation and for monochromatization of the X-ray beam, respectively.
Instrument operating conditions were as follows:
Target Copper
Radiation CuK
Potential 35 kV
Current 15 mA
Detector Scintillation
Detector Voltage 835.2V 15
Baseline 2.80
Window 4.00
Gain 128
Range 1000 cps
Time Constant 1 or 2
Scan Speed 2°2 e/min
Chart Speed 1 in/min
Diffractograms were normally obtained over a total scan range of 2°2 0 to 30°26 for the Mg-saturated clays at room temperature, and from 2°2 0 to 15°2 0 for all other cation and glycerol treatments. CHAPTER II
DESCRIPTION OF THE STUDY AREA AND SOIL CHARACTERISTICS
2.1 Location and Physiography
Four pedons from Warren County in southwestern Ohio
(Figure 1) were selected and studied in this investigation.
Pedons WA-69 & 70 (Dana and Xenia) are located on the
Shaker School Campus Farm in Otterbein about 4.8 km west of
Lebanon. Pedons WAr-71 & 72 (Rossmoyne and Dana Variant) are located on the Liston Burton Farm about 11.2 km east of
Lebanon near Huffman Cemetery.
Most of the county is mantled with Peoria loess which is underlain by either Illinoian or Wisconsinan glacial till (Figure 2). In the western and northern parts of the county, the Wisconsin till plain is nearly level to undu lating. Pedons WA-69 & 70 occur on a nearly level,
Wisconsinan ground moraine. In the southern and south eastern parts of the county, the Illinoian till plain is level to gently sloping but highly dissected. Pedons WA-71
& 72 occur on the nearly level, Illinoian ground moraine about 200 m south of Spring Hill. Spring Hill is a lime stone bedrock high about 50 m in height.
16 WISCONSINAN
ILLINOIAN
Figure 1. Location of Warren County, study sites, and glacial deposits in Ohio. 18
WARREN COUNTY, OHIO
NO. PARENT MATERIAL 1 WISCONSINAN GLACIAL TILL 2 ILLINOIAN GLACIAL TILL 3 OUTWASH AND ALLUVIUM
NORTH
Lebanon
O WA-69 & 70 • WA- 71 & 72
0 1 2 3 4 5
Scale -- miles
Figure 2. Parent material, glacial boundary, and site selection in Warren County, Ohio. 19
Most of the county is drained by the Little Miami
River and/or one of its tributaries, including Todd Fork and Caesars Creek. The northwestern part of the county is drained by the Miami river.
The elevation in the county ranges from 183 m above sea level in the Little Miami Valley to 305 m on the upland about 2 miles northeast of Ft. Ancient (Wolford, 1927).
2.2 Bedrock Geology
Detailed information about the bedrock geology of the study area is available (Orton, 1878; Fenneman, 1916;
Wolford, 1927; Stout, 1941; Goldthwait, 1961). Based on these sources, the underlying bedrock of Warren County is largely composed of two formations, the Maysville and
Richmond of the Ordovician system. Both the Maysville and
Richmond Formations are composed of alternating layers of calcareous limestone and shale (Fenneman, 1916). The
Maysville Formation covers the southwestern portion of the county and the valley floors of the Miami and Little Miami rivers in the northern portion (Stout, 1941). The Richmond
Formation covers the northern part of the county and is present on the uplands in the southern portion (Stout,
1941). Since both study sites occur largely in the north central part of the county and in upland positions, the underlying bedrock of both study sites is probably 20
Ordivician-age interbedded limestone and calcareous shale of the Richmond Formation.
2.3 Glacial Geology
The oldest glacial drift recognized in the county is of Illinoian (Wolford, 1927). Leverett (1902) described this till as compact, harder than Wisconsin till, and indurated as a result of partial cementation with lime.
The depth of leaching of the Illinoian drift is usually 1.8 to 2.8 m whereas the leaching depth of the Wisconsinan drift is less than 1.0 m (Leverett, 1902; Rogers, 1936;
Rosengreen, 1974).
The northern and western parts of the county was also glaciated during the late Wisconsinan period (Wolford,
1927) . This ice-sheet left material which covers the northwestern portion of Warren county to a line extending roughly from the northeastern to the southwestern corner of the county. A terminal moraine, 0.8 to 3.2 km wide forms a continuous fringe along this glacial lobe (Wolford, 1927) .
Wisconsinan loess mantles both the Illinoian and
Wisconsinan till plains in southwestern, Ohio (Leverett,
1902). The loess mantle on the Xenia, Dana, Rossmoyne, and
Dana Variant pedons is 76, 84, 86, and 102 cm in thickness, respectively. 21
2.4 Climate and Vegetation
The climate of Ohio is classified as humid and temper
ate. Data from Franklin, in the northwestern part of
Warren County, indicate an annual average temperature of
11 C. The average annual precipitation at Franklin is
92.6 cm. The natural vegetation (i.e. prior to settlement)
of the study areas consisted of a mixed hardwood forest and
prairie grassland (Gordon, 1969). According to the offi
cial USDA/SCS series descriptions, the native vegetation
for Dana is prairie grass, and for Rossmoyne and Xenia it
is hardwood trees.
The organic carbon content of the two Mollisols and
Alfisols may have been influenced by the native vegetation.
Prairie vegetation annually contributes a large amount of
organic matter to soils much of which is composed of roots which are easily incorporated into the soil. This results
in a high organic carbon content. On the other hand, soils with mixed hardwoods vegetation are lower in organic carbon
content because mixed hardwoods annually contribute a lower
amount of leaves and roots to soils, and the roots and
leaves are slowly decomposed and incorporated.
At time of sampling, the Otterbein and Burton sites were presently under cultivation with soybeans and corn,
respectively. 22
2.5 Soil Characteristics
2.5.1 Morphological Description
Detailed morphological descriptions of the Dana,
Xenia, Dana Variant, and Rossmoyne pedons were made from field observations and are presented in Appendix A. The family, subgroup, and series classification of the four pedons are presented in Table 2. A brief morphological description of each pedon is presented below.
The Dana and Xenia series are deep, moderately well- drained soils formed in Peoria Loess and the underlying calcareous Wisconsinan glacial till. Both pedons have a silt loam epipedon and clay loam, silty clay loam, and loam textures in the underlying horizons. The Dana and Xenia pedons have solum thicknesses of 96 and 89 cm, respectively and abundant clay films on ped faces in the B horizons.
Dominant matrix colors in the A horizon is 10YR 3/2 and
10YR 4/2, and in the B horizons, and 10YR 5/4 and 10YR 4/4.
The Dana pedon does not have an argillic horizon (see
Section 2.5.2), whereas the Xenia pedon does. Consequent ly, the Dana pedon which classifies as a Hapludoll is a taxajunct of the Dana series.
The Rossmoyne and Dana Variant pedons consist of deep, moderately well-drained soils formed in a mantle of Peoria
Loess and underlying Illinoian glacial till. Both pedons have a silt loam epipedon and silty clay loam and silty clay textures in the B horizons. The Dana Variant and I
Table 2. Classification of the soils studied.
Pedon Family Subgroup Series
WA-69 fine-silty, mixed, mesic Typic Hapludoll Dana
WA-72 fine silty, mixed, mesic Typic Hapludoll Dana Variant
WA-70 fine-silty, mixed, mesic Aquic Hapludalf Xenia
WA-71 fine-silty, mixed, mesic Aquic Fragiudalf Rossmoyne
to u> Rossmoyne pedons have solum thicknesses of 152 and 188 cm,
respectively. The dominant matrix colors of the Ap hori
zons derived from loess are 10YR 4/2 and 10YR 3/2, and in
the B horizons 10YR 5/6. The Rossmoyne pedon has an
argillic horizon extending from the loess and into the
till, and also a fragipan in the till. The Dana Variant
pedon lacks an argillic horizon (see Section 2.5.2) and
classifies as a Hapludoll. There are abundant clay film
coatings on ped faces throughout the B horizons derived
from loess in both pedons. However, only the Rossmoyne pedon has silt and ferromagans coatings on ped faces in the
B horizons.
In addition to field and laboratory data, official
USDA/SCS series descriptions and a Warren County soil survey report were consulted to review morphological data.
A modern soil survey is available (Garner et al. 197 3) for
Warren County which provides detailed information regarding the properties of the soils in the county.
2.5.2 Particle Size Analysis
The particle size analysis data for each pedon is presented in Appendix B. The upper portions of all pedons are dominated by silt which is typical for soils derived from loess. The lower horizons which are derived from
Wisconsinan and Illinoian till exhibit a sharp increase in sand content. The fine and total clay distributions are presented in Figures 3 and 4. The upper most horizons of
the Alfisols (Xenia and Rossmoyne) contain less fine and
total clay contents than the associated Mollisols. In the
Xenia pedon, the total clay maximum occurs in the loess,
whereas in the Rossmoyne pedon, the total clay maximum
occurs in the Illinoian till. In general, the fine and
total clay contents in the solum of both Alfisols are
similar to that reported by Rutledge (1975a). The fine and
total clay contents in the B and C horizons are similar for
both the Alfisols and Mollisols.
There are argillic horizons in the loess layers of the
Alfisols. The presence of an argillic horizon in both
Alfisols is supported by the following: (1) a 69 to 86%
increase in total clay content of the B horizon relative to
the overlying A horizons, (2) an increase in the fine to
total clay ratio with depth (Table 3), and (3) presence of
clay films in the B horizons.
Whereas field observations suggest that an argillic horizon occurs in the Mollisols, particle size analysis does not support the occurrence of an argillic horizon in either Mollisol because: (1) the fine and total clay contents remain fairly uniform throughout the solum, and
(2) the fine to total clay ratios decrease with depth
(Table 3). iue . Fineand total claydistribution Figure 3. for the Xenia and
DEPTH [m| 0.5 1.0 0 Dana pedons. 10 20 CLAY % 30 40 ei F O O O— FC Xenia aa C —☆ ☆— FC Dana C • TC 50 C □ TC
60 26
iue . Fineand total claydistribution Figurefor 4. the Rossmoyne
DEPTH |m| 3.0 2.0 1.0 0 andDana Variant pedons. 10
20 CLAY %
30
aa ain F A ☆ A— FC Variant Dana oson F O O O— FC Rossmoyne 40
C □ TC 50
27
28
Table 3. Fine to total clay ratios^ of select horizons from Dana, Dana Variant, Xenia and Rossmoyne pedons.
Fine Clav Soil Horizon Depth Total Clay
- cm -
Dana Ap 0-15 0.53 Bt2 56-68 0.35
Dana Variant Ap 0-25 0.54 Btl 58-61 0. 34
Xenia Ap 0-20 0. 24 Bt2 48-61 0.35
Rossmoyne Apl 0-20 0.23 Bt2 51-68 0.29
Ratios were calculated using data in Appendix B.
It is important to note that the fine and total clay contents are 2 to 5 times greater than and fine to total clay ratios of the surface horizons of Mollisols are approximately 2 to 2.5 times greater than the corresponding values in the associated Alfisols. Because contiguous pedons at both sites formed in the same loess mantle, pedogenic processes must be responsible for the differences noted between the Alfisols and Mollisols at each site.
Two possible hypotheses could explain the contrasting fine and total clay contents in the surface horizons of the contiguous Alfisols and Mollisols. One hypothesis is that the fine and total clay contents within each pedon were mainly influenced by clay translocation. This hypothesis is rejected because it does not adequately explain the lack of any difference in the clay content between the B hori zons of the Alfisols and Mollisols. If clays are trans located in the Alfisols, the clays should accumulate in the argillic horizon. Hence, the clay contents in the argillic horizon of the Alfisols should be higher than in comparable zones of the Mollisols. Instead, the clay contents in the argillic horizons of the Alfisols are essentially the same as the clay contents in the Mollisols. This suggests that the accumulation of translocated clays in the B horizons is essentially the same in both Alfisols and Mollisols.
Therefore, clay translocation does not seem to be causing the contrasting clay contents in the surface horizons of adjacent Alfisols and Mollisols.
An alternate hypothesis for the differing clay contents in the surface horizons may be that clay minerals in Alfisols and Mollisols are weathering at dissimilar rates with clay minerals in Mollisols being more resistant to weathering. An explanation for this hypothesis may be that clay minerals in Mollisol surface horizons are more resistant to chemical dissolution due to greater sorption of soil humic substances. There is approximately 1 to 2 times more organic matter in the surface horizons of the
Mollisols than associated Alfisols. It has been shown that a large portion (> 50%) of the organic matter in soils is sorbed by mineral constituents, notably the clay fraction (Anderson, 1979). Humic substances sorbed to clays may decrease their susceptibility to decomposition. The increased resistance to mineral breakdown attributed to humic substances may be related to their capacity to buffer reactions or to the formation of a cation:organic crust around the particles which limits proton penetration and diffusion of released cations. The magnitude of mineral protection is probably dependent on the proportion of the mineral surface area covered. One might hypothesize that clay minerals in Alfisols have less sorbed organic matter which results in less mineral protection. With less protection, clay minerals in Alfisols would be more susceptible to dissolution.
2.5.3 Chemical Properties
Data for the chemical analyses are presented in
Appendix B. The Dana and Dana Variant Ap horizons have a organic carbon contents of 2.49 and 1.96%, respectively.
The organic carbon contents of the Xenia and Rossmoyne Ap horizons are 1.48 and 1.10%, respectively. All pedons have a near neutral pH throughout the solum, except the Dana and
Xenia surface horizons which are fairly acidic. It should be noted that the pH values of the Rossmoyne and Dana
Variant pedons are higher than typical for Illinoian age glacial soils. Carbonate-enriched seepage from Spring Hill has contributed to the near neutral pH and high base status 31
of these pedons. All pedons also have a high base status
in the solum, but only the Dana and Xenia pedons contain
free carbonates (at 84 to 89 cm, respectively).
2.5.4 Mineralogy
A. Clav Mineralogy of the Fine Clay Fraction
Representative XRD patterns for the fine clay
fractions of selected horizons from the Dana, Xenia,
Rossmoyne, and Dana Variant (WA-69, 70, 71, and 72) pedons
are presented in Figures 5 through 15. Clay mica can be
identified in all four pedons based on the presence of 1.0
and 0.5 nm peaks with Mg-25°C treatment. In all four
pedons, the presence of kaolinite is indicated by a 0.71 nm peak with the K-25°C treatment which is destroyed when heated to 530°C. The absence of any 1.4 or 0.7 nm peaks
after heating to 53 0°C indicates that chlorite is not present. Goethite can be identified based on the presence
of 0.42 nm unit layer spacing peak with Mg-25°C treatment.
Vermiculite and smectite are routinely distinguished by XRD using Mg saturation and solvation with either
ethylene glycol or glycerol (Mg-Gly). After glycerol
treatment, Mg-smectite will exhibit a reflection between
1.7 to 1.8 nm. Glycerol was used as the solvation agent in
this study. Expansion to a unit layer spacing of 1.8 nm which is indicative of smectite was observed in the fine
clay fraction of all four pedons. A plateau between 1.4 32
1.4 nm 0.32 0.49 1.0 0.35 0.72 Dana
Bt2
Xenia
Bt2
Figure 5. X-ray diffractograms of the fine clay fraction from selected horizons of WA-69 & 70, Mg-25 C treatment. 33
< 0 .2 um Mg-GLY
1.4 1.8nm
0.71
Dana
Ap
Bt2
Xenia
Ap
Bt2
Figure 6. X-ray diffractograms of the fine clay fraction from selected horizons of WA-69 & 70, Mg-Glycerol (Mg-Gly) treatment. 34
1.4 nm < 0.2 urn K -25°C 0.71
Dana Ap
Bt2
Xenia
Bt2 2°20
Figure 7. X-ray diffractograms of the fine clay fraction from selected horizons of WA-69 & 70, K-25°C treatment. 35
< 0.2 um
1.0 1.4 nm 0.71
Dana
Xenia
Bt2 2°20
Figure 8. X-ray diffractograms of the fine clay fraction from selected horizons of WA-69 & 70, K-350°C treatment. 36
1.4 nm
0.71
Dana
Bt2
Xenia
B 12 2°20
Figure 9. X-ray diffractograms of the fine clay fraction from selected horizons of WA-69 & 70, K-530°C treatment. 37
<0.2um Mg- 25°C
Dana Var. 1.4 nm 1.0 0.33 0.36 0.71 0.5 0.42
Bt1
Rossmoyne
Ap1
Bt2
2°20
Figure 10. X-ray diffractograms of the fine clay fraction from selected horizons of WA-71 & 72, Mg-25 C treatment. 38
<0.2um 1.0 14 1.8nm Mg-GLY 0.71
Dana Var.
Bt1
Rossmoyne
Ap1
Bt2 2°20
Figure 11. X-ray diffractograms of the fine clay fraction from selected horizons of WA-71 & 72, Mg-Glycerol (Mg-Gly) treatment. 39
1.0 1.4nm
0.71
Dana Var.
Ap
Bt1
Rossmoyne
Ap1
Bt2 2°20
Figure 12. X-ray diffractograms of the fine clay fraction from selected horizons of WA-71 & 72, K-25 C treatment. 40 1.0 1.4 nm
0.71
Dana Var.
Bt1
Rossmoyne
Ap1
Bt2 2°20
Figure 13. X-ray diffractograms of the fine clay fraction from selected horizons of WA-71 & 72, K-350°C treatment. 41
1.0 0.71 1.4 nm < 0.2 um K-530 °C
Rossmoyne
Ap1
Bt2 2°20
Figure 14. X-ray diffractograms of the fine clay fraction from selected horizons of WA-71, K-530°C treatment. 42
0.71 1.0 1.4nm
Dana Var.
Ap
Bt1 2°20
Figure 15. X-ray diffractograms of the fine clay fraction from selected horizons of WA-72, K-530 C treatment. and 1.8 nm which is indicative of smectite interstratified with other 2:1 minerals was also observed in the fine fraction of all four pedons. The plateau is more observeable in the Ap than the Bt horizons. Henceforth, smectite clay minerals includes any components expandable to > 1.4 nm with glycerol. A unit layer spacing of 1.4 nm with Mg-Gly treatment, which later collapsed to 1.0 nm with heating to 350°C which is indicative of vermiculite, is also observed in the fine clay fraction of all four pedons.
Aluminum interlayered 2:1 vermiculite can be identified in the fine clay fractions of all four pedons based on presence of a broad peak from 1.4 to 1.0 nm for K-25°C
(Figures 7 and 12) treatment that shifted toward 1.0 nm upon heating to 350 and 5300C (Figures 8, 9, 13, 14, and
15) .
Semi-quantitative estimates of clay mineralogy were made for the fine clay fractions from selected horizons of
WA-69, 70, 71, and 72 (Table 4). Vermiculite is the domi nant mineral in the fine clay fraction of all four pedons followed in decreasing amounts by clay mica, kaolinite, smectite, and goethite. No quartz or feldspars are detected. Clay mica content is lower in the Dana and Xenia pedons (5-15%) than in Rossmoyne and Dana Variant pedons
(15-25%). Smectite content is lowest in the Dana pedon
(< 5%), intermediate in the Xenia and Rossmoyne pedons (5-
15%), and highest in the Dana Variant pedon (15-25%). Table 4. Semi-quantitative estimates^ of clay mineralogy^ of fine clay fraction from selected horizons of WA-69, 72, 70, and 71 (Dana, Dana Variant, Xenia, and Rossmoyne).
Site Depth Horizon CM V§ S* K Q GF
- cm -
WA-69 0-15 Ap XX XXXXX XXXX (Dana) 56-69 Bt2 XX XXXXX XXXX
WA-72 0-25 Ap XXX XXXXX XXXXX ___ X (Dana V . ) 58-81 Btl XXX XXXXX XXXXXX
WA-70 0-20 Ap XX XXXXX XX XX ______- X (Xenia) 48-61 Bt2 XX XXXXX X XX X
WA-71 0-20 Apl XXXXXXXXXX XX X (Rossmoyne) 51-69 Bt2 XXX XXXXX XXX X
^Abundances are X = <5%, XX = 5-15%, XXX = 15-25%, XXXX = 25-35%, XXXXX = 35-45%. * CM = clay mica, V = vermiculite, S = smectite, K = kaolinite, Q = quartz, G = goethite, .and F = feldspars. ^Indicates relative degree of Al-interlayering. Indicates smectite interstratified with other 2:1 minerals. 45
It is interesting to note the high proportion of smectite-type clay minerals in the fine clay fraction of pedons at the Burton site. The high proportion of smectite-type clay minerals in these pedons may be due to carbonate-enriched groundwater creating an environment favorable for smectite formation and/or stability.
Smectite-type clay mineral are thermodynamically stable and form in a base rich environments.
Overall, there are no major differences in the fine clay mineralogy between Alfisols and Mollisols within each site. However, considerably more aluminum interlayered vermiculite and less smectite occurs in the fine clay fraction of the Dana and Xenia pedons than in the Rossmoyne and Dana Variant pedons. Because all four pedons formed in the same loess mantle, the low clay mica and smectites contents and slight increase in aluminum interlayering suggest that clays at the Otterbein site (Dana and Xenia pedons) are slightly more weathered than those at the
Burton (Dana Variant and Rossmoyne pedons) site.
B. Clay Mineralogy of the Coarse Clay Fraction
Representative XRD patterns of coarse clay fraction from selected horizons of the Dana, Xenia, Rossmoyne, and
Dana Variant pedons (WA-69, 70, 71, and 72) are presented in Figures 16 through 26. Various clay minerals were identified as described in Section 2.5.4.A, for the fine 46
2-0.2 um 0.33 Mg-25°C 1.4nm 0.36 Dana 0.42 1.0 0.71 0.5
Bt2
Xenia
Bt2 2°20
Figure 16. X-ray diffractograms of the coarse clay fraction from selected horizons of WA-69 & 70/ Mg-25 C treatment. 47
2-0.2 um Mg-GLY
1.8nm 1.4 0.71
Dana
Bt2
Xenia
Bt2
^ n y l-av H i f f ractocjrams of the coarse clay fraction Figure 17- teaA r i z o n a o£ WA-69 6 70, Mg-GlyceTol (Mg-Gly) treatment. 48
2-0.2 urn K-25°C 1.0 0.71 1.4 nm Dana
Ap
Bt2
Xenia
Bt2 2°20
Figure 18. X-ray diffractograms of M a i M c h y g ^ ti0n from selected horizons of WA-69 & 70, ^ treatment. 49
2-0.2 um K -350 °C 1.0 0.71 1.4 nm Dana
Bt2
Xenia
Ap
Bt2 2°20
Figure 19. X-ray diffractograms of the coarse clay fraction from selected horizons of WA-69 & 70, K-3500C treatment. 50
-0.2 um 1.0 1.4 nm Dana 0.71
Bt2
Xenia
Bt2 2°20
Figure 20. X-ray diffractograms of the coarse clay fraction from selected horizons of WA-69 & 70, K-53 00C treatment. 51
2-0.2 um Mg-25°C
1.4 nm Dana Var. 0.36 0.42 1.0 0.33 0.5 0.71
Bt 1
Rcsr .icyne
Ap1
Bt 2 2°20
Figure 21. X-ray diffractograms of the coarse clay fraction from selected horizons of WA-71 & 72, Mg-25°C treatment. 52
2-0.2 urn Mg-GLY 1.8 nm 0.71 1.0
Dana Var.
AP
Bt1
Rossmoyne
Ap1
Bt2
2°20
Figure 22. X-ray diffractograms of the coarse clay fraction from selected horizons of WA-71 & 72, Mg-Glycerol (Mg-Gly) treatment. 53
2 -0 .2um 1.0 1.4 nm K-25°C 0.71
Dana Var.
Bt1
Rossmoyne
Ap1
Bt 2 2°20
Figure 23. X-ray diffractograms of the coarse clay fraction from selected horizons of WA-71 & 72, K-25°C treatment. 54
2-0.2 um K-350°C
1.0 1.4nm Rossmonyne
0.71
Ap1
Bt2 2°29
Figure 24. X-ray diffractograms of the coarse clay fraction from selected horizons of WA-71, K- 350°C treatment. 55
2-0.2 um K-350°C 1.0 1.4nm
Dana 0.71 Var.
Ap
Bt1
Figure 25. X-ray diffractograms of the coarse clay fraction from selected horizons of WA-72, K- 350°C treatment. 0.71 1.0 1.4 nm
Dana Var.
Ap
Bt1
Rossmoyne
Ap1
Bt2 2°20
Figure 26. X-ray diffractograms of the coarse clay fraction from selected horizons of WA-71 & 72, K-530°C treatment. 57 clay fraction. In addition, quartz was identified based on the presence of a 0.426 nm peak whereas feldspars were identified by 0.325 and 0.320 nm peaks.
In all four pedons, clay mica, vermiculite, and kao- linite can be identified in both the Ap and Bt2 horizons.
Although, smectite is not a common constituent of the coarse clay fraction in most soils, it is present in both horizons of all four pedons. Also observed was expansion from 1.4 to 1.8 nm in the Mg-Gly treatments of all four pedons which is indicative of smectite interstratified with other 2:1 minerals. Quartz and feldspars also occur in most horizons of all four pedons.
Semi-quantitative estimates of the clay mineralogy were made for the coarse clay fractions from selected horizons of WA-69, 70, 71, and 72 (Table 5). In all four pedons, the dominant clay minerals are vermiculite and smectite in about equal amounts followed closely by kaolinite and clay mica in about equal amounts. Quartz and feldspars occur in minor amounts and are constant with depth. Smectite type minerals increase with depth in all four pedons. Kaolinite and smectite type minerals are more prevalent in the coarse than in the fine clay fractions.
No goethite or Al-interlayered minerals can be de tected in the coarse clay fraction of any pedon. Overall, there are no differences in the coarse clay mineralogy between Alfisols and Mollisols within each site, and I
Table 5. Semi-quantitative estimates'*' of clay mineralogyf for the coarse clay fraction from selected horizons of WA-69, 72, 70, 71 (Dana, Dana Variant, Xenia and Rossmoyne).
Site Depth Horizon CM V s§ K Q GF
- cm -
WA-69 0-15 Ap XXX XXXXXXXXXXXX X (Dana) 56-69 Bt2 XXX XXXXXXXXXXXXX X
WA-72 0-25 Ap XXX XXXXXXXXXXX ___ X (Dana V.) 58-81 Btl XXX XXXX XXXX XXX XX X
WA-70 0-20 Ap XXX XXXX XXX XXX XXX (Xenia) 48-61 Bt2 XXX XXXXXXXXXXXXX X
t WA-71 0-20 Apl XXX XXXXXXXXXXX X (Rossm.) 51-69 Bt2 XXXXXXX XXXXXXXXX ------X
^Abundances are X = <5%, XX = 5-15%, XXX = 15-25%, XXXX = 25-35%, XXXXX = 35-45%. tcM = clay mica, V = vermiculite, S = smectite, K = kaolinite, Q = quartz, G = goethite, and F = feldspars. ^Indicates smectite interstratified with other 2:1 minerals.
U1 00 59 between soils at the Otterbein (Dana and Xenia) and Burton sites (Rossmoyne and Dana Variant).
C. Clay Mineralogy of the Total Clay Fraction
Representative XRD patterns of the Mg-Glycerol treatments for the total clay fractions from selected horizons of the Dana, Xenia, Rossmoyne, and Dana Variant pedons (WA-69, 70, 71, and 72) are presented in Figures 27 and 28. Various clay minerals were identified as described in Section 2.5.4.A, for the total clay fraction. Semi- quantitative estimates of clay mineralogy are presented in
Tables 6 and 7. Similar clay mica contents occur in the loess portions of Dana, Xenia, and Rossmoyne pedons, but the clay mica content is slightly higher in the loess of the Dana Variant pedon. Clay mica content tends to increase with depth in the Dana and Xenia pedons with higher contents evident in the Wisconsinan till than the overlying loess. Clay mica content in the Illinoian till of the Rossmoyne and Dana Variant pedons is essentially the same as that of the overlying loess.
Vermiculite content of the Dana and Xenia pedons is similar and increases with depth. In contrast, higher vermiculite contents occur in the solum of Rossmoyne than the Dana Variant pedon. Vermiculite contents also tend to increase with depth in the Rossmoyne and Dana Variant 60
2 Ourn Mg GLY 0.71
Figure 27. X-ray diffractograms of the total clay fraction from (1) Dana Ap, (2) Dana Bt2, (3) Xenia Ap, and (4) Xenia Bt2 horizon, Mg-Glycerol (Mg-Gly) treatment. 2 - O u m M g -GLY
I.Snm
1.4
0.71
Figure 28. X-ray diffractograms of the total clay fraction from (1) Dana Variant Ap, (2) Dana Variant Btl, (3) Rossmoyne Apl, and (4) Rossmoyne Bt2 horizon, Mg-Glycerol (Mg-Gly) treatment. I f
Table 6. Semi-quantitative estimates^ of clay mineralogy^ for the total clay fraction from selected horizons of WA-69 and 70 (Dana and Xenia).
Site Depth Horizon CM v§ a ' c K Q G
- cm -
WA-69 0-15 Ap XXX XXX XXXX XX XX X (Dana) 15-33 A XXX XXXXXXX --- XX XX X 56-69 Bt2 XXXXXXXXXXX --- XXXX X 84-97 2BC XXXX XXXX XXX --- XXXX X 97-122 2C XXXXX XXXX XX XX XXXXX
WA-70 0-20 Ap XXX XXXXXX --- XX XX X (Xenia) 20-36 AB XXX XXXXXX --- XX XX X 48-61 Bt2 XXXX XXXX XXXX --- XXXXX 76-89 2BC XXXXXXXXXX --- X XXX 125-152 2C3 XXXXX XXXX ---- XX X XX X
* Abundances are X = < 5%, XX = 5-15%, XXX = 15-25%, XXXX = 25-35%, XXXXX = 35-45%. * CM = clay mica, V = vermiculite, S = smectite, C = chlorite, K = kaolinite, Q = quartz and G = goethite. Indicates the relative degree of Al-interlayering. ^Indicates smectite interstratified with other 2:1 minerals.
G\ to I
Table 7. Semi-quantitative estimates'" of clay mineralogy^ for the total clay fraction from selected horizons of WA-71 and 72 (Rossmoyne and Dana Variant).
Site Depth Horizon CM V§ s* c K Q G
- cm -
WA-71 0-20 Apl XXX XXX XXX X XX (Rossmoyne) 51-69 Bt2 XXX XXXXXXXX --- XX XXX 86-112 2Btxl XXX XXXX XX --- XXXX 155-168 2Bt2 XXXXXXXXXX --- XX XXX 188-208 2Bt4 XXX XXXXX XX --- XXXX 254-272 2Bt7 XXXXXXXXXX --- XXXX 310-328 2Bt9 XXXXXXXXXX —— XX XX ---
WA-72 0-25 Ap XXXX XXX XXXX --- XX XX X (Dana V.) 25-43 A XXXX XXX XXXXX --- XXXXX 59-81 Btl XXXX XXXXXXXX --- XX XXX 102-122 2Bt3 XXXX XXXX XXX --- X XX X 140-152 2Bt5 XXX XXXXX XX --- X XX X
^Abundances are X = < 5%, XX = 5-15%, XXX = 15-25%, XXXX = 25-35%, XXXXX = 35-45%. TCM = clay mica, V = vermiculite, S = smectite, C = chlorite, K = kaolinite, Q = quartz, .and G = goethite. ^Indicates the relative degree of Al-interlayering. Indicates smectite interstratified with other 2:1 minerals.
LO 64
pedons. Al-interlayered vermiculite occurs in upper
horizons of all four pedons.
In the total clay fraction, Mollisols tend to have a higher smectite content than the Alfisols. In addition, some Al-interlayered smectitic clays occur in both Alfisols
(Xenia and Rossmoyne) but only one Mollisol (Dana). This suggests that smectite type clay minerals are more stable in the Mollisols than Alfisols. On the other hand, the weathering environment in the Alfisols may be more intense than that in the Mollisols which results in the decomposition of smectitic clays and causes Al- interlayering in the fraction remaining. These findings support the hypothesis that clay minerals, notably smectitic clays are more protected from weathering reactions in Mollisols than Alfisols. Additional evidence for the dissimilar rates of clay mineral weathering were shown by Mollisols having a higher fine and total clay content in surface horizons than associated Alfisols.
Maximum smectite contents occur in the Btl and Bt2 horizon of the Dana Variant and Dana, Xenia, and Rossmoyne pedons. Smectite contents in both the Wisconsin and
Illinoian glacial tills are similar. Chlorite minerals were not detected except in the lowest horizons of both
Dana and Xenia pedons. Kaolinite and goethite are both a minor constituent in all four pedons. The quartz contents in all four pedons is similar. 65
In summary, the clay mineralogy of the horizons derived from loess material at both sites are dominated by vermiculite, smectite, and clay mica wrth low amounts of kaolinite. Similar conclusions were reached by Rutledge et al. (1975b) who found that the clay mineralogy of Alfisols formed in loess in southwestern Ohio was high in vermicu lite and clay mica.
The clay mineralogy of both tills are dominated by clay mica and vermiculite, with lower amounts of smectite and kaolinite. Rutledge et al. (1975b) reported that the clay mineralogy of Illinoian till underlying Wisconsinan loess was dominated by vermiculite and smectite. The amount of smectite found in the Illinoian-age glacial material in this study is lower than that reported by
Rutledge et al. (1975b).
In conclusion, there is a higher smectite (includes other 2:1 minerals expandable to greater than 1.4 nm with glycerol) content and a lower amount of Al-interlayering in the smectite clays in the Mollisols total clay fraction from horizons of the loess layer. In addition, Mollisols have a higher total and fine clay content in the loess layers than the Alfisols. The stability of smectite type clay minerals in Mollisols may be due to sorbed organic matter. On the other hand, smectite type clay minerals are not as stable in the loess layers of both Alfisols. The increased degree of weathering has resulted in Alfisols at both sites having a lower smectite, fine and total clay contents in the loess layers relative to Mollisols. The vermiculite contents in the loess layers of the Dana,
Xenia, and Dana Variant pedons are similar whereas, the vermiculite content of the Rossmoyne pedon is higher. In the loess layers, soils at the Otterbein site exhibit more
Al-interlayered vermiculite in the total clay fraction than soils at the Burton site. Finally, the amounts of clay mica, chlorite, kaolinite, quartz and goethite in the total clay fractions of all four pedons are somewhat similar. CHAPTER III
SOIL ORGANIC MATTER CHARACTERIZATION
3.1 Introduction
Soil organic matter is composed of a broad spectrum of
organic constituents, however two major types of compounds
can be distinguished: non-humic and humic substances.
Non-humic substances are composed of fresh organic residues
which have not undergone biotic or abiotic decomposition.
Humic substances are composed of compounds which have
undergone biotic or abiotic transformation and bear little
similarity to the parent compounds. Humic substances are
usually high-molecular-weight, brown to black in color and
have a heterogeneous composition (Stevenson, 1982). In
order to study the chemical nature and reactive properties
of soil humic compounds, they first must be separated from
non-humic and inorganic soil components. About 200 years
ago, Europeans discovered that humic substances could be
extracted from soils with alkaline reagents. Soil humic •
substances were then fractionated into three types of
components: humic acids (HA), fulvic acids (FA), and
humin. Early attempts to qualitatively characterize these
fractions were largely unsuccessful because of their complex and heterogeneous composition. Recently,
67 68 researchers have made progress in elucidating the chemical composition of humic and fulvic acids due to the introduction of advanced spectroscopic techniques. Topics covered in this chapter include (1) biochemistry of humus formation, (2) extraction, fractionation, purification and distribution of soil humic and fulvic acids and (3) characterization of soil humic and fulvic acids.
3.2 Literature Review
3.2.1 Biochemistry of Humus Formation
Soil organic matter is an accumulation of fresh (non- humic) , partially decayed and partially resynthesized plant and animal (humic) components. Typically, plant and animal residues are composed of carbohydrates, simple and crude proteins, hemicellulose, cellulose, lignins, fats and waxes. When incorporated into the soil, organisms begin decomposing the residue as a source of energy. Compounds are decomposed at considerably different rates. For instance, carbohydrates and proteins are decomposed quickly, while hemicellulose and cellulose are decomposed somewhat slower. The longer chained aliphatic and condensed aromatic components such as lignin, fats, and waxes tend to be fairly resistant to decomposition. Subse quently, a wide variety of humic by-products of smaller molecular weights are formed. These degradation products 69
undergo further enzymatic and chemical reactions to form
new colloidal polymers called humus.
Humus is an amorphous, colloidal, heterogeneous mix
ture of low to high molecular weight compounds which have
been modified or synthesized by soil organisms, or by
secondary chemical reactions not directly mediated by soil
organisms. Several pathways have been proposed for the
formation of humic substances by soil microorganisms during
the decay of plant and animal remains. These pathways which have been summarized by Stevenson (1982) are pres
ented in Figure 29.
The classical lignin-protein theory, which was popu
larized by Waksman (1936), was that humic substances repre sent modified lignin material (pathway 4). According to
Waksman (1936), ligneous material was incompletely decom posed by microorganisms and the residue condensed with proteins to form a lignin-protein complex. The structural modifications in lignin necessary for this to occur
involve the loss of methoxyl (OCH3) groups with the generation of o-hydroxy-phenols and oxidation of aliphatic sidechains to form carboxylic acid (COOH) groups
(Stevenson, 1982). It was postulated that the lignin- protein complex was further subjected to unknown reactions which yielded first humic and then fulvic acids (Stevenson,
1982). Most investigators have since rejected Waksman's 70
PLANT RESIDUES
TRANSFORMATION BY MICROORGANISMS MODIFIED LIGNINS
AMINO LIGNIN SUGARS POLYPHENOLS COMPOUNDS DECOMPOSITION PRODUCTS
QUINONES QUINONES
HUMIC SUBSTANCES
Figure 29. Mechanism for the formation of soil humic substances (after Stevenson, 1982). theory in favor of other possible pathways (Stevenson,
1982) .
Pathway 2 and 3 (Figure 29) form the foundation for the more favored polyphenol theory. In pathway 2, poly phenols are synthesized by microorganisms from nonlignin carbon sources. The polyphenols are enzymatically oxidized to quinones which can polymerize in the presence or absence of amino compounds. In pathway 3, lignin decomposition products (phenolic aldehydes and acids) undergo enzymatic conversion to quinones which can polymerize with or without amino compounds (Stevenson, 1982). In pathway 1, reducing 71
sugars and amino acids from the by-products of microbial
metabolism undergo nonenzymatic polymerization to form
brown-colored nitrogenous polymers.
According to Stevenson (1982) pathway 1 is thought to be insignificant, whereas pathways 2, 3, and 4 may be more
important mechanisms for humus formation. Nevertheless, a
completely satisfactory scheme for the formation of humic
and fulvic acids under diverse geologic environments has yet to be established (Stevenson, 1982).
Formation of humic acid-type compounds has also been
linked to nonbiological sources. There have been several reports regarding the formation of humic substances in the
laboratory. These generally involve reactions between organic materials and inorganic substances. For instance, hydroquinone has been catalytically oxidized to humic acid like polymers by Mn oxides and to a lesser extent by Fe and
Al and Si oxides (Shindo and Huang, 1982, 1985b). In another study, Shindo and Huang (1985a) demonstrated that primary minerals were also capable of accelerating the polymerization of hydroquinone into humic acid type poly mers. The formation of humic substances in the environment probably involves reactions mediated by both biological and nonbiological agents. 72
3.2.2 Extraction. Fractionation. Purification, and Distribution of Soil Humic and Fulvic Acids
The extraction of humic substances from soils and
sediments is often the first task that must be undertaken
in studies of organic matter. Schnitzer and Khan (1972)
indicate that the "ideal" extractant should physically
remove all of the humic material without altering its
physical and chemical properties. In addition, Stevenson
(1982) indicates that the ideal extractant should be appli
cable to all soils and should yield a product which is
representative of that found in the organic pool. However,
the search for the "ideal" extractant for soil organic
matter continues today.
A wide variety of reagents has been employed for
organic matter extraction (Stevenson, 1982). The choice of
extractant will depend on the type of material chosen for
isolation. For instance, amino acids and amino and simple
sugars can be extracted from soils using hot acids and bases; clay-bound biochemicals with hydrofluoric acid (HF);
fats, waxes, and resins with alcohols and ethers
(Stevenson, 1982); and alkanes and alkanoic acids by
supercritical gas extraction with n-pentane (Schnitzer and
Preston, 1987). Dilute aqueous NaOH remains the most commonly used and quantitatively the most effective reagent used for humic substances extraction (Schnitzer and Khan, 1972). In fact, dilute NaOH can be considered as the "classical" reagent used for extraction of humic substances. Over 200 years ago, Achard (1786) treated peat with alkali and obtained a dark, amorphous fraction which precipitated when acidified.
The alkali-soluble, acid-insoluble portion of soil humic components eventually became known as humic acid. This definition of humic acid has remained unchanged to the present day.
Typically, 0.1 to 0.5N NaOH solutions with a soil to extractant ratio of 1:5 or 1:10 (g/ml) have been used for extraction. The yield of humic substances is affected by the concentration of NaOH. Levesque and Schnitzer (1966) reported that the highest contents of C and N were extracted from a Podzol Bh horizon using 0.1N or 0.15N
NaOH. Extraction efficiencies as high as 80% of the total carbon content have been reported using dilute NaOH
(Stevenson, 1982).
There have been a few reports of structural modifica tions to humic substances following alkali extraction.
Tinsley and Salam (1961) reported that hydrolysis of amino acids and sugar compounds, or conversely, condensation reactions between amino compounds and aldehydes and pheno- lie compounds, can occur in alkaline solution. Autooxid ation of humic compounds during alkaline extraction is promoted by high pH and the presence of air. Bremner
(1950) demonstrated that O was taken up by a alkali- solution during the extraction of humic substances. Swift and Posner (1972) found that the breakdown of humic acids under alkaline conditions was increased in the presence of
0. These researchers reported that the breakdown involved an increase in the cation exchange capacity as well as the oxidation state of the humic acid-polymer. To minimize chemical changes during alkali extraction of humic materi al, the International Humic Substance Society (IHSS) has recommended that the procedure should be performed under a
N2 atmosphere.
Conversely, several researchers have reported that humic material is not altered by extraction in alkaline solution. For instance, Rydalevskaya and Skorokhod (1951) reported that there was no significant difference in the elementary composition and carboxyl group content between humic acids extracted under a O2 environment from different soils and peats with 1% sodium fluoride (NaF) and 0.4% NaOH solutions. Smith and Lorimar (1964) reported that humic acid extracted under an O2 environment from peat with dilute sodium pyrophosphate (Na4P207) solution was quite similar to that extracted with dilute NaOH. Forsyth (1947) found that fulvic acid extracted with dilute NaOH under an O2 environment had identical properties to fulvic acid
extracted with water. Schnitzer and Skinner (1968) com pared fulvic acid extracted from a Podzol Bh horizon using
0.5N NaOH and 0.1N HC1 under an 02-free environment. Both extracts were characterized by elemental and functional group analysis, infrared (IR) spectrophotometry, and by gel
filtration. The elementary composition and the oxygen- containing functional groups of both extracts v/ere nearly
identical. In addition, the IR spectra and fractionation behavior on Sephadex gels of both extracts were almost identical. From these reports, it is apparent that the evidence regarding the modification of humic substances by alkaline solutions is not conclusive. Neutral salts of mineral or organic acids have also been used for the extraction of humic substances from soils and sediments.
Included are salts of complexing agents (e.g., Na4P2C>7) and
EDTA), organic complexing agents in aqueous media (e.g., acetylacetone), dilute acid mixtures containing HF, and organic solvents of various types. An advantage of using these milder extractants is that there is less potential for chemical modification of humic substances (Stevenson,
1982). Unfortunately, these extractants are much less effective than alkali metal hydroxides in removing organic matter.
Stevenson (1982) has reviewed the use of these extractants. Sodium pyrophosphate is a commonly used reagent for organic matter extraction. The amount of
organic matter recovered (< 30%) is considerably less than with caustic alkali, but less alteration occurs (Stevenson,
1982) . Organic chelating agents such as, acetylacetone, and 8-hydroxyguinoline have been used to extract humic compounds from the illuvial horizon of Spodosols. The organic matter in these soils is commonly complexed with Al and Fe, and the complexing of these metals by chelating agents releases the organic matter to soluble forms
(Stevenson, 1982). Dilute mineral acids containing HF have also been used to extract soil organic matter. Hydro fluoric acid may cause the release of organic matter from inorganic combinations through the dissolution of hydrated silicate minerals and the formation of complexes with Fe and Al (Stevenson, 1982).
The isolation of specific humic fractions has been described in detail by Stevenson (1965) and Kononova
(1966). The classical method of fractionation of humic substances from soils and sediments following extraction with NaOH or Na4P207 is based on differences in solubility at various pH levels. As noted earlier, the fractions commonly obtained include: humic acid, soluble in alkali, insoluble in acid; fulvic acid, soluble in alkali, soluble in acid; and humin, insoluble in alkali. The typical extraction and fractionation scheme for isolation of humic and fulvic acids, and humin is presented in Figure 30. 77
Typical Fractionation Scheme for Humus :
HU UUS
extract wl h alkali
(Insolublel (Soluble! HUMIN treat with acid f, 1 } , IPrecipitatedl INot precipitated!
HUMIC ACID FULVIC ACID HA FA
Figure 30. Typical fractionation scheme for humus.
Several factors influence the yield obtained with the typical fractionation scheme. Sequi et al. (1975) demon strated that metals (Fe and Al) contained in soil organic matter extracts affect organic matter precipitation by acid and hence the yields of humic and fulvic acids. Gascho and
Stevenson (1968) found that the yield of humic and fulvic acids was also influenced by the strength of the extractant. They reported that increased yields of humic and fulvic acids were obtained with decreasing concentra tions of NaOH or Na4P2C>7.
After fractionation, humic and fulvic acids may con tain considerable quantities of inorganic constituents (salts, Si, sesquioxides, and clay) and additional nonhumic impurities such as proteins and carbohydrates. Proteins and carbohydrates are considered nonhumic substances because they resemble fresh, undecomposed organic com pounds. The humic and fulvic acids may contain up to 10-
70% ash. It has been recommended by the IHSS that both humic and fulvic acids can be considered purified if the ash content is less than 1%. Removal of inorganic components can be accomplished by the use of high speed centrifugation, ion exchange and nonionic resins, and by treatment with mineral acids. Schnitzer and Skinner (1968) employed Amberlite IR-120 exchange resin in the H-form to reduce the ash content of a Podzol fulvic acid to less than
1%. Treatment of humic and fulvic acid solutions with HF can also reduce the ash content to less than 1%. Gascho and Stevenson (1968), Lowe (1969), and Khan (1971) have used dilute solutions of HF or HC1-HF to purify humic and fulvic acids by dissolving inorganic colloids. Lowe (1969) and Dormaar et al. (1970) reported, however, that high concentrations of HF, or the use of HF in the absence of
HCl, may cause modifications of humic material.
In recent years, high recoveries of organic compounds from aquatic sources have been obtained with macroreticular 79
Amberlite XAD resins (Leenher and Huffman, 1976; Malcolm et al., 1977; Thurman et al., 1978; Aiken et al., 1979). This resin has been recommended by the IHSS for the purification of fulvic acids from soil and aquatic sources. Typically, unpurified fulvic acid preparations are passed through XAD resins several times. It is postulated that XAD resins remove coprecipitated or coabsorbed nonhumic substances
(e.g., proteins and carbohydrates) from unpurified fulvic acid preparations.
XAD-8 resin consists of nonionic, macroporus acrylic ester copolymers of intermediate polarity, which possess a large surface area. The resin retains the humic sub stances, while the nonhumic impurities are removed during flushing with water. The retained humic substances are released from the resin by back-eluting with dilute NaOH.
Leenher and Huffman (1976) have reviewed the sorption process of XAD resins in detail. For the final purifica tion step, fulvic acids are passed through H-saturated cation exchange resins several times.
The quantity of humic and fulvic acids extracted from soils varies considerably from one soil to another. Typi cally, the quantity of humic and fulvic acids extracted from soils is reported as a ratio of HA/FA. Several researchers (Schnitzer and Gupta, 1964; Kononova, 1966;
Lowe, 1969; Anderson et al., 1974b; Tan, 1978; Anderson,
1979; Zhang et al., 1988) have reported HA/FA ratios for 80
soil formed under different vegetative communities. How
ever because HA/FA ratios are dependent on the methods of
extraction and separation, the ratios obtained by different
researchers should only be compared with caution.
Kononova (1966) made an extensive survey of HA/FA
ratios of several major soil groups in Russia and found
that forest soils normally had lower ratios than grassland
soils. Lowe (1969) reported that Podzols had a narrow
HA/FA ratio, whereas Chernozems had a wide ratio in
Alberta. Tan (1978) reported that more humic acid was
extracted from Mollisols than Alfisols or Ultisols and that
there was 6-7 times more fulvic acids than humic acids
extracted from Alfisols. In general, humus of forest soils
is characterized by a high content of fulvic acids, whereas
humus of peat and grassland soils is high in humic acids.
3.2.3 Chemical Characterization of Humic and Fulvic Acids
In the early nineteenth century, Sprengel made the
first comprehensive study on the chemical nature of humic
substances. Sprengel's studies on the acidic nature of
humic acid (cited by Stevenson, 1982) was a major contribu
tion to humus chemistry. During this same period,
Berzelius (1839) also examined the chemical nature of humic
substances. Berzelius isolated two humic fractions,
apocrenic and crenic acids, from mineral waters and sedi ment which he believed were distinct chemical compounds. 81
In fact, he suggested chemical formula for crenic and
apocrenic acids as C24H12O6 anc* C24H6°12' respectively.
German (1845) (cited by Kononova, 1966) challenged the
concept that humic compounds (HA, crenic acid, and
apocrenic acid) were chemically distinct substances. He
isolated numerous humic substances differing in elementary composition. In addition, he concluded that N was a constituent of humic substances.
Mulder, a student of Berzelius, made further advances
in the isolation and chemical characterization of humic compounds. Mulder (1862) divided humic substances into the following categories based on solubility and color: (1) ulmin or humin-insoluble in alkali; (2) ulmic acid (brown) and humic acid (black)-soluble in alkali; and (3) crenic and apocrenic acids-soluble in water. Mulder postulated that these groups were chemically individual compounds, which did not contain N.
During the early part of the twentieth century, sever al researchers were again studying the chemical nature of humic substances. Some again challenged the popular opin ion that humic substances were individual compounds and that N was a contaminant. In a series of articles,
Schreiner and Shorey (cited by Stevenson, 1982) identified many organic compounds in humic substances including organ ic acids, hydrocarbons, fats, sterols, aldehydes, carbohy drates, and specific N-containing substances. This work 82 was particularly important because it showed that humic
substances contained organic compounds of known chemical composition. Nevertheless, humic substances still remained
largely undefined.
In 1930, Shmook suggested that the components of humic substances were not specific compounds, but were mixtures of closely related substances having similar structural features, and that N was a structural component. Based on his studies, Shmook concluded that humic acid contained two major components: an organic N-containing compound (pro tein) and an aromatic portion, possibly derived from lignin
(Stevenson, 1982).
Also in the 1930's, Waksman's theories on the origin and structure of soil humic compounds became very popular.
Waksman (1936) regarded humus as a mixture of plant-derived materials which includes fats, waxes, resins, hemi- cellulose, cellulose, and lignoproteins. His theories had a dominating influence on humus chemistry for several years.
By the 1940's, many investigators realized that the various soluble fractions were not chemically discrete components, but were group designations (Stevenson, 1982).
The terms humic acid, fulvic acid, and humin were suggested and accepted as operational definitions by soil organic matter chemists. The fractions do not represent discrete 83
compounds but instead represent groups of substances pos
sessing a common structural framework of similar organic
compounds.
Before the 1950's, humic chemists were limited by
analytical capabilities in their endeavors to determine the
chemical nature of humic substances. However by the 1950's
advanced analytical instruments such as Infrared Spectro
photometry (IR) and Gas Chromatography/Mass Spectrometry
(GC/MS) were available. Interpretation of data from these
instruments was, however, difficult. By the 1970's, even more advanced instrumentation, such as carbon and proton
Nuclear Magnetic Resonance (NMR) and Electron Spin Reso nance (ESR) were available for use by humic chemists. NMR and ESR represent the state-of-the-art forms of instrumen tation presently available to study humic substances.
Several methods are available to study functional groups and core components of soil humus. These include both wet chemical and spectroscopic techniques. Spectro scopic techniques include: E^/E q ratio (ratio of absor bance at 465 and 665 nm), IR, 13C NMR, ESR, and GC/MS.
Schnitzer (1982) has outlined extensive wet chemical procedures to measure the functional groups present in humic and fulvic acids. Stevenson (1982) has reported
functional group contents in alkaline extracted humic and fulvic acids from typical soils. 84
Perhaps the simplest characterization of humic and fulvic acids is an elemental analysis. Schnitzer (1975) reported (Table 8) average ranges for elemental contents of soil humic and fulvic acids. Humic acids were character ized as having higher C and N contents but, lower 0 con tents when compared to fulvic acids. Similar ranges for S and H contents occur in both humic and fulvic acids. The ranges of the data in Table 8 are quite narrow. This suggests that soil humic and fulvic acids have some degree of structural homogeneity.
Atomic ratios are also useful in the characterization of humic and fulvic acids. H/C, 0/C, and N/C ratios of humic substances, calculated from elemental data, aid in the identification of humic substances and the determina tion of structural formulas (Steelink, 1985). Steelink
(1985) has reviewed H/C, 0/C, and N/C ratios of humic substances and concluded that 0/C ratios for soil humic and fulvic acids cluster around 0.50 and 0.70, respectively.
Schnitzer (1975) reports higher fulvic acid 0/C ratios (1.0 to 1.1) than Steelink (0.7) ratio. The typical H/C ratio of soil humic and fulvic acids were clustered around 1.0.
There has not been as much emphasis placed on the N/C ratio as the H/C and 0/C ratio.
The E4/E6 absorption ratio has been used by humic chemists to elucidate the degree of condensation in soil 85
Table 8. Elemental analysis of humic substances (Schnitzer, 1975).
Element HA FA
— % of dry, ash-free wt. —
C 50-60 40-50
H 4-6 4-6
N 2-6 < 1-3
S 0-2 0-2
0 + 30-35 44-50
^Calculated by difference. humic components. Kononova (1966) suggested that the E4/ E 6 ratio is related to the degree of condensation or complexity of the aromatic carbon network. A high ratio is indicative of a low degree of aromatic condensation and infers the presence of a relatively large proportion of aliphatic structures. Conversely, a low ratio is indicative of a relatively high degree of condensation of aromatic humic constituents. The ratio is known to vary with soil order and climate (Kononova, 1966; Schnitzer and
Khan, 1972; Schnitzer, 1977). For example, according to
Kononova (1966), E4/E6 ratio's for humic acids extracted from Spodosols, Inceptisols or Oxisols are about 5.0, but are only about 3.5 for Boralfs and Haploborolls. For fulvic acids, the ratio ranges from 6.0 to 8.5 (Kononova,
1966). Schnitzer (1977) reports that the ratios for humic and fulvic acids range from 3.8-5.8, and 7.6-11.5, respec 86 tively. The latter are somewhat higher than those reported by Kononova (1966).
The introduction of IR spectroscopy in the 1950's to the study of humic chemistry was heralded as a major break through. Humic chemists made extensive use of IR analyses for humic and fulvic acid characterizations (Schnitzer and
Gupta, 1964; Dormaar, 1967; Tan and Clark, 1969; Schnitzer,
1971; Stevenson and Goh, 1971; Tan, 1977, 1978), but were somewhat disappointed with the results. Schnitzer and
Gupta (1964), Wagner and Stevenson (1965), and Schnitzer
(1971) found IR to be useful in the identification of humic and fulvic acid functional groups. However, the use of IR to study the structural core of humic and fulvic acids was less successful. Extensive overlapping of absorption peaks occur in IR spectra of humic and fulvic acids due to their heterogeneous composition. Because different organic groups have absorption maxima at the same wavelength, group assignments to specific regions are somewhat tenuous.
Nevertheless, IR does provide a good method for gross chemical characterization of humic and fulvic substances.
IR has also proven useful in the study of absorption mechanisms between humic substances and metals (reviewed by
Schnitzer, 1971) and humic substances and clay (Farmer,
1971). Stevenson (1982) has reviewed the principals of IR
absorption by organic molecules. Group assignments for IR
absorption bands for humic and fulvic acids have been
discussed by Stevenson (1982) and Germasimowicz and Byler
(1985). These peak assignments are provided in Table 20 of
section 3.5.2.I.D. A typical IR spectral pattern of soil
humic or fulvic acids consists of a series of broad absorp
tion bands with extensive overlapping of these bands. The main functional groups identifiable in IR spectra are hydrogen-bonded hydroxyls (3380 cm-1); aliphatic C-H groups
(2920 and 2850 cm-1); carboxyl and ketonic carbonyl groups
(172 0 cm-1); guinone carbonyl groups (1660 cm-1); and C=C of aromatic rings (1610 and 1510 cm-1), (Theng et al.,
1966).
A prominent feature in the IR spectral pattern of both humic and fulvic acids is the broad absorption band in the
3300 to 3400 cm-1 region (Dormaar, 1967; and Tan, 1977).
This peak has been attributed to the 0-H stretching of absorbed water or of OH-groups attached to carbon skeletons
(Stevenson and Goh, 1971). Stevenson and Goh (1971) suggested that the sample should be heated prior to analy sis to eliminate absorption in this region due to adsorbed water. Nevertheless, Tan (1977) found that heating the sample did not significantly reduce the intensity of this peak. He concluded that absorption in this region was not 88 due to free or hygroscopic water, but to OH-groups bonded to carbon skeletons.
The origin of a peak at 1020 cm-1 in humic acids is uncertain. Schnitzer and Khan (1972) suggest that this peak is due to silicate impurities. Tan (1977), however, reported the appearance of the 1020 cm-1 peak in fulvic acid samples which contained little or no Si02. Samples high in polysacharrides are known to have absorption peaks at 1,200 to 1000 cm-1 (Mortensen, 1960, 1965; Tan and
Clark, 1969). Thus absorption at 1020 cm-1 may be due to carbohydrates.
Notable differences occur in the IR spectra of humic and fulvic acids. The most obvious dissimilarity is the intensities of the bands at 2900 to 2800 and at 1725 cm-1.
Humic acids generally have a more intense 2900 cm-1 peak than fulvic acids due to the presence of more aliphatic groups (Schnitzer, 1971). The 1725 cm-1 band is usually the dominant peak in a typical fulvic acid IR spectral pattern, but is only of moderate intensity in humic acid patterns. This can be attributed to the preponderance of carboxylic acid groups in fulvic acids. Additionally, fulvic acids have a characteristic strong absorption peak between 1100-1000 cm-1 (Tan and Clark, 1969; Stevenson and
Goh, 1971) which is attributed to C-C, C-OH, and C-O-C vibrations. This absorption peak is identical to that exhibited by soil polysaccharides (Mortensen, 1960, 1965) . 89
Tan and Clark (1969) and Tan (1977) suggested that the IR
spectral pattern of humic acids is dominated by a strong
absorption band at 2900 to 2800 cm-1 whereas, fulvic acids
have strong IR absorption bands at 3400, 1620, and 1050-
1020 cm- 1 . As Tan (1977) indicates one of the most signif
icant value of IR is its capability to distinguish between
humic and fulvic acids.
Recently, 1H (proton) and 13C nuclear magnetic reso
nance spectroscopy have become important tools for organic
matter structural analysis. Advanced designs in NMR
spectrometers allows for humic substances to be examined in both liquid and solid state (Hatcher et al., 1980b). In this investigation, humic substances were analyzed in the
solid state; thus subsequent discussions will be limited to the solid state.
A brief and simplified review of elementary quantum mechanics will be needed to understand the principles of nuclear magnetic resonance. Atomic nuclei are constantly
in motion which can be described as rotational or vibra tional. These motions exist in distinct quantum states that correspond to discrete energy levels. Quantum numbers are used to characterize the energy levels. Molecules and parts of molecules may jump between quantum states and the energy *E absorbed or released during these changes in energy states is related to frequency v by Plank's constant h: 90
*E = hv (1)
For example, a transition between the quantum states illus
trated as El and E2 is possible by the absorption of
energy.
E2 hv = *E (2) El
Specific energy inputs are required to achieve excitation
from the lower energy level (El) to the higher one (E2).
The motion of molecules involved in NMR spectroscopy
is that of nuclear spin. Spinning nuclei possess a spin
angular momentum which is expressed by [h/2] [1 (1+1)I1/2,
where I is the spin number (Wilson, 1981). It has been
found experimentally that I is an odd integer multiple of
1/2 for nuclei of odd atomic mass numbers (isotope number),
zero for nuclei of even atomic mass numbers and even
nuclear charges (atomic number), and an integer for nuclei
of even atomic mass numbers and odd nuclear charges
(Wershaw, 1985). Therefore, the spin number may have
values of 0, 1/2, 1, 3/2, 2, etc. In particular, for
protons and 13C, I = 1/2, but for 12C, 1=0. Charged
nuclei which spin about their axis produce magnetic fields.
Consequently, the spinning nuclei behave as small bar magnets which have a magnetic moment. In a magnetic-free
environment these magnetic moments are oriented in random
fashion, however if placed in a magnetic field it may take 91
up one of 21 + 1 orientations with respect to the direction
of the magnetic field (Wilson, 1981). For instance, for
the nuclei and 13C I = 1/2, hence there are (21+1) =2
possible orientations which may be represented as +1/2 and
-1/2. For 13c and 1h it is possible to induce the spinning
nuclei to resonate between these two orientations.
To recapitulate, the spinning nuclei of l3c and 1h have magnetic moments which have two possible orientations.
Both orientations have a different energy state. For example, in an applied magnetic field, the magnetic moments tends to align with the field which is the lowest energy state. A nuclei with a magnetic moment aligned against the
field is in a higher energy state. Energy of the proper frequency or wavelength can be absorbed by the nucleus in the lowest energy state in order to resonate to the higher energy state (Streitwieser and Heathcock, 1985). Transi tions between energy states is possible by energy of frequency v, given by:
Cv = HO/2 tt (3) where HO is, the magnetic field strength at the nucleus and C is the gyromagnetic ratio for a particular nuclear type.
In practice, NMR can be induced by either (1) sweeping v (called continuous wave) while keeping HO fixed or (2) by fixing v and varying HO. In practice, most spectrometers are operated at constant HO because frequency differences 92 can be measured more precisely than differences in magnetic field strength (Streitwieser and Heathcock, 1985).
However, an alternative method co induce NMR in humic substances is called pulse Fourier transformation (FT—NMR).
Using this technique, nuclei are subjected to short intense pulses of radiation and all nuclei resonate simultaneously.
The resulting spectrum, which is a plot of signal strength against time, is commonly called free induction decay. To increase sensitivity, a large number of free induction decays are collected. Next, the information is sent to a computer and the data is averaged by a Fourier transforma tion (Wilson, 1981). The contact time and the pulse repe tition time must be optimized in order to obtain quantita tive NMR data. The averaging of spectra from numerous scans increases the signal to noise ratio which allows for more detailed assignments of structural groups (Wilson,
1981).
The success of an NMR determination is dependent on obtaining an adequate signal. In general, signal intensity is dependent on (1) the magnetic moment and (2) the abun dance of the species in the sample (Wershaw, 1985). Pro tons have a very high magnetic moment and natural abundance relative to 13C. Signal intensity is low for 13C due to 1) a low natural abundance (only 1.1%) and 2) the low reso nance of 13C atoms which is about 6000 times weaker than that of 1-H (Pavia et al., 1979). Nevertheless, 13C NMR 93
spectroscopy represents state of the art instrumentation
for determining the structural characteristics of soil humic substances.
In NMR spectroscopy, structures are identified by a particular resonance frequency. This resonance frequency will be influenced by the magnetic moments of other pro tons, magnetic fields of electrons and nuclei of functional groups. For example, the motion of electrons will generate their own small magnetic field which will have a shielding effect upon nuclei. As a result of this shielding, the energy required to obtain resonance differs from that of unshielded nuclei. Protons and functional groups in organ ic molecules in different electronic environments experi ence different degrees of shielding, thus energy absorption required to obtain resonance varies with the environment
(Streitwieser and Heathcock, 1985). These changes in energy levels required for resonance are referred to as chemical shifts. Consequently, it is possible to identify functional groups by determining the specific chemical shifts. Spectroscopists use chemical shifts to identify organic compounds (Wilson, 1981).
In proton or 13C NMR determinations, the common prac tice is to measure resonance frequencies of nuclei relative to a standard such as tetramethylsilane (TMS). A unitless measure (6) is used to describe the difference in energy between a sample and TMS: 6i = vi - vTMS x 106 (4) VO where 6i is the chemical shift of proton i, vi is the
resonance frequency of the proton, vTMS is the resonance
frequency from the standard, and vo is the operating fre quency of the instrument (Streitwieser and Heathcock,
1985). Since the resulting number is small, it is multi plied by 106 so as to be convenient to use. Consequently, chemical shifts in and ^3C NMR are measured in 6 scale.
Operationally, solid-state samples are packed into vials and placed into the NMR instrument. NMR is induced by either continuous wave or pulse FT. Adequate proton NMR spectra can be obtained by continuous wave measurements whereas 13C NMR spectra is usually obtained by pulse FT
(Wershaw, 1985). A baseline is then established using TMS.
Absorption of energy by the sample is recorded by the NMR instrument as a peak. The peak area and chemical shift in scale from TMS is measured. The chemical shifts are then compared to known standards in order to identify structures.
In order to obtain information from the NMR experi ments, excited nuclei must lose energy (called relaxation) and return to their previous equilibrium distribution among the energy states. If relaxation does not occur then eventually the populations in these states will become equal and no more energy will be absorbed (called satura
tion) (Wershaw, 1985). No NMR information can be gathered when a system is in a state of saturation (Wershaw, 1985).
The relaxation time of 13C nuclei can vary from less than a second to several minutes, and not all 13C nuclei necessar
ily have time to relax to their equilibrium distributions before the next pulse of energy (Wilson, 1981). Therefore, the optimum pulse spacing of energy must be optimized prior to NMR analysis. Normally, a pulse spacing of 1 to 1.5 sec is used. Wilson (1981) reported that once the optimum pulse spacing has been established for any individual humic acid, relaxation effects do not contribute too much of a problem for quantitative measurements.
Because of the heterogenous structures present in soil humic substances, a typical 13C NMR spectral pattern is composed of a series of broad peaks. Initially, very little structural information was resolved from these spectra, however recent advances have overcome these prob lems. The extreme line broadening in 13C NMR has been attributed to (1) static dipolar interactions (called dipole-dipole coupling) of the 13C atoms with adjacent protons and (2) chemical shift anisotropy (Wershaw, 1985).
Dipole-dipole coupling is caused by the direct interaction of the magnetic moments of protons attached to the 13C nuclei. Specifically, adjacent protons influence the resonance frequency of the ^3C nuclei. The net coupling effect produces a broad and sometimes split NMR peak. The line broadening may be minimized to a large extent by a technique called cross-polarization (CP). The theory of
CP-NMR has been explained by Pines (1973), Wilson (1981), and Wershaw (1985). Briefly, protons are irradiated at their resonance frequency so that they become activated and are no longer coupled to the 13C nuclei. In addition, some energy is transferred between the excited protons and 13C nuclei which causes an increase in the rate of relaxation of the 13C nuclei (Wershaw, 1985). After the transfer of energy, the number of excited nuclei in the less abundant
13C species approaches that of the more abundant species 1H
(Wershaw, 1985). Hence, the population of excited 13C nuclei increases which results in an increase in sensitivi ty and signal intensity. Chemical shift anisotrophy arises from the fact that the chemical shift of a given 13C atom in a molecule will vary to some extent as a function of the orientation of the molecule in the magnetic field (Wershaw,
1985). This anisotrophy results in a large deviation from the true line width and gives an erroneous chemical shift.
This effect can be eliminated by spinning the sample at the so called magic angle (MAS) (Wershaw, 1985). This method involves spinning the sample while in the NMR instrument at
57.74 degrees. 97
13c NMR spectral quality is vastly improved by em
ploying cross polarization and magic angle spinning tech
niques (Hatcher et al., 1980c). Recently, it has become
common to employ CPMAS techniques to pulse FT l3c NMR
instrumentation. All 13C NMR spectra collected in this
study have employed these techniques.
Several workers have used proton NMR to elucidate the
structure of soil humic acids (Grant, 1977; Wilson et al.,
1978; Ruggiero et al., 1979; Hatcher et al., 1981b) and soil fulvic acids (Stuermer and Payne, 1976; Ruggiero et al., 1979; Hatcher et al., 1980c). Proton NMR has an advantage of aiding in the recognition of long chain versus highly-branched aliphatic structures. A disadvantage of proton NMR is that limited information is gathered about the actual carbon framework of the humic substances.
In contrast, Wilson (1981) reported that 3-3c NMR spectroscopy has some inherent advantages over proton NMR for structural studies of soil humic substances. First in
13c NMR, the carbon skeleton is observed rather than adja cent protons. This technique allows four and sometimes five carbon types to be distinguished in humic substances by characteristic chemical shifts (Wilson et al., 1981a).
Typical assignments for these chemical shift regions are presented in Table 9. In general, each spectra is divided into five regions; aliphatic-C (0-50 ppm), oxygen alkyl-C
(50-110 ppm), aromatic-C (110-160 ppm), carboxylic and Table 9. Chemical shifts and structural assignments in the CPMAS 13C NMR spectrum of humic and fulvic acids (modified from Saleh et al. 1983).+
Observed Range of Resonance chemical Chemical Shift reaions peaks shift Assianment ------ppm ------
aliphatic carbons 12-17 13-17 terminal methyl carbon or methyl (0-50 ppm) carbon or further aromatic ring 21-36 19-35 methyl carbon to aromatic ring or methylene carbon or from terminal methyl group 37-49 37-53 methylene carbons of branched alkyls
oxygen alkyl carbons 55, 56 54-56 methoxy carbons (50-110 ppm) 6 6 , 72, 81, 60-101 carbons atoms of carbhydrates, 84, 92, 98 ethers, and certain amino acids
aromatic and alkenic carbons 117, 118 108-118 aromatic carbon ortho to ether (110-160 ppm) oxygen or hydroxy group 130, 139, 118-150 aromatic and substituted aromatic 148, 150 carbons
carboxylic, or amide 172-174 160-190 carbons in carboxylic acids, carbons (160-190 ppm) 178 esters or amides
carbonyl carbons 190, 220, 190-240 carbons atoms in aldehyde and (190-240 ppm) 240 ketone groups
+Some overlap of shifts are likely. 99
amide-C (160-190 ppm), and carbonyl-C (190-240 ppm). The
boundaries may vary 5-10 ppm between interpretators.
Secondly in 13C NMR, chemical shifts indicative of specific
carbon nuclei are spread over a wider range which
contributes to better signal resolution.
There have been many reports of wide variations in the
relative abundance of aliphatic vs aromatic components in
soil humic acids (Hatcher et al., 1980a). Using 13C NMR
spectroscopy Hatcher et al. (1981c), Sohn (1985), and Saiz-
Jimenez et al. (1986) found that soil humic acid exhibited high aromatic character and low aliphatic and carbohydrate content. Hatcher et al. (1981c) found that aromatic-C comprised about 50 and 92% of alkaline extracted humic acids from Mollisols and Inceptisols, respectively. They also reported that there was, however, significant amounts of aliphatic structures present. Saiz-Jimenez et al.
(1986) reported that humic acids extracted from a Typic
Xerochrept and a Typic Rendoll from Spain, contained 2 5 and
34% aromatic character, respectively. Sohn (1985) reported that humic and fulvic acids isolated from estuarine sedi ments contained 36 to 40% aromatic-C and low amounts of carbohydrates-C. In contrast to the previous reports,
Grant (1977), Wilson and Goh (1977), Wilson et al. (1978),
Hatcher et al. (1980c, 1981b), Wilson et al. (1981a),
Preston and Ripmeester (1982), and Bayer et al. (1984) 100
report that soil humic acids are dominated by aliphatic and
carbohydrate groups and not aromatic-C.
Several workers have found that the dominant carbon
structure in soil humic acid varies with soil types. In
their work with Australian soils, Skjemstad et al. (1983)
found that aromatic C content of humic acids ranged from
10-45%. They concluded that the content of aromatic groups
in humic acids is variable. Lobartini and Tan (1988)
reported that humic acids from a tropical Inceptisol and
Ultisol had combined carbohydrate and polysaccharide carbon
contents of 59.5-67.6%, and that humic acids extracted from
several temperate soil orders were dominated by aromatic
and carboxyl C groups. In particular, the aromatic C
content of humic acids extracted from a Mollisol and
Spodosol was 37.2 and 54.9%, respectively.
The effectiveness of 13C NMR spectroscopy for esti mating the abundance of phenolic-OH carbon in soil humic
substances is uncertain. Hatcher et al. (1981c) did not
find any evidence of phenolic-OH groups in the 13C spectra of fulvic acid extracted from a Spodosol or humic acids extracted from a Mollisol or Ultisol. Thus they concluded that phenolic-OH groups were not an important structural component of humic and fulvic acids. In contrast, Hatcher et al. (1980b) identified a prominent phenolic-OH peak in 101 the 13C CPMAS NMR spectra of humic acid extracted from peat. Lobartini and Tan (1988) reported that the phenolic-
OH carbon content of humic acid extracted from an Incept- isol, Mollisol, and Ultisol was very low. Hatcher et al.
(1981c) suggested that phenolic-OH carbon in humic sub stances may be undetectable by 13C CPMAS NMR spectroscopy because stable free radicles associated with phenolic-OH groups may broaden the signal and render it invisible.
This suggestion is substantiated by Schnitzer and Preston
(1986) who reported inconsistencies between data obtained by 13C NMR and chemical methods for estimating the contents of phenolic-OH groups. They showed that 13C NMR spectros copy underestimated the content of phenolic-OH groups.
The dominant carbon structure in fulvic acids as revealed by 13C NMR spectroscopy has not been subject to as much debate as that of humic acids. The dominant carbon structures found in soil fulvic acids are aliphatic, carbo hydrate, and carboxylic acids (Stuermer and Payne, 1976;
Ruggiero et al., 1979; Preston and Ripmeester, 1982;
Blondeau, 1986).
3.3 Methods of Soil Organic Matter Extraction
In the present study, organic fractions were extracted from four soils using the procedures approved by the IHSS.
In the Dana, Xenia and Rossmoyne soils the organic matter was extracted from the surface horizons. However in the
Dana Variant pedon, the organic matter was extracted from both the surface Ap horizon and underlying A horizon.
Field observations revealed that the surface horizon of the
Dana Variant soil was lighter in value than the underlying
A horizon which suggests that the Ap horizon has been influenced by either cultivation-induced organic matter oxidation or by the addition of sediments by erosion or cultivation practices. The latter could result in contamination of the indigenous organic fraction with organic components from other sites. Therefore, the darker
A horizon was also extracted. Each soil was extracted twice and is referred to as batch one or batch two.
One to 1.2 kg of air-dried, 2 mm sieved soil was placed into a 15 L plastic bucket. Two 1/4-inch holes were bored into the lid, through which two one hole rubber stoppers were inserted. Nitrogen gas was introduced through a glass capillary tube placed in one of the rubber stoppers. A teflon-coated plastic stirrer was inserted through the other rubber stopper. The stirrer rod was connected to a slow speed electric motor. Headspace gases were allowed to escape through the loose fitting hole provided for the stirrer. Deionized water was added to the soil to give a water to soil ratio of 1.5:1. The slurry was then stirred at a slow speed for 10 min. After 3 0 min, the pH of the slurry was determined with an Orion pH meter with a combination electrode. 103
3 . 3 .1 Soil Pre-treatment
To maximize the amount of humic and fulvic acids
extracted, the mixture was acidified with dilute HC1 to
remove Ca and Fe which interfere with the solubilization of
organic matter. The pH of the soil slurry was lowered to 1
with IN HC1. The soil slurry was brought to a final
solution to soil ratio of 10:1 with 0.1N HC1. The
acidified soil slurry was stirred for 2 hrs and then
allowed to settle overnight.
3.3.2 Alkaline Extraction
The slightly yellow-colored acid supernatant was removed by siphoning and discarded prior to extraction.
Minimal amounts (usually < 5%) of humic substances are lost by the acidification pretreatment (Stevenson, 1982). The pH of the soil slurry was raised to 7.0 with IN NaOH, and immediately 0.1N NaOH was added, under a N2 atmosphere to give a final extractant to soil ratio of 10:1. Humic substances are usually extracted from soil under a N2 atmosphere to minimize artifact production. The amount of
N2 was regulated so that a match would not burn inside the plastic bucket. The soil was stirred at a low speed under the N2 atmosphere for 24 hrs. The slurry was then filtered through glasswool to remove floatable debris and immedi ately acidified with 6N HC1 to a pH of less than 1. After insoluble material was allowed to settle (about 5 min), the 104
dark colored supernatant was removed by siphoning and was
transferred to a plastic bucket, covered, and allowed to
settle overnight. The remaining soil which contained
unextractable material (humin) was discarded, but a small
sample was saved for organic carbon analysis.
3.3.3 Separation of Humic and Fulvic Acids
After sitting overnight, the dark colored humic mate
rial precipitated and the yellow colored fulvic material
remained in suspension. Fulvic acids were removed by
siphoning and stored in a plastic jug. To insure an ade quate separation of these two fractions, the humic precipi tate was centrifuged to remove as much solution as possible. The additional solution following centrifugation which contained fulvic material was added to the fulvic acid container. The humic material was redissolved by raising the pH to greater than 11 with IN KOH under a N2 atmosphere. Additionally, enough KCl was added to attain a
0.3M K solution. Potassium helps in purification by floc culating the inorganic sediments. The humic solution was stirred for an additional 2 hrs.
The black-colored humic solution was then centrifuged at high speeds to remove inorganic sediments. The super natant was treated with 6N HC1 to lower the pH to less than
2. The solution was allowed to stand for 1 hr and then centrifuged at high speeds. The clear supernatant was 105 discarded, and the humic material was collected and stored in plastic bottles.
3.3.4 Purification of Humic Acids
The humic acids were poured into Union Carbide Dialy sis tubing and were dialyzed in deionized water with an
Oxford Multiple Dialyzer. The dialysis tubing had a cutoff point of 12,000. This implies that humic material with a molecular weight greater than 12,000 will be retained, whereas material less than 12,000 will diffuse through the membrane. The humic acid was allowed to dialyze for 8 to
24 hrs. The water was periodically changed and tested for
Cl ion with 0.1N silver nitrate. When a negative Cl ion test was obtained, small duplicate aliquots of humic acid were collected for the determination of ash content. The aliquots were transferred to 5 ml teflon coated vials, and evaporated to dryness at 80°C. The humic acids were trans ferred to a acid-washed crucibles and ashed for 24 hrs at
550°C. The crucibles were allowed to cool in a desiccator over P2O5. The ash was then weighed. When the ash content was greater than 1%, the humic acid was removed from the dialysis bags and transferred to a plastic bucket. A 0.3M
HF-0.1M HC1 solution was added to the humic acids and stirred at high speeds. After 24 hrs the solution was allowed to settle, and the dilute HF-HCl was siphoned off.
The humic acids were then dialyzed for another 8 to 24 hrs 106
or until a negative Cl ion test was found. The ash test
was repeated and treatment with the HF-HCL continued until
a one percent ash content was achieved. This usually
required two to three treatments with the HF-HCl solution.
The purified humic acids were then transferred to plastic beakers and freeze-dried. The humic acids were ground to a fine powder with an agate mortar and pestle and
stored in a desiccator.
3.3.5 Purification of Fulvic Acids
The fulvic acids were rotary evaporated on a Buchler
Flashevaporator at 40°C to reduce their volume from 8 L to
150 ml. The fulvic acids were dialyzed in deionized H2O
for 24 hrs to reduce the amount of salts and then stored in a small glass vial. The methods for purifying fulvic acids using exchange resins have been outlined by the IHSS. Some modifications were required to complete the purifications, and are explained below.
The fulvic acids were filtered through a series of three columns which contained two types of exchange resin.
Two columns, were packed with Amberlite XAD-8 No. A-6525
20-60 mesh, a nonionic polymeric absorbant. Prior to filtration, the XAD-8 resin was cleaned in a Soxhlet ex tractor with sequential washings of methanol, acetonitrile, ether and deionized water. Both columns were rinsed with several liters of deionized water to insure removal of all organic reagents. The fulvic acids were leached through 107
the first column at a flow rate of 8 ml/min. The column
was then flushed with 100 ml of deionized water, which was
discarded. The outlet of the column was connected to a
peristalic pump, and was back-eluted with 350 ml of 0.1N
NaOH followed by 450 ml of deionized water at 8 ml/min.
The golden colored effluent was collected and acidified with 6M HC1 to lower the pH to less than 1. Concentrated
HF was then added to attain a 0.3M HF solution and was
stirred overnight at a low speed. The fulvic acids were then rotary evaporated to about 50 ml.
The fulvic acids in HF were then leached through the second column containing XAD-8 resin at 2 ml/min. The column was then flushed with 160 ml of deionized water which was discarded. The column was back-eluted with 250 ml of 0.1N NaOH, followed by 250 ml of deionized water.
The fulvic acids were collected and stored in a plastic jug.
A third column was packed with Bio-Rad Ag MP-50 50-100 mesh H+ form analytical grade cation exchange resin. The resin was charged using 1 L of 3N HC1. The resin was flushed with deionized water until a negative Cl ion test was found in the effluent. The fulvic acids were passed through the AG MP-50 exchange resin four times. Between each passage the resin was recharged with 500 ml of 3N HC1, and flushed with deionized water until free of Cl ion. 108
An attempt was made to determine the ash content by
weight in the fulvic acids. Four ml of the fulvic acids
were transferred to an acid-washed crucible and evaporated
to dryness on a hot plate. They were then weighed, and
placed in a 550°C muffle furnace overnight. The crucibles were removed and were allowed to cool in a desiccator. The
amount of ash in these samples was to small for this
researcher to measure accurately. However, samples were sent to Galbraith Laboratories for percentage ash determi nation. The fulvic acids were freeze-dried.
3.3.6 Purification of Humin
After the initial extraction (Section 3.3.2), a sample of the insoluble humin was saved. The humin was dried in an oven overnight at 90°C. The dried humin was ground in a ceramic mortar and pestle. About 10 g of humin was placed in a porous porcelain funnel and placed on top of a 1000 ml buchner flask. The humin was flushed with deionized water, until the leachate was fairly clear. After leaching, the humin was dried at 40°C and was ground in a glass mortar and pestle. A 1.0 g portion of the humin was analyzed for organic carbon content by dry combustion according to methods outlined in Section 1.3.5. 109
3.4 Methods of Humic and Fulvic Acid Characterization
3.4.1 Functional Group and E4/E5 Ratio Analyses
Total acidity, carboxylic acid (COOH), phenolic acid, and E4/E6 ratio of all humic acid samples were measured using methods as described by Schnitzer (1982). For total acidity measurements, 0.2N Ba(OH)2 is added to the humic acids, shaken for 24 hrs and then filtered. The excess
Ba(OH)2 is titrated with standard HC1 to pH 8.4. Likewise, carboxylic acid groups are determined by adding IN Ca(OAc)2 to the humic acids, then shaken for 24 hrs and filtered.
The excess Ca(OAc)2 is then titrated with standard NaOH to pH 9.8. A blank consisting of either Ba(OH)2 or Ca(OAc)2 is also titrated. The quantity of total acidity or carboxylic acid content is calculated from the difference in sample and blank titrations. The quantity of phenolic-
OH groups is then determined by difference between the amount of total acidity and carboxylic acid content.
Sufficient fulvic acids were not available for the total acidity, carboxylic acid, or phenolic-OH measurements. The
E4/E6 absorbance ratio was measured by weighing humic or fulvic acids into flasks and then adding 0.05N NaHC03. The absorbances at 465 and 665 nm were measured and the ratio then determined. All tests were performed in triplicate. 110
3.4.2 Elemental Analyses
One to two hundred mg of humic and fulvic acids were
sent to Galbraith Laboratories for elemental analysis (C,
O, H, and N), percent ash, and percent water by the Karl
Fischer method.
3.4.3 Infrared Spectrometry
Infrared absorption (IR) spectral patterns for all humic and fulvic acids were obtained by methods as de
scribed by Schnitzer (1982). However, several modifica tions will be presented.
One mg of humic or fulvic acids was ground with 400 mg of spectroscopically pure oven-dried (80°C) potassium bromide (KBr) in a WIG-L-BUG Grinder for 60 sec. The mixture was transferred to a Beckman K-13 KBr die which had been previously heated in a 80°C oven. The die with mix ture was heated for 15 min at 80°C. The die was removed and was attached to a vacuum pump. While evacuating, the die was placed on a Carver Laboratory press, and a pressure of 83,640 pascals was applied for 5 min. The humic or fulvic acid-KBr pellet was removed and stored in a desiccator over P2°5* Ill
The IR absorption spectra for each humic or fulvic
acid was determined on a Beckman IR double beam spectro
photometer. Each sample was analyzed against a 400 mg
blank KBr pellet.
3.4.4 13c CPMAS NMR Spectroscopy
Five hundred mg of humic acid from each sample were
sent to the University of Guelph, where Mrs. Sue Shipitalo
performed the 13C solid-state NMR analysis. The 13C NMR
spectra were collected at 22.6 MHz on a Bruker CXP-100
spectrometer. Samples were spun at the magic angle (54.7°)
in an Andrew-Beams spinning apparatus with air at approxi
mately 3-3.5 KHz. The spinners were made of Kel-F with an
internal volume of 460 microliters. Samples were packed by
hand into the spinners, approximately 300 to 400 mg were
used. The contact time was 1 ms and 400 data points were
collected. The number of scans collected for each spectrum was 12,000.
One to two hundred mg of fulvic acid from each soil was sent to the Regional NMR Center at The Colorado State
University, Ft. Collins, Colorado for 13C CPMAS solid-state
NMR analysis. The 13C NMR spectra for fulvic acid were
collected at a carbon frequency of 15 MHz, sweep width of
10,000, acquisition time of 26 ms, and recycle time of 1
sec, on a Nicolet NT-150 spectrophotometer. Samples were
spun at the magic angle at 3800 rps. The contact time was 112
1.0 ms and 2048 points in full frequency domain were col
lected. The number of scans collected for each sample was
9000.
3.5 Results and Discussion
3.5.1 Extraction of Humic and Fulvic Acids
The organic carbon contents of initial soil, humin
material and yield of purified humic and fulvic acids are
presented in Tables 10 and 11, respectively. These data
have been used to calculate extraction efficiencies.
Yields of humic substances have been expressed as a per
centage of the total organic carbon (TOC) content (Table
12) and alkali-extractable organic carbon (Table 13).
Twentyfour to 4 0 percent of the TOC content of the soil was
collected during the extraction. Lowe (1969) reported that
23 and 38% of the total C content of a Orthic Gray Wooded
and a Orthic Black soil respectively, were extracted with
0.IN NaOH. In this study, the %T0C extracted from the
Alfisols and Mollisols are similar to Lowe (1969). With
the exception of the Dana Variant A horizon, as the soil
organic matter content decreased, the yield of humic sub
stances decreased. It should be noted that the Alfisols
(Xenia and Rossmoyne) yielded less humic substances on a
percentage basis than the Mollisols (Dana and Dana
Variant). Closer examination of the data in Table 12
reveals that the difference in yields of total humic sub- 113
Table 10. Organic carbon content of soil and humin.
Organic Carbon SamDle Batch Soils Humin
Dana Ap B1 2.49 0.82 Dana Ap B2 2.49 0.82
Dana V. Ap B1 1.96 0.56 Dana V . Ap B2 1.96 0.49
Dana V. A B1 2.10 1.04 Dana V. A B2 2.10 0.89
Xenia Ap B1 1.48 0.71 Xenia Ap B2 1.48 0.72
Rossmoyne Apl B1 1.10 0.48 Rossmoyne Apl B2 1.10 0.40
Table 11. Yield of purified humic and fulvic acids extracted by alkaline solution.
Initial amount of soil Yield Soil Batch extracted FA HA - m g ------
Dana Ap B1 1000 140 9610 Dana Ap B2 1100 462 10,663
Dana V . Ap B1 1200 176 7250 Dana V. Ap B2 1200 136 8913
Dana V. A B1 1200 85 7381 Dana V. A B2 1200 208 6984
Xenia Ap B1 1100 287 2336+ Xenia Ap B2 1100 61+ 5240
Rossm. Apl B1 1200 115 2817 Rossm. Apl B2 1200 201 3570
’'"Part of sample was lost due to spillage. 114
Table 12. The average humic to fulvic acid (HA/FA) ratio and yield of humic substances expressed as a percentage of total organic carbon (TOC) content.
Humic Substances Fractionation Soil Yield HA/FA Soil OC FA HA Total Ratio % — % of TOC in soil —
Dana Ap 2 .49 1.2 38.8 40 32:1
Dana V. Ap 1.96 0.7 34.3 35 53:1
Dana V. A 2.10 0.6 28.5 29 49:1
Xenia Ap 1.48 1.1 23.2 24 21:1
Rossmoyne Ap 1.10 1.2 24.2 25 19:1
Table 13. Yield of purified humic substances expressed as a percentage of alkali-extractable organic carbon.
Yield of Purified Soil Humic Substances i i i i i 1 1 1 1 1 1 i
Dana U1 VO
Dana VariantAp 52
Dana Variant A 56
Xenia 31
Rossmoyne 42 115
stances between Mollisols and Alfisols is due to differ
ences in yields of humic acids. In fact, the Mollisols
yielded approximately 1.6 to 2 times more humic acid and
exhibited HA/FA ratios 1.5 to 2.8 times greater than the
Alfisols. Lowe (1969), Tan (1978), and Stevenson (1982)
also reported more humic acid extracted from grassland
soils (Mollisols) than forest soils (Alfisols). Kononova
(1966) and Lowe (1969) reported that Mollisols typically
have a higher HA/FA ratio than Alfisols. Thus the results
of this study are consistent with prior work.
HA/FA ratios ranged from 19 to 53 which is consider
ably higher than that reported by Stevenson (1982) and
Zhang et al. (1988) who reported that HA/FA ratios for
Alfisols and Mollisols typically range from 0.7 to 3.
However, these ratios must be compared with caution because
HA/FA ratios are dependent on methods of extraction and purification.
The yield of purified humic substances (HA+FA) calcu
lated as a percentage of alkali-extractable organic carbon ranged from approximately 31 to 59%. Mollisols yielded nearly double that of the Alfisols. In general, the yield of purified humic substances decreased as the OC content of the soil decreased.
As shown in both Tables 11 and 12, the yields of fulvic acids are much lower than those of humic acids and the yields are very similar for both the Alfisols and Mollisols. Lowe (1969) and Stevenson (1982) reported that
Alfisols have a higher proportion of fulvic than humic
acids. The Alfisols in this study do not conform to this
pattern. The low recovery of humic and fulvic acids from
the pool of alkali-extractable organic carbon is due to the
method of obtaining purified humic substances as recom
mended by the IHSS which requires extensive dialysis and
passage through exchange resins. For example, approximate
ly 70% of the alkali-extractable organic carbon material in
Alfisols was lost during the purification process. A
factor contributing to the low recovery of humic substances
may be the inappropriate molecular weight cutoff point of
the dialysis tubing. Stevenson (1982) reported that the
average molecular weight of a fulvic acid polymer ranged
from 275-2110 daltons. Assuming that the average molecular weight of fulvic acid polymers in both Alfisols and
Mollisols are approximately 2000 daltons, a large portion
of the polymers would not be retained by the dialysis
tubing (molecular weight cutoff of 12,000 daltons). In
fact, when fulvic acids were being dialyzed, the dialysis water was often a dark gold color. The low humic acid yields from Alfisols may also be due to the loss of the
less than 12,000 daltons molecular weight fraction during dialysis. This suggests that: (1) some proportion of the humic and fulvic acids had diffused through the dialysis membrane and were discarded, and (2) the humic and fulvic 117
acids recovered in this study have a molecular weight of
greater than 12,000 daltons. To reduce humic substances
lost during the purification part of future humic
substances studies dialysis tubing with a molecular weight
cutoff point of less than 12,000 daltons should be used.
The ash content in purified and unpurified humic and
fulvic acid is presented in Table 14. The ash content of
unpurified humic and fulvic acids ranged from 45 to 77 and
40 to 91%, respectively. The ash content in purified humic
acids was lowered to less than 2% by repeated treatment
with 0.3M HF-0.1M HCl. A low ash content (generally less
than 1%) in humic or fulvic acids has been recommended by
the IHSS in order to obtain reliable quantitative data.
Several samples of humic and fulvic acids were sent to
Galbraith Laboratories for determination of ash content.
The ash content in humic acids as determined by Galbraith
Laboratories is very similar to that determined in this
study. The ash contents of fulvic acids as determined by
Galbraith Laboratories ranged from 3.75-11.5%. Further
purification was needed, although additional purification
would result in more fulvic acid loss. To avoid further
fulvic acid losses, additional purification was judged
unnecessary. In summary, more humic substances and a
higher extraction efficiency were obtained from the
Mollisols than Alfisols. The main difference between the
Alfisols and Mollisols was in the amount of humic acids 118
Table 14. Ash content in unpurified (unpur.) and purified (pur.) humic and fulvic acids.
Humic Acids Fulvic Acids Soil Batch UnDur. Pur.t UnDur. Pur.
Dana Ap B1 67.1 1.7 84 --- Dana Ap B2 66.8 <1 (0.7) 87 (11.5)
Dana V. Ap B1 ---- <1 83 (3.7)
Dana V. Ap B2 ---- <1 83 ---
Dana Var. A B1 ______1.6 40 --- Dana Var. A B2 ---- 1.3(2 .0 ) 81 ---
Xenia Ap B1 61.6 <1 (0.7) — (7.1) Xenia Ap B2 77.3 1.0 89 ---
Rossmoyne Apl B1 ---- 1.6 91 (7.5) Rossmoyne Apl B2 45 <1 (1.4) 87
^Values in parentheses have been analyzed by Galbraith L abs. extracted with the latter providing considerably more humic acids. Both the Alfisols and Mollisols yielded similar amounts of fulvic acids. Overall, the humic substances extracted from all pedons are dominated by humic acids.
With the exception of one horizon, as the organic matter content of the pedon decreased, the yield of humic substances also decreased. Purification of humic substances resulted in substantial loss of humic components. No differences in extraction efficiency were observed between the Otterbein and Burton sites. 119
3.5.2 Humic and Fulvic Acid Characterization
A. Elemental and Atomic Ratio Analyses
Elemental compositions and atomic ratios of humic acids extracted from the four pedons are presented in Table
15. The C content of humic acid ranged from 52.8-58.3%; 0 content from 33.8-38.0%; H content from 3.7-4.9%; and N content from 3.1-4.8%. The C content of humic acids from soils on the Illinoian surface (Dana Variant and Rossmoyne) is lower and the O content is higher than those of humic acids from soils on the Wisconsinan surface (Dana and
Xenia). Stevenson (1982) reported that during the process of humification, (organic matter diagenesis) organic matter loses polysaccharides, proteins, and oxygen containing functional groups. These changes are accompanied by an increase in the C and decrease in the 0 contents. This data suggests that the organic matter in the surfaces horizons of soils on the Wisconsinan surface exhibit a higher degree of organic matter humification (diagenesis) than the organic matter on the Illinoian surface. This condition may be due to: (1) the acidic pH of the surface horizons which favors a microbial population capable of rapidly decomposing organic debris, or (2) an increase in the portion of humic substances available for decomposition because of lower amounts of Ca and Mg ions in soils on the
Wisconsinan surface. Calcium and Mg ions normally protects 120
Table 15. Elemental composition (%) and atomic ratios of alkaline extracted humic acid samples.
Soils C HN 0 H/CO/C N/C — %
Dana Ap 58.3 3.7 3.1 33.8 0.77 0.44 0.05
Dana V. A 52.8 4.1 3.9 36.1 0.93 0.51 0. 06
Xenia Ap 56.6 4.8 4.0 35.0 1.01 0.46 0.06
Rossm. Apl 52.8 4.9 4.8 38.0 1.10 0.54 0.08
^Calculated on a dry, ash-free basis.
humic substances from decomposition by binding the humic
substances to solid phases.
The humic acids extracted from the Alfisols (Rossmoyne
and Xenia) have wider H/C ratios, and higher H and N con
tents than the Mollisols (Dana and Dana Variant). In
addition, the N/C and the H and N contents increase with a
decrease in OC content of the soil. This condition may be
explained by: (1) the N containing substances (i.e., amino
acids, amino sugars) are mineralized at a faster rate in
Mollisols than Alfisols, and (2) the N containing sub
stances would accumulate in the Alfisols due to the slower
rate of mineralization. Visser (1983) reported that humic
substances with a wide H/C ratio are composed of less- condensed or more open aromatic-type structures and have a higher content of aliphatic components. This implies that the humic acids extracted from the Alfisols are not as 121 structurally condensed or complex as humic acids from the
Mollisol. Overall, the H/C, 0/C and N/C atomic ratios for humic acids in this study are very similar to those re ported by Hatcher, as cited by Steelink, (1985). Schnitzer
(1977) reported the elemental composition for an "ideal" humic acid by synthesizing numerous analyses of a large number of humic acids extracted from soils from widely differing climatic and geologic environments. The ideal humic acid is composed of 56.2% C, 4.7% H, 3.2% N, and
35.5% 0, (Schnitzer, 1977). Humic acid composition found in this study is similar to that for the "ideal" soil humic acid. The data are also similar to that for humic acids extracted from a Gray Solonetz and a Brown Solod reported by Lowe (1969). The H content, however, is about 1% lower than that reported for humic acid extracted from a Gray
Wooded soil by Schnitzer and Gupta (1964). However, a comparison of such data is somewhat moot because of differ ences in extraction and purification methods. Neverthe less, the elemental composition of humic acid extracted from these pedons appears to be compatible with that reported in the literature.
The elemental composition and atomic ratios of fulvic acids extracted from the four pedons are presented in Table
16. The C content ranged from 40.5-46.0%; 0 content from
29.1-37.1%; H content from 2.8-3.5%; and N content from
2.0-2.9%. There appears to be no definable trends in the 122
Table 16. Elemental composition (%) and atomic ratios of alkaline extracted fulvic acid samples.+
Soil C H N 0 H/C O/C N/C -- %
Dana Ap 41.8 3.5 2.0 35.0 1.01 0.63 0.04
Dana V. A 40.5 3.2 2.8 37.1 0.96 0.69 0.04
Xenia Ap 41.6 2.8 2.2 29.1 0.81 0.53 0.05
Rossm. Apl 46.0 3.3 2.0 34.4 0.86 0.56 0.05
"^Calculated on a dry, ash-free basis.
data except that the 0 content in Mollisols is slightly higher than that in Alfisols. This condition may be due
to: (1) the low amount of fulvic acids recovered, or (2)
the loss of fulvic acid components during purification which may have contributed to differences in the elemental
composition between these pedons. Schnitzer (1977)
reported the elemental composition for an "ideal" fulvic acid as: 45.7% C, 5.4% H, 2.1% N, and 44.8% 0. In this
study, the H and 0 content of fulvic acids are lower than those reported for the "ideal" fulvic acid. The low 0 and
H contents of fulvic acids may be due to (1) an increase in moisture of the sample during analysis, or (2) a high ash content. First, Huffman et al. (1985) reported that 0 and
H measurements in humic substances are difficult to determine because of the presence of moisture. They noted that the moisture content of the sample must be accurately 123
known, because any gain or loss of moisture after weighing
the sample is by far the most critical with 0 and H
determinations. Second, the method employed by Galbraith
Labs for 0 determination only measures organically bound
oxygen. Thurman (1985) reported that there is some error
in the elemental composition of humic substances when the
ash contents exceeds 5%. Low 0 contents in the fulvic acid
samples may be due to a portion of the 0 being associated
with metals in the ash (Thurman, 1985). The C and N
content of fulvic acids are close to those for the "ideal"
fulvic acid.
In the fulvic acids, the H/C ratio ranged from 0.81-
1.01, 0/C ratio from 0.53-0.69, and N/C ratio from 0.04-
0.05. Steelink (1985) reported that the O/C, H/C, and N/C
ratios for fulvic acids typically cluster around 0.7, 1.0 and 0.05, respectively. The H/C, O/C, and N/C ratios for
fulvic acids from the Mollisols (Dana and Dana Variant) are similar to values reported in the literature; however, except for the N/C ratio, the ratios for fulvic acids from the Alfisols (Xenia and Rossmoyne) are somewhat lower.
Fulvic acids extracted from Mollisols exhibit wider H/C and
O/C ratios than fulvic acids extracted from Alfisols. This suggests that fulvic acids from the Mollisols may be com posed of more open-type aromatic structures and may have a higher content of oxygenated functional groups such as carboxylic acid, OH and OCH3 than those of the Alfisols 124
(Visser, 1983). The latter conclusion cannot be verified
because insufficient sample was available to do a function
al group analyses of the fulvic acids. The N/C ratios of
all the fulvic acids extracted in this study are similar
which is opposite to that found in the humic acids. This
suggests that: (1) the fulvic acids contain somewhat
similar amounts of N-containing compounds, or (2) the
dialysis tubing preferentially retained similar N con
taining structures. Atomic ratios of fulvic acids were
similar between soils at the Otterbein and Burton sites.
B. E4/E5 Absorption Ratio
The mean E4/E6 absorption ratio for humic and fulvic acids is presented in Table 17. The E4/E6 ratio is the ratio of absorbance at 465 and 665 nm, and has been widely used for characterization purposes. This ratio is believed to serve as an index to the degree of aromaticity in humic substances. A low ratio is indicative of a relatively high degree of condensation or complexity of aromatic constituents, conversely a high ratio reflects a low degree of aromatic condensation and infers the presence of more aliphatic structures (Stevenson, 1982). Kononova (1966) indicated that the E4/E6 ratios for humic acids are usually less than 5; whereas those for fulvic acids range from 6.0 to 8.5. Zhang et al. (1988) reported that the E4/E6 ratios of humic acids from the surface horizon of a cultivated 125
Table 17. Mean E4/E6 absorbance ratios for humic and fulvic acids.'5'
Soil Batch Humic Acids Fulvic Acids
Dana Ap B1 4.1 (0.23) 7.7 (0 .1 ) Dana Ap B2 4.1 (0.46) 10.6 (0.71)
Dana V . Ap B1 4.0 (0 .01) 16.2 (0.45) Dana V. Ap B2 3.9 (0.19) 15.3 (0.72)
Dana V. A B1 3.9 (0.04) 15.9 (0.60) Dana V. A B2 4.0 (0.19) 11.9 (0.83)
Xenia Ap B1 6.1 (0.15) 12.2 (1 .0 ) Xenia Ap B2 4.2 (0.33) 11.9 (1.45)
Rossmoyne Apl B1 4.6 (0.24) 12.6 (0.51) Rossmoyne Apl B2 4.5 (0.58) 13.2 (1.16)
+Standard deviations are reported in parentheses.
Iowa Alfisol and Mollisol were 3.9 and 3.3, respectively.
In this investigation, the E^/Eq ratios for humic acids
extracted from the Mollisols ranged from 3.9 to 4.1; whereas, ratios for those extracted from Alfisols ranged
from 4.2-6.1. Thus the E4/E6 ratios for the humic acids
(except for one value) generally are consistent with those
reported by Kononova (1966), but are higher than those reported by Zhang et al. (1988). The lower E4/E6 ratio in humic acids from Mollisols indicates that they are more structurally condensed or complex than humic acids from
Alfisols. This is in agreement with interpretations of 13C
NMR data which will be presented later and the H/C atomic ratios previously presented in Table 15. As shown in Table 17, the E4/E6 ratios for fulvic acids from the Mollisols ranged from 7.7-16.2 whereas, those from the Alfisols ranged from 11.9-13.3. The E4/E6 ratios of fulvic acids from the Mollisols and Alfisols are somewhat similar. This suggests that, in general, fulvic acids from the Mollisols and Alfisols are similar in structure. This is contrary with the H/C and O/C atomic ratios observed in Mollisol fulvic acids. Zhang et al.
(1988) reported that the E4/ E 6 ratio of fulvic acids from the surface horizons of cultivated Iowa Alfisols and
Mollisols were 4.9 and 2.4, respectively. In addition,
Chen et al. (1977) reported E 4/ E 6 ratios for fulvic acids between 11 to 14. The ratios for fulvic acids in this study are much larger than those reported by Kononova
(1966), and Zhang et al. (1988). The high E4/E6 ratios for fulvic acids from pedons in this study may be due to a preponderance of nonaromatic components. Overall, the
E4/E6 ratios for fulvic acids are much larger than those of humic acids. This finding is in agreement with that reported by Kononova (1966). This suggests that, in general, the fulvic acids are not as structurally condensed as the humic acids.
C. Functional Group Analyses
Functional group analyses of humic acids are presented in Table 18. Functional group analyses were not performed 127
Table 18. Functional group analysis for all humic acid samples.-^
Phenolic Soils Batch Total Aciditv^ COOH OH§ mol/kg
Dana Ap B1 6.79 (0.36) 4.45 (0.22) 2.34 Dana Ap B2 7.57 (0.32) 4.46 (0.09) 3.11
Dana V. Ap B1 6.95 (0 .21) 4.60 (0.10) 2.35 Dana V. Ap B2 7.15 (0.34) 4.76 (0 .12) 2.39
Dana V. A B1 ,7.53 (0.41) 5.20 (0.04) 2.33 Dana V. A B2 7. 36 (0.06) 5.13 (0.05) 2.23
Xenia Ap B1 5.18 (0.27) 3.94 (0 .02) 1.24 Xenia Ap B2 5.77 (0.07) 4.00 (0.04) 1.77
Rossmoyne Apl B1 5.74 (0.24) 3.95 (0.06) 1.79 Rossmoyne Apl B2 5.68 (0.09) 3.72 (0.08) 1.96
^Means of three replicates are reported tStandard deviations are reported in parentheses determined by difference between total acidity and carboxylic acid groups on fulvic acids because of insufficient sample. Humic acids extracted from Mollisols (Dana and Dana Variant) contained higher quantities of total acidity, carboxylic acid, and phenolic-OH groups than those extracted from the
Alfisols. Functional group contents in Mollisol humic acids ranged from 6.79-7.57 mol/kg for total acidity, 4.45-
5.20 mol/kg for carboxylic acid groups, and 2.33-3.11 mol/kg for phenolic-OH groups; whereas, contents in Alfisol
(Xenia and Rossmoyne) humic acids ranged from 5.18-5.77 mol/kg for total acidity, 3.72-4.0 mol/kg for carboxylic 128
acid groups, and 1.24-1.96 mol/kg for phenolic-OH groups.
Schnitzer (1977) reported that the "ideal" humic acid
contains 6.7, 3.6 and 3.9 mol/kg total acidity, carboxylic acid and phenolic-OH groups, respectively. In addition,
Lowe (1969) reported that the total acidity, carboxylic acid, and phenolic-OH contents of humic acid from an Orthic
Brown and Orthic Black soil (both Mollisols) ranged from
7.2-7.8, 2.34-2.43, and 4.78-5.37 mol/kg, respectively.
Humic acids extracted from Mollisols in this investigation contain similar contents of total acidity but have higher contents of carboxylic acid groups than reported by Lowe
(1969) and Schnitzer (1977).
The total acidity and phenolic-OH contents for humic acids from the Alfisols in this study are lower than the
"ideal" humic acid (Schnitzer, 1977), but carboxylic acid groups are similar and all contents are lower than those reported by Lowe (1969). Lowe (1969) reported contents of
2.3, 5.9, and 7.89 mol/kg for carboxylic acid, phenolic-OH and total acidity for an Orthic Gray Wooded (an Alfisol) soil.
A slight difference in functional group distribution between the humic acids extracted from the Dana Variant Ap and A horizons is evident in Table 18. The humic acids extracted from the A horizon exhibits greater total acidity and carboxylic acid content relative to the Ap horizon.
There appears to be no difference in functional group 129 contents of humic acids between the Otterbein and Burton sites.
D. Infrared Absorption Spectra
IR absorption spectra of soil humic substances are discussed with reference to the assignment of specific absorption bands to rotational and vibrational movements of specific chemical bonds. Assignments for the various absorption bands are tabulated in Table 19.
A problem with IR is that several functional groups have absorption maxima in the same region. In this study, only a rough chemical identification was obtained from the
IR spectra because treatments which improve the spectra were not included. The identification of specific groups is facilitated by treatments which remove other groups absorbing at the same frequency.
1. Humic Acids
The IR absorption spectra for the Dana and Xenia, and
Dana Variant and Rossmoyne humic acids are presented in
Figures 31 and 32, respectively. Overall, the spectral patterns are similar. The IR absorption pattern for Dana
Ap B1 humic acid is of poor quality relative to the others.
But major peaks are still evident. Two broad bands at 3400 and 2900 cm-1 which are generally attributed to OH stretching of hydrogen-bonded hydroxyls and aliphatic C-H Table 19. Main infrared (IR) absorption bands for humic and fulvic acids (after Stevenson, 1982).
Wave No. (cm-1) Assignments
3400-3300 O-H stretching, and N-H stretching (trace) 2949-2900 Aliphatic C-H stretching 1725-1720 C=0 stretching of COOH and ketones (trace) 1660-1630 C=0 stretching of amide groups, quinones C=0 and/or C=0 of H-bonded conjugated ketones 1620-1600 Aromatic C=C, strongly H-bonded C=0 of conjugated ketones 1590-1517 COO- symmetric stretching, N-H deformation + C=N stretching 1460-1450 Aliphatic C-H stretching 1400-1390 OH deformation and C-0 stretching of phenolic-OH, C-H deformation of alkyl and methyl groups, COO- antisymmetric stretching 1280-1200 C-0 stretching and OH deformation of COOH, C-0 stretching of aryl ethers 1170-950 C-0 stretching of polysaccharide-like substances, Si-0 of silicate impurities
stretching are evident in all of the humic acids. No major absorption bands are evident between 2800-1900 cm-1.
A series of absorption bands between 1720-1610 cm-1 are evident in all samples. In most cases, these peaks are of about equal intensity. The absorption peak at/or near
1720 cm-1 has been attributed to C=0 stretching of carboxylic acid and ketonic groups. The bands between 1650 and 1630 cm'*1 which most samples exhibit is attributed to
C=0 stretching of amides and quinones. The absorption peaks between 1630-1610 cm"1 are attributed to C=C aromatic ring stretch and to strongly H-bonded C=0 of conjugated ketones. iue 1 I seta pten 40-0 c-1) o ( ) (1 for ) 1 cm- (4000-900 pattern spectral IR 31. Figure %Trans. 0 0 4 3 0 5 2 3 0 8 3 3 0 0 9 0 2 9 2 0 5 2 3 0 0 2 3 n () ei p 2 hmc cd samples. Bl, acids Ap humic Xenia (3) B2, Ap B2, Xenia Ap (4) Dana (2)and Bl, Ap Dana 0 2 9 2 0 2 9 2 0 0 9 2 0 0 4 2 aeN. m-1 No. cm Wave 0 2 7 1 0 2 7 1 0 2 7 1 0 2 7 1 04 0 7 1 0 0 0 10 61 1 0 5 6 1 0 5 6 1 0 3 6 1 5 2 6 1 5 2 6 1 0 1 6 1 0 0 4 1 0 0 4 1 0 0 4 1 0 9 3 1 0 0 3 1 0 0 1 1 0 4 2 1
00 10 1 0 3 2 1 0 4 2 1 0 3 2 1 f 0 8 0 1 0 8 0 1 0 5 0 1 0 0 9 0 5 0 1 0 0 9 2 0 0 9 0 0 9
131 132
1 7 2 0 2 9 2 0 1 6 3 0 1 4 0 0 1 6 5 0 1 2 3 0 3 4 2 0 1 0 4 0 1100 900
2 9 0 0 1710/ 1620 14'00 1230 3 4 1 0 1 0 4 0 1 0 9 5 900
3400 2 9 0 0 \ ifi5n 1400 1705 ^ 1620 1650 1220 / 1Q20 1 0 9 5 9 0 0
4 2 9 0 0 1 4 0 0 3 4 1 0 (/) 17,0 I 'l620 1220 C 1 6 5 0 0) 1 0 9 5 1— H 9 0 0
1720 1 4 0 0 1 2 3 0 1 6 5 0 1® 3 0 2910 1100 1 0 4 0 9 0 0 3 4 3 0
2 9 1 0 1720 / 'l640 1400 1 2 3 0 3 4 2 0 1 6 5 0 1100
4000 3 2 0 0 2 4 0 0 1 7 0 0 1 3 0 0 9 0 0 Wave No. c m '1
Figure 32. IR spectral pattern (4000-900 cm-1) for (1) Dana Variant Ap Bl, (2) Dana Variant Ap B2, (3) Dana Variant A Bl, (4) Dana Variant A B2, (5) Rossmoyne Apl Bl, and (6) Rossmoyne Apl B2, humic acids samples. All samples have a weak absorption band at 1400-
1390 cm-1 which is due to OH deformation and C-0 stretching
of phenolic-OH, C-H deformation of aliphatic groups, or to
COO-antisymmetric stretching. All samples show similar
absorption bands at approximately 1240-1220, 1100-1080, and
900 cm-1. Some samples also exhibit a weak band between
1050-1020 cm-1. The bands at 1240-1220 cm-1 are due mainly
to C-0 stretch and OH deformation of carboxylic acid
groups, with minor contribution from aryl ethers (Wagner
and Stevenson, 1965; Stevenson and Goh, 1971; and Piccolo
and Stevenson, 1982). The bands at 1100-1080 and 1050 cm-1 have been assigned to polysaccharide-like substances or to
silicate impurities (Stevenson, 1982). Schnitzer and
Desjardins (1969) working with purified humic acids (< 1.4% ash) also reported that IR absorption bands at/or near 1070 and 1050 cm-1 are due to silicate impurities. The ash contents for all humic acids (< 2%) are very similar to that of Schnitzer and Desjardins (1969). Therefore, it is possible that absorption in this region is due to either poly-saccharide-like components or silicate impurities.
The sharp absorption peak at 900 cm-1 is assigned to =C-H out-of-plane bending of aromatic and alkene structures
(Pavia et al., 1979).
No major differences are evident among any of the humic acids collected in this study. The humic acids show evidence for aromatic, aliphatic, carbonyl (as carboxylic 134
acid, ketones, quinones and phenols) and polysaccharide
like components and are similar to those shown by Schnitzer
and Gupta (1964) and Stevenson and Goh (1971).
2. Fulvic Acids
The IR absorption spectra for the Dana and Xenia, Dana
Variant and Rossmoyne fulvic acids are presented in Figures
33 and 34, respectively. Overall, the spectra are similar.
In all samples, there is a prominent broad band between
3420 and 3360 cm-1 which is attributed to OH stretching of hydrogen bonded hydroxyls. The fulvic acids from most soils exhibit a weak absorption band between 2960 and 2920 cm-1. This peak is attributed to aliphatic C-H stretch ing.
All fulvic acids yield strong absorption bands at 1720 and weak bands between 1630-1610 cm"1. The bands at 1720 and 1630-1610 cm-1 have been assigned to C=0 stretching of carboxylic acid and ketones, and C=C of aromatics and H- bonded C=0 of conjugated ketones, respectively. In contrast, to the humic acid IR spectra, the absorption bands between 163 0 and 1610 cm"1 are relatively weak and imply that there is less aromatic and ketonic structures in the fulvic acids.
All samples exhibit absorption bands between 1420-
1390, and 1230-1200 cm"1. The peaks at 1420-1390 cm"1 are attributed to OH deformation and C-0 stretching of iue3. R pcrl atr (0090 m1 fr 1) (1 for cm-1) (4000-900 pattern spectral IR 33. Figure
0 0 0 4 %Trans. 0 2 4 3 0 2 4 3 0 0 4 3 n () ei p 2 fli cd samples. acids Bl, Ap fulvic Xenia B2, (3) Ap B2, Xenia Ap (4) Dana (2)and Bl, Ap Dana 0 0 4 3 0 0 2 3 0 2 9 2 0 2 9 2 0 0 4 2 Wave cm.-1 No. 0 3 7 1 0 3 7 1 0 2 7 1 0 0 3 1 0 0 7 1 0 2 7 1 0 4 6 1 0 3 6 1 0 3 6 1 0 3 6 1 0 0 2 1 0 2 4 1 0 0 4 1 0 0 4 1 0 9 3 1 1200 1200 1110 10 0 8 0 1 1110 1110
/ 0 9 0 1 0 8 0 1
0 0 9
135
136
9 0 0 2 9 3 0 1100 3 4 0 0 1 2 3 0
1 7 2 0
2 9 6 0 1610 9 0 0 1 4 0 0 3 4 0 0 1200
1 7 2 0 1 1 0 0 1 0 8 5
2 9 3 0 1 6 3 0 1 4 0 0 9 0 0 1 2 3 0 / I I 3 4 0 0 1 7 2 0 1 0 8 5 5 2 9 6 0 1 6 3 0 3 3 8 0 1 4 2 0 1110 1200 1 7 2 0 2 9 6 0 1 6 3 0 3 3 6 0 1 4 0 0 1200 / I 1085 1 7 2 0 i mo. 4 0 0 0 3 2 0 0 2 4 0 0 1 7 0 0 1 3 0 0 9 0 0 Wave No. cm ~1 Figure 34. IR spectral pattern (4000-900 cm-1) for (1) Dana Variant Ap Bl, (2) Dana Variant Ap B2, (3) Dana Variant A Bl, (4) Dana Variant A B2, (5) Rossmoyne Apl Bl, and (6) Rossmoyne Apl B2, fulvic acids samples. 137 phenolic-OH and COO- antisymmetric stretching. The peaks at 1230-1200 cm-1 are attributed to C-0 stretching and OH deformation of carboxylic acid, and C-0 stretching of aryl ethers. Most fulvic acids, exhibit a weak absorption band at 1110-1100 cm-1 and another between at 1090 and 1085 cm-1. These peaks are attributed to C-0 stretching of polysaccha ride-like components or to Si-0 stretching of silicate impurities. Unlike the humic acids, there are no bands at/or near 1050-1040 cm-1. There is a sharp band at 900 cm-1 in both Dana Variant Ap and A Bl samples, whereas the others lack this band. This band has been assigned to =C-H out-of-plane bending of aromatic and alkene structures (Pavia et al., 1979). Little differences can be detected among any of the fulvic acids examined in this study. IR analysis did, however, reveal that fulvic acids are not as aromatic as humic acids and are composed of carbonyl (as carboxylic acid, ketones, phenolic, and quinones), aliphatic, aromatic, and polysaccharide-like components. Furthermore, the IR spectral patterns for fulvic acids in this study are similar to those reported by Schnitzer and Gupta (1964), and Stevenson and Goh (1971). 138 E. 13C CPMAS NMR Spectra Interpretation of 13C NMR spectra is based on chemical shift data of known organic compounds and accompanying functional groups. As previously mentioned, (see Section 3.2.3) solid-state 13C cross polarization magic angle spin ning nuclear magnetic resonance (CPMAS NMR) allows four, and sometimes five, C types to be distinguished by charac teristic chemical shifts (Wilson et al., 1981a). A typical 13C CPMAS NMR spectrum of a humic acid is presented in Figure 35. As shown in Figure 35, the spectrum can be divided into five regions (see Table 9): aliphatic-C (0-50 ppm), oxygen alkyl-C (50-110 ppm), aromatic C (110-160 ppm), carboxylic and amide-C (160-190 ppm), and carbonyl-C (190-240 ppm). In the following section, the most signifi cant peaks in each spectrum are identified. The spectra were evaluated in a manner similar to that of Hatcher et al. (1983), Saleh et al. (1983), and Skjemstad et al. (1983). The areas for the previously defined chemical shift regions were delineated by dropping verticals to an arbi trarily drawn baseline. The defined areas were measured by tracing on plastic sheets and expressed as a percent of the total organic carbon content. Vila et al. (1976) cautioned that the chemical shifts associated with C in humic substances can only be assumed to be reliable to + 5-10 ppm. Furthermore, they reported that it is hazardous to 139 13 C NMR chemical shift region assignments: 1. Carbonyl 2. Carboxyl 3. Aromatic 4. Oxygen-alkyl 5. Aliphatic £ 200 0 ppm Figure 35. Representative 13C CPMAS NMR spectra of humic acid showing chemical shifts and divisions into regions for intensity analysis. conclude that signal intensities correspond linearly to absolute concentrations of various classes of C atoms, or that the absence of a certain signal implies the absence of a specific chemical structure. However, Hatcher et al. (1980a) reported a + 5% error for absolute area calcula tions on well-defined peaks and + 10% for peaks which were not well resolved. At best, 13C NMR data can only be interpreted as semi-quantitative estimates. Although the 140 intensity measurements are semi-quantitative, 13C CPMAS NMR spectra provide qualitative information regarding the nature of a sample and permit comparisons with other sam ples characterized by the same method (Dereppe et al., 1980; Wilson et al., 1981a). 1. Humic Acids The 13c CPMAS NMR spectra for the humic acids analyzed in this study are presented in Figures 36 through 40. The distribution of C in all humic acids is presented in Table 20. To facilitate spectra interpretation, spectra patterns are discussed by the chemical shift regions defined earli er. a. Aliphatic Region (0-50 ppm) The aliphatic-C region is attributed (see Table 9) to C occurring in long chain polymethylene structures or branched alkyl-C or methyl-C positions attached to aromat- ic-C rings. In the aliphatic-C region, spectra of humic acids from Dana and Xenia exhibit a broader range of chemi cal shifts (18-32 ppm) relative to Dana Variant and Rossmoyne which show a relatively well defined peak at 32- 34 ppm. These data suggest that; (1) the humic acids contain methyl-C in long chains and alkyl-C bonded to aromatic ring structures and (2) the Dana and Xenia humic acids may contain a more heterogeneous mixture of Figure 36 13C CPMAS NMR spectra of Bl and B2 humic B2 and Bl of spectra 13C NMR CPMAS 36 Figure 220 200 cd fo aaA (WA-69) pedon. Ap Dana from acids ppm 8 100 0 B2 141 142 in 200 100 0 8 ppm Figure 37. 13C CPMAS NMR spectra of Bl and B2 humic acids from Xenia Ap (WA-70) pedon. 143 C*5 B2 05 C5 m 200 O 100 0 oppm Figure 38 . •L^C CPMAS NMR spectra of Bl and B2 humic acids from Dana Variant Ap (WA-72) pedon. 200 / " * 100 0 6 ppm Figure 39. 13C CPMAS NMR spectra of Bl and B2 humic acids from Dana Variant A (WA-72) pedon. 14 5 104 CO 200 100 0 8 ppm Figure 40. 13C CPMAS NMR spectra of Bl and B2 humic acids from Rossmoyne Apl (WA-71) pedon. Table 20. Distribution of C in alkaline extracted humic acid samples as determined by 13C CP-MAS NMR analysis. Soil Batch Aliphatic Oxygen alkyl Aromatic Carboxylic Carbonyl ------% c ------ Xenia Ap Bl 26.5 22.3 30.1 17.6 3.5 Xenia Ap B2 24.6 21.8 32.3 17.2 4.1 Dana Ap Bl 18.1 19.4 42.6 18.51 1.4 Dana Ap B2 21.3 21.3 41.4 14.4 1.6 Rossmoyne Apl Bl 22.9 30.0 32.6 13.0 1.5 Rossmoyne Apl B2 20.6 29.6 33.2 14.9 1.7 Dana Variant Ap Bl 20.9 31.3 29.6 11.3 6.9 Dana Variant Ap B2 19.9 23.4 40.2 13.6 3.0 Dana Variant A Bl 17.3 21.6 40.0 16.6 4.4 Dana Variant A B2 16.7 23.7 41.6 14.3 4.7 147 aliphatic-C structures relative to Dana Variant and Rossmoyne humic acids. Schnitzer and Preston (1986) reported absorption peaks in spectra of some humic acids at 16.5 and 32.5 ppm which they attributed to methyl-C occurring as long chain and terminal positions, respectively. Lobartini and Tan (1988) also reported a major absorption peak at 30 ppm in spectra from humic acids of two Mollisols which was attributed to aliphatic-C groups. Carbon occurring in aliphatic structures (Table 20) in the humic acids ranges from 16.7-26.5% of the total C. The C content occurring in aliphatic-C structures for all humic acids are lower than that reported in the literature. Skjemstad et al. (1983) reported that aliphatic-C contrib utes 25 to 30% of the total C in some Alfisol humic acids. Grant (1977) reported that polymethylene chains comprise up to 30% of the total organic matter of a Podzol A horizon. Lobartini and Tan (1988) also reported that 32.2% of the C in humic acid from a Mollisol occurred in aliphatic-C structures. In summary, the humic acids from Alfisols have a higher amount of C occurring as aliphatic structures than the Mollisols. The high amount of aliphatic-C groups in the Xenia humic acids may be due to: (1) degradation of larger humic compounds or (2) or may be a metabolic by product of microbial metabolism (Preston et al., 1987). It 148 is interesting to note that the Dana Variant Ap horizon has a higher aliphatic-C content than A horizon. Assuming that the Dana Variant A horizon is not cultivated, the higher content of aliphatic-C groups in the Ap horizon may be due to cultivation. Preston et al. (1987) reported that cultivation will cause an increase in the aliphatic-C content of organic matter. b. Oxvoen Alkvl Region (50-110 ppm) This region of the spectra is attributed to C oc curring as oxygen substituted alkyl-C structures (Saleh et al., 1983). Some typical structures which resonate in this region are: amino acids, carbohydrates-C, and methoxy-C (OCH3) groups. The most prominent peaks in the alkyl-C region in the spectra of all the humic acids (Figures 34 to 38) occur at 57-58 and 73-74 ppm. In some of the spectra, a 104 ppm peak is observable. The peak at 55-58 ppm is attributed to methoxy-C groups associated with lignin and lignin-like products (Hatcher et al., 1980c; and Piotrowski et al., 1984). The peak at 72-74 and 104 ppm is character istic of carbohydrates and ether bound alkyl carbons (Voelter and Breitmaier, 1973; Preston and Ripmeester, 1982.) Skjemstad et al. (1983) speculated that carbohy drates contribute more to this region than do ether type structures. The proportion of the methoxy-C peak (58 ppm) vs carbohydrate-C peak (74 ppm) in spectra of humic acids 149 from Alfisols are greater than those in the humic acids from Mollisols. This suggests that humic acids from Alfisols exhibit more lignin and lignin-like character than humic acids from Mollisols. In the humic acids of the Dana Variant A and to a lesser extent in the Dana Ap, the 57-58 ppm peaks are weak. Peaks between 60 and 65 ppm which are due to amino acid-C are not very prominent in any of the spectra. A decrease in signal intensity at 58 ppm in the Dana Variant A horizon humic acids spectra was noted relative to the Ap horizon which suggests more methoxy-C groups in humic acids from the Ap horizon. This condition may be explained by the Ap horizon receiving fresh residue (corn and soybean debris) containing lignin and lignin-like components. The percentage of total C occurring in oxygen alkyl-C structures (Table 20) ranged from 19.4 to 31.3% which is similar to that reported in the literature. Skjemstad et al. (1983) reported that the oxygen alkyl-C content of humic acids ranged from 16-37%. Lobartini and Tan (1988) reported that the oxygen alkyl-C content of humic acid from a Mollisol was 18.8%. In summary, peaks at 58, 73-74 and 104 ppm in spectra of the humic acids suggest that carbohydrates-C, carbohydrate-like, lignin and lignin-like substances are present. The amounts of C as oxygen-alkyl structures does not differ between the Alfisols and Mollisols at both 150 sites. However, the proportion of lignin (58 ppm) vs car bohydrate (74 ppm) does vary between Alfisols and Mollisols. Humic acids from Alfisols exhibit more lignin and lignin-like character than humic acids from Mollisols. This suggests that Alfisols may have formed under forest vegetation whereas Mollisols formed under grasses. c. Aromatic Region (110 to 160 ppm) This region of the spectra is attributed to aromatic-C and substituted aromatic-C structures (Saleh et al., 1983). The most intense peaks in the humic acid spectra occur in the region between 129-134 ppm. According to Schnitzer and Preston (1986) this suggests that the aromatic-C structures are substituted with alkyl-C groups. Nearly all of the Mollisol humic acids are higher in aromatic character than the Alfisols humic acids. The E4/EQ absorption ratio, and IR pattern all support the high aromatic character of the Mollisols. The aromatic-C content for all humic acids ranged (Table 20) from 29.6-41.6%. Hatcher et al. (1981c), Preston and Schnitzer (1984), and Schnitzer and Preston (1986) reported that 40-50% of the C extracted from Mollisols of widely different climatic conditions occurs as aromatic-C structures. Lobartini and Tan (1988) reported that the proportion of C in humic acids from a Mollisol occurring as aromatic-C was 37.2%. The percentage of C in Mollisol humic acids occurring in aromatic-C structures was 151 similar to that reported by Hatcher et al. (1981c), Preston and Schnitzer (1984), Schnitzer and Preston (1986), Lobartini and Tan (1988). Some spectra show a minor peak at 153-157 ppm. These peaks which are attributed to phenolic-OH bonded carbons seem to be better expressed in the humic acids extracted from the Alfisols. As previously mentioned (see Section 3.2.3), the presence of phenolic-OH C in soil humic acids is subject to some debate. Some researchers (Hatcher et al., 1981c) conclude that phenolic-OH C is not an important component of soil humic acids, whereas others (Hatcher et al., 1980b; and Schnitzer and Preston, 1986) conclude that phenolic-OH C is an important component. Schnitzer and Preston (1986) only observed minor phenolic-OH C peaks in the 13C NMR spectra of soil humic acids which showed large concentrations of phenolic-OH C (4.7-7.5 mol/kg) by chemical methods. Schnitzer and Preston (1986) concluded that chemical methods for phenolic-OH C quantification were high because of some inclusion of carboxylic acid groups. The results of this study are very similar to those of Schnitzer and Preston (1986). Peaks due to phenolic-OH C were nearly absent in the NMR spectra of Mollisol humic acids; however, chemical methods (see Table 18) suggest a large concentration (2.33-3.11 mol/kg) of phenolic-OH C relative to humic acids from Alfisols. It is possible that detection of phenolic-OH C in Mollisol humic acid is 152 complicated because signals arising from phenolic-OH C may be shifted down field so that they overlap with resonance of carboxylic acid groups (Schnitzer and Preston, 1986). Thus, the current study is inconclusive regarding this debate. In summary, results presented in this investigation support the conclusion of Schnitzer (1981) and Stevenson (1982) that aromatic-C is a major structural component of both Alfisol and Mollisol humic acids. Humic acids from Alfisols and Mollisols contain an average aromatic-C con tent of 32 and 40%, respectively. Phenolic-OH groups were not detected in spectra of humic acids from Mollisols whereas, humic acids from Alfisols did contain phenolic-OH structures. This indicates that phenolic-OH groups in Mollisols humic acids does not favor detection by 13C CPMAS NMR possibly due to an overlap of resonance signals with carboxylic acid groups (Schnitzer and Preston, 1986). The aromatic-ring of Mollisol humic acids is probably substituted with either alkyl-C or carboxylic acid groups. d. Carboxyl Region (160-190 ppnU The 160-190 ppm region of the spectra is due to carboxyl, ester-C, quinone-C and amide-C structures (Skjemstad et al., 1983). Carboxylic acids probably ac count for most of the signal in this region, but ester-C and amide-C contributions can not be ignored (Hatcher et 153 al., 1981c). This chemical shift area will be referred to as the carboxylic acid region due to the dominance of absorption by this group. Typically, peaks in this region occur between 172 and 174 ppm (Saleh et al., 1983). At present, 13C NMR cannot effectively differentiate carboxylic acid, ester-C or amide-C groups in complex mixtures because their resonances in this region overlap (Skjemstad et al., 1983). Nevertheless, ^3C NMR spectroscopy may effectively summarize the contributions of these groups. A single peak between 173-175 ppm was recorded for all the humic acids. This signal is more intense in the spectra of Alfisol humic acid than the Mollisol humic acid spectra. In addition, this signal is nearly of equal intensity relative to the aromatic-C peak (130-134 ppm) in the Alfisol spectra; whereas for Mollisol spectra, the carboxylic acid peak is approximately one-half the magnitude of the aromatic-C peak intensity. The more intense peaks observed in Alfisol than Mollisol humic acids may be due to Alfisol humic acids having more carboxylic acid functional groups attached to similar C skeletal structures. The carboxylic acid content of the humic acids ranged (Table 20) from 11.3-18.5% of the total C which is similar to that reported in the literature. Skjemstad et al. (1983) reported that C occurring in carboxylic acid groups in soil humic acids can range from 8-14%. Lobartini and 154 Tan (1988) reported that the distribution of carboxylic acid groups in humic acids from a Mollisol and a Ultisol was 11.8 and 14.9%, respectively. The data suggest that Alfisol and Mollisol humic acids contain similar amounts of C as carboxylic acid groups, however Alfisol humic acids may have more carboxylic acid functional groups attached to similar C skeletal structures. e. Carbonyl Region (190-240 ppm) The 190-240 ppm region of the spectra is attributed to carbonyl-C occurring as aldehyde and ketone-C structures (Saleh et al., 1983). The spectra exhibit only weak, poorly defined peaks between 190-240 ppm. Carbon occurring in aldehydes and ketones ranged (Table 20) from 1.4-6.9% of the total C. These data are similar to those presented by Hatcher et al. (1980a) who reported that C occurring as aldehydes and ketones was generally less than 5% of the total organic carbon content in soils. Schnitzer (1976), however, estimated an aldehyde and ketone content of about 12% of the total organic carbon for soil humic acids. Both Xenia and Dana Variant humic acids have more C occurring in aldehyde and ketone struc tures than humic acids from the Dana and Rossmoyne soils. The data suggests that aldehyde and ketone structures are a minor component of Alfisol and Mollisol humic acids and 155 that there are no major differences between the Mollisols and the Alfisols. 2. Fulvic Acids The 13C CPMAS solid-state NMR spectra of the fulvic acids were interpreted in a manner similar to that of the humic acids as described by Hatcher et al. (1983), Saleh et al. (1983), and Skjemstad et al. (1983). The spectra were partitioned into five regions which were attributed to the following organic moieties: aliphatic-C (0-50 ppm), oxygen alkyl-C (50-110 ppm), aromatic-C (110-160 ppm) carboxylic-C and amide-C (160-190 ppm), and carbonyl-C (190-240 ppm). For each spectra, significant peaks are identified and the area for each region determined and reported as a percent age of the total carbon. The 13C CPMAS NMR spectra for the fulvic acids are presented in Figures 41 and 42. The interpreted C content in the various moieties (expressed as a percentage of the total C) is presented in Table 21. a. Aliphatic Region (0-50 ppm) The aliphatic-C region is attributed to C occurring in long chain polymethylene-C structures or branched alkyl-C or methyl-C positions attached to aromatic-C rings (Saleh et al., 1983). Spectra for all the fulvic acids have multiple peaks between 30 and 41 ppm but only the Alfisols show a peak at 17-18 ppm. This suggests that all the fulvic acids contain long chain polymethylene and alkyl-C 156 f-CN CO to 200 100 0 ppm Figure 41 13C CPMAS NMR spectra of (1) Xenia (WA-70) Ap Bl and (2) Dana (WA-69) Ap B2 fulvic acids. 157 200 100 0 ppm Figure 42. 13C CPMAS NMR spectra of (1) Rossmoyne (WA-71) Apl and (2) Dana Variant (WA-72) Ap B2 fulvic acids. Table 21. Distribution of C in alkaline extracted fulvic acid samples as determined by 13C CPMAS NMR analysis. Soil Batch AliDhatic Oxvaen alkvl Aromatic Carboxvlic Carbonvl ------% c Xenia Ap B1 23.8 28.8 18.0 24.2 5.2 Dana Ap B2 24.6 30.4 19.3 23.2 3.5 Rossmoyne Apl B1 27.3 23.7 18.8 25.5 4.7 Dana Variant Ap B2 25.2 23.8 18.8 28.0 4.2 159 groups attached to aromatic rings (Saleh et al., 1983). The signals at 17-18 ppm in spectra of the Alfisol fulvic acids suggests some differences between the fulvic acids of the Mollisols and Alfisols. Blondeau's (1986) analysis of - fulvic acids extracted from a forest soil yielded spectra with a series of bands between 25-45 ppm which is very similar to the spectra reported herein. The C content in aliphatic structures ranges from 23.8-27.3% (Table 21) with fulvic acids from the Burton site containing somewhat higher aliphatic-C character than fulvic acids from the Otterbein site. b. Oxygen Alkyl Region (50-110 ppm) The 50-110 ppm region of the spectra is attributed to C occurring as oxygen substituted alkyl-C structures (Saleh et al., 1983). Some typical structures which resonate in this region are: amino acids, carbohydrates, and methoxy-C groups. The most prominent peak in this region of the spectra occurs at 72 ppm. Peaks occurring between 70 and 72 ppm have been attributed to carbohydrates and carbohydrate-like components (Hatcher et al., 1983). The strong absorption peak at 72 ppm suggests that carbohy drates and carbohydrate-like components are major compo nents of all fulvic acids in this study. Preston and Ripmeester (1982), Hatcher et al., (1983) and Blondeau (1986) all also reported that soil fulvic acids are largely 160 dominated by carbohydrates and carbohydrate-like compo nents. According to Hatcher et al. (1980a), additional substitution of O and N compounds to rings may contribute to the broad series of peaks between 70 and 84 ppm such as those presented in the spectra of the Rossmoyne fulvic acids (Hatcher et al., 1980a). All spectra exhibit a weak to very weak peak at 56 ppm. A 56 ppm peak has been at tributed to methoxy-C groups of lignin (Hatcher et al., 1983). The low intensity of this peak in the current study suggests that the content of lignin in the fulvic acids is minor. The spectra of the Alfisol fulvic acids also exhib it a very weak peak at 96 ppm which may be due to amino acids or to ethers. The C content in the oxygen alkyl-C region ranges from 23.8-30.4% (Table 21) with fulvic acids from soils at the Burton site being higher in oxygen alkyl-C structures, notably carbohydrates and carbohydrate-like substances, than fulvic acids from soils at the Otterbein site. c. Aromatic Region (110-160 ppm) The 110-160 ppm region of the spectra is attributed to aromatic-C and substituted aromatic-C structures (Saleh et al., 1983). Spectra of fulvic acids are dominated in the aromatic region by a peak occurring between 130-137 ppm. According to Hatcher et al. (1980a) this indicates that all the fulvic acids in this study have some aromatic-C charac ter and are substituted with C functional groups, namely alkyl moieties. All spectra also show a very weak peak between 146 and 149 ppm. This signal is generally attrib uted to O-substituted aromatic-C structures most likely phenolic-OH C groups (Saleh et al., 1983); however, contri butions from N-substituted aromatic-C structures can not be precluded (Schnitzer and Preston, 1986). The weak signal intensity for phenolic-OH C groups suggests that either they are not detectable with 13C NMR or are only a minor constituent. Low detectability of phenolic-OH C groups by 13C CPMAS NMR has been reported previously by Hatcher et al. (1981c) and Schnitzer and Preston (1986). Minor peaks are also present at 110-124 ppm and using interpretations of Saleh et al. (1983) may be attributed to aromatic car bons ortho to ether 0 or OH groups. The C content in aromatic-C and substituted aromatic-C groups ranges from only 18-19.3% (Table 21). These data suggest that the fulvic acids are not dominated by aromatic-C or substituted aromatic-C groups and that there is no difference between the Mollisols and Alfisols. This finding supports the conclusion of Wilson and Goh (1977), Preston and Ripmeester (1982), Hatcher et al. (1983), and Blondeau, (1986) that soil fulvic acids are not dominated by aromatic-C structures. It is interesting that the aromatic character of fulvic acids is much lower than that 162 of humic acids. The low aromatic character of fulvic acids relative to humic acids as determined by NMR analysis is supported by IR analysis. d. Carboxvl Region (160-190 Pt>rol The 160-190 ppm region of the spectra is due to carboxyl, ester, quinone and amide-C structures (Skjemstad et al., 1983). Carboxylic acid groups probably account for most of the intensity in this region, but ester and amide-C contributions can not be ignored (Hatcher et al., 1981c). This region of the spectra will be referred to as the carboxylic acid region due to the dominance of absorption by this group. The most intense peak in the entire spectra of all the fulvic acids occurs between 172 and 174 ppm. This peak consumes the entire region of all the spectra. Saiz- Jimenez et al. (1987) also noted a dominate carboxylic acid absorption peak in spectra from several soil fulvic acids. The C content in carboxylic acid groups ranges from 24.2- 28% (Table 21). The carboxylic acid character of the Alfisol and Mollisol fulvic acids is similar at each site, however; it is slightly higher at the Burton site than the Otterbein site. e. Carbonvl Region (190-240 ppm) The 190-240 ppm region of the spectra is attributed to carbonyl-C occurring as aldehyde and ketone-C structures 163 (Saleh et al., 1983). All spectra exhibit some minor peaks in this region, but the peaks are weak and barely discern- able above background noise. Both Mollisol spectra show one peak between 220 and 224 ppm whereas, both Alfisol spectra show a series of small peaks. The latter suggests that the Alfisol fulvic acids may be composed of a hetero genous mixture of aldehyde and ketone-C structures. The one peak in the Mollisol fulvic acid spectra suggests that they may be composed of a more homogeneous mixture of aldehyde and ketone-C structures. The C content in aldehyde and ketone structures of the fulvic acids ranges between 3.5-5.2% (Table 21). Fulvic acids from both Alfisols and Mollisols seem to have approximately the same aldehyde and ketone character. 3.6 Summary More humic substances were extracted from Mollisols than Alfisols with the majority being humic acids. Fulvic acids, however, occur in only minor amounts in all pedons because of methods employed in this study. Inappropriate size dialysis tubing used in purification procedures employed in this study contributed to a substantial loss (from 40-70%) of the humic substances most notably observed in the Alfisols. Data from IR and 13C CPMAS NMR spectroscopy indicated that humic acids from both Alfisols and Mollisols are 164 dominated by alkyl substituted aromatic structures, fol lowed by aliphatic, carbohydrate, and carboxylic acid groups with minor amounts of aldehyde and ketone groups. Mollisol humic acids contain a higher aromatic-C content than Alfisol humic acids. The low aromatic-C character of Alfisol humic acids is supported by wide H/C ratios and high E4/E6 ratios. No phenolic-OH groups were detected in any Mollisol humic acids specimens. Also 13C NMR spectra showed that humic acids from the Alfisols contain some phenolic-OH character and more lignin, and lignin-like products than those of Mollisols. Fulvic acids from both Alfisols and Mollisols as indicated by 13C NMR spectroscopy contain mostly aliphatic, carbohydrate, and carboxylic acid structures, followed by aromatic groups and minor amounts of aldehyde and ketone groups. The aromatic structures in both Alfisol and Mollisol fulvic acids are largely alkyl substituted with minor amounts of phenolic-OH substitution. The proportion of nonaromatic and aromatic structures in the fulvic acids is similar for Alfisols and Mollisols; however, fulvic acids from soils on the Wisconsinan till plain are higher in carbohydrates than the soils on the Illinoian till plain. In conclusion, some differences in the chemical and structural properties of humic acids from contiguous Alfisols and Mollisols in southwestern, Ohio were documented. CHAPTER IV HUMIC SUBSTANCES IN WHOLE SOILS 4.1 Introduction Most often, the chemical structures comprising soil organic matter are determined following extraction with either alkali or acid. The main disadvantage of this approach is that the organic matter may be modified during extraction and that only a portion of the organic matter is examined. 13C NMR spectroscopy is a technique capable of providing direct characterization of soil organic matter; thus eliminating the need for extraction. The character ization of organic matter associated with mineral size fractions will be presented in this chapter. 4.2 Literature Review 4.2.1 13C NMR Analysis of Unfractionated Soils 13c CPMAS NMR spectroscopy is a technique which can give structural information on the types and forms of C in carbonaceous material. Recently, the CPMAS NMR technique has been used to obtain 13C spectra of solid samples, including peats (Preston and Ripmeester, 1982; Wilson et 165 al., 1983a, 1983b; and Preston et al., 1987) and unfrac tionated whole mineral soils (Barron et al., 1980; Wilson et al., 1981b, 1983a, 1983b; and Preston and Ripmeester, 1983) . In these studies, carboxylic, aromatic, oxygen alkyl, aliphatic, and carbonyl contents were estimated in humic substances without prior chemical extraction. Preston and Ripmeester (1982, 1983), and Preston et al. (1987) , reported that the 13C NMR spectra of organic matter in a mineral and organic soil showed mostly aliphatic and carbohydrate structures with a low quantity of aromatic and carboxylic moieties. In a similar study with some New Zealand soils, Wilson et al. (1981, 1983a, 1983b) reported that the C in organic matter was distributed in the follow ing order; carbohydrate > aliphatic > aromatic > carboxylic > carboxyl. It appears that most of the C in organic matter of mineral and organic soils from several different environments, occurs as aliphatic and carbohydrate type structures and to a lesser extent aromatic and carboxylic structures. 4.2.2 13C NMR Analysis of Fractionated Soils The low natural abundance of 13C (1.1%) and the low organic carbon content of mineral soils is often a limita tion in obtaining well resolved spectra during solid state 13C NMR studies of whole soils (Preston and Ripmeester, 1983; and Oades et al., 1986). Preston and Ripmeester 167 (1983) reported that the 13C CPMAS NMR spectra of a clay loam soil (3.1 % OC) showed a poor signal/noise ratio and lack of resolution. They attributed the poor spectral patterns to a low organic carbon content. In several other studies which utilized samples with higher organic carbon contents, well resolved 13C NMR spectra were obtained (Barron and Wilson, 1981; Preston and Ripmeester, 1982; and Wilson et al., 1981). In order to improve the resolution of 13C NMR spectra of whole soils with low organic carbon contents, the organ ic matter can be concentrated in the material by particle size fractionation and/or sedimentation. Such an approach was used by Preston and Ripmeester (1983) who improved spectral resolution by collecting the less than 50 ym size fraction from a mineral soil. They reported that the organic matter in this size fraction was dominated by aliphatic and carbohydrate type structures. Sonification and sedimentation techniques have also been used by Turnchenek and Oades (1979) to study organic matter associated with certain mineral size fractions. In this study, systematic changes in the chemistry of organic matter were observed across a range of particle size frac tions. Oades et al. (1986) fractionated a red-brown soil into many size fractions which was analyzed directly by solid-state 13C NMR. They found that carbohydrate and aliphatic structures were dominate in the medium to coarse 168 sand and clay size fractions, respectively. The amount of aromatic-C character was low, but was best expressed in the silt fraction (2-20 urn). In summary, well resolved 13C NMR spectra can be obtained of organic matter of whole soils and associated with mineral size fractions. Using 13C NMR spectroscopy, humic substances in whole soils was found to be largely composed of aliphatic and carbohydrate type structures. Aliphatic and carbohydrate type substances were most often found associated with sand and clay-size material whereas aromatic and carboxylic acid type structures were associ ated with silt-size material. 4.3 Material and Methods 4.3.1 Sonification of Whole Soils In general, the surface horizon of pedons in this study have a low organic carbon content (< 3%). Several researchers (see Section 4.2.2) have noted that 13C NMR spectral resolution for patterns of whole soils with low organic carbon contents is poor. 13C NMR spectral resolu tion of whole soil can be improved by obtaining an organic carbon enriched fraction relative to the whole soil by sonification and sedimentation techniques (see Section 4.2.2). Henceforth, to improve NMR spectral resolution, our goal was to obtain an organic carbon enriched fraction. Oades et al. (1987) obtained many size and density frac tions with enriched C contents by first sonifying a whole soil sample and then using sedimentation and density gradi ent techniques. In this study, sonification methods were employed to obtain organic carbon enriched fractions. Whereas the methods were similar to those of Oades et al. (1987), several procedures were modified. Five 20 g por tions of 2 mm air-dried whole soil from the surface horizon of Dana, Xenia, Rossmoyne, and Dana Variant pedons were placed into 200 ml plastic beakers. Eighty ml of deionized water and a teflon-coated stirring bar were added to the beaker. This beaker was then placed inside a 700 ml beaker and crushed ice was packed around the outside of the smaller beaker. The soil slurry was cooled in order to minimize any possible alteration of the organic matter due to excess heat generated during the sonification procedure. The soil slurry was mechanically dispersed with a ultrason ic probe (Branson Sonifer Cell Disruptor Model W 185) for 10 minutes at 80 watts. 4.3.2 Collection of Organic Carbon Enriched Soil All soil slurries were combined and then transferred to a tall 1000 ml glass beaker, shaken, and allowed to settle for 24 hours. The temperature of the suspension was recorded, and the upper 9 cm of the suspension was removed by siphoning. Both fractions (the suspended upper 9 cm and everything else) were later freeze dried. Using Stokes Law it was determined that after 24 hours, the less than 1.2 m fraction still remained suspended, while material greater than 1.2 m in diameter settled below 9 cm. These two fractions will arbitrarily be referred to as the fine and coarse fractions, respectively. The experiment was pre formed twice on Dana, Xenia and Rossmoyne pedons (called Batch 1 [Bl] and Batch 2 [B2]), and once on the Dana Variant pedon (called Bl). Particle size analysis (see Section 1.3.3) and total organic carbon content (see Section 1.3.5) were determined on both the fine and coarse fractions. All analyses were preformed in duplicate. 4.3.3 13C CPMAS NMR Analysis of Organic Carbon Enriched Soil About 500 mg of the fine fraction was sent to the Nuclear Magnetic Center in Ft. Collins, CO for solid-state 13c CPMAS NMR analyses. The ^ C NMR spectra for the fine fractions were collected at a carbon frequency of 15 MHz, sweep width of 10,000, acquisition time of 28 msec, and a recycle time of 1.0 sec on a Nicolet NT-150 spectrophoto meter. Samples were spun at the magic angle at 3000 rps. The contact time was 1.0 msec and 2048 points in full frequency domain were collected. The number of scans collected for each sample was 36,000 to 60,000. 171 Spectral patterns of fine fractions were interpreted in a manner similar to the spectra of the humic and fulvic acids (see Section 3.5 E). 4.4 Results and Discussion 4.4.1 Particle Size Analysis of Fractions Collected by Sonification and Sedimentation The yield of fine and coarse fraction and particle size analysis for the fine fraction collected is presented in Table 22. The largest yield of fine material occurs in both Dana Variant samples followed by Xenia, Dana, and Rossmoyne samples. Approximately 77 to 89% of the material collected after 24 hours was clay and of this approximately 48-49% was fine clay. Calculations suggest that the suspended fraction should be composed entirely of particles Table 22. Yield and particle size analysis of the fine fraction obtained by sonification and sedimentation Particle size Yield Soils 1.2-0.2 Urn < 0.2 Pm Coarse Fine g ----- Dana Ap 77.8 48.5 96.4 2.3 Dana V. Ap 85.9 48.1 85.6 12.0 Dana V. A 89.4 49.6 85.9 11.4 Xenia Ap 86.1 49.8 93.0 6.4 Rossmoyne Apl 83.9 48.2 98.8 1.7 •j* , Average results of duplicate analyses. of less than 1.2 pm. The inclusion of coarser particles suggest that some particles greater than 1.2 ym were collected during siphoning. It is likely that some coarse material was inadvertently collected while siphoning close to the 9 cm mark. 4.4.2 Percent organic carbon in collected fractions The carbon content in the fine and coarse fractions obtained from sonified soils are presented in Table 23. Oades et al. (1987) employing sonification and sedimenta tion techniques recovered size and density fractions which ranged from 2 to 18 times the organic carbon content of the whole soil (a Rhodoxeralf). Preston and Ripmeester (1983) Table 23. Total C contents in fine and coarse fractions and in whole soils. Oraanic carbon content’*' Soils Batch Fine Coarse Whole ----- % Dana Ap Bl 6.9 2.5 2.5 B2 6.9 2.0 2.5 Dana Variant Ap Bl 3.9 2.4 2.0 Dana Variant A Bl 4.0 1.9 2.1 Xenia Ap Bl 4.3 1.6 1.5 B2 3.9 1.3 1.5 Rossmoyne Apl Bl 3.2 1.3 1.1 B2 3.8 1.3 1.1 4* Average of two replicates. 173 recovered a less than 50 ym size fraction by sieving which had approximately twice the organic carbon content of the whole soil (a Orthic Humic Gleysol). The organic C con tents of the fine and coarse fractions ranges from 3.2 to 6.9% and 1.3 to 2.5%, respectively. The fine fractions contained 2 to 4 times the C content as the whole soil. The fractionation procedure utilized in the current study resulted in a significant enrichment of organic carbon in the fine fraction relative to the whole soil. The fine fraction collected from the Dana horizon contained the highest C content (6.9), whereas the Rossmoyne fine frac tion contained the lowest (3.2). The C content of the coarse fraction did not vary much from that of the whole soil. 4.4.3 13C NMR Spectra of Fine Material from Whole Soils The 13C CPMAS NMR spectra of the fine fraction col lected from whole soils are presented in Figure 43. The fine fraction from the Rossmoyne pedon was not analyzed because of the low C content. Overall, the spectral pat terns are of poor quality with a low signal to noise ratio which makes interpretation very difficult. Therefore interpretation must be made with caution. Although the spectral quality is not as good as with the humic and fulvic acid spectra, many of the same peaks are evident in the fine fraction. The Dana Variant and 174 Dana Var. Ap CM 3.9 % OC Xenia Ap . 4.3% OC Dana Ap 6.9% OC CO 200 100 s ppm Figure 43. 13c CPMAS NMR spectra of fine material collected from sonified whole Dana Variant Ap, Xenia Ap, and Dana Ap soils. 175 Xenia fine fraction show a well defined 29-30 ppm peak, whereas the same peak in the fine fraction of Dana is less well expressed. The peak at 29-3 0 ppm is indicative of aliphatic groups. All three spectra also display peaks at 54-60 ppm which are indicative of methoxy groups. In addition, all three spectra show peaks between 72-79 and 93-101 ppm which indicates that carbohydrates, ethers and certain amino acids are present. In general, peaks are not well expressed in the aromatic region (110-160 ppm) of all spectra. Weak peaks at 113 and 132-133 ppm are present in the Dana Variant and Dana and Xenia pedons, respectively. Evidence for the presence of aromatic groups in these soils is weak. Evidence for phenolic-OH groups (149-150 ppm) is present in the spectra for Dana and Xenia, but lacking in Dana Variant. All spectra exhibited peaks at 173-178 ppm which are indicative of carboxylic acid groups. Weak peaks between 191-198 and 210-215 ppm which is indicative of carbonyl groups occur in the spectra of all three samples. In conclusion, the organic matter associated with the fine fraction in this study consisted of aliphatic, carbo hydrate, and carboxylic acid with low proportions of aro matic and carbonyl groups. Similar conclusions were reported by Wilson (1981b, 1983a, 1983b); Preston and 176 Ripmeester (1982); Preston et al. (1987). Overall, the spectra were of low quality due to low signal to noise ratio which may be caused by low carbon content or peak broadening from paramagnetic Fe species. No attempt was made to analyze fine fraction with NMR after Fe removal. No significant increase in spectral quality or resolution was noted in the Dana samples with the highest organic C content. This finding is in contrast to that of Preston and Ripmeester (1983). 4.5 Summary Sonification and sedimentation techniques resulted in an enrichment in organic carbon C of 2 to 4 times in size fractions relative to the whole soil. The organic C en riched fine fraction was 77 to 89% clay of which approxi mately 50% was less than 0.2 ym clay. Solid-state 13C CPMAS NMR spectra were of low quality which makes interpre tation difficult. In the fine fraction spectra of Dana Variant and Xenia peaks indicative of aliphatic, carbohy drate and carboxylic acid structures are evident. Spectra of the Dana fine fraction are similar except there is little evidence for aliphatic structures. There is little evidence of aromatic structures in the organic matter of the Dana Variant pedon, but some evidence of aromatic character in that of the Dana and Xenia pedons. There is some evidence for carbonyl character in all three samples. The spectral resolution of the fine fractions did not improve significantly with an increase in organic carbon content. CHAPTER V INFLUENCE OF SORBED HUMIC SUBSTANCES ON THE DECOMPOSITION OF TOTAL CLAY FRACTION BY SULFURIC ACID 5.1 Introduct i on Minerals in soils are vulnerable to attack by physical and chemical processes which may transform them into pro ducts which bear little resemblance to the original materi als (Loughnan, 1969). Agents such as acids are active participants in the weathering of minerals. Protons from acids decompose minerals by breaking bonds which results in the release of soluble products such as Ca, Mg, and Na ions. Soluble cations may be complexed by humic substances. The complexation of metals plays an important part in the genesis of soils, as it may increase the concentrations of metal ions in aqueous solutions to levels that are far in excess of their normal solubilities which increases mineral dissolution (Huang and Keller, 1970). Pedologists are interested in the ability of organic acids to chelate and transport metals within soils because these reactions have been implicated in Spodosol formation (McKeague, 1983). In addition to chelating metal ions and hydrous ox ides, humic substances sorb to the surfaces and to the interlayer regions of minerals. The sorption of humic 178 179 substances to minerals may inhibit the rate of mineral decomposition by forming a diffusion limiting or an armor- ized surface layer (Krauskopf, 1967). The objective of this chapter is to evaluate the role of humic and fulvic acids in the weathering of soil clays. Sections of the literature review include: (1) weathering of minerals in soils, (2) sorption of humic substances by minerals, and (3) sorption mechanism of soil humic sub stances by minerals. 5.2 Literature Review 5.2.1 Weathering of Minerals in Soils Rocks and primary minerals are subject to weathering reactions which produce secondary minerals which contrib ute to the formation of soils. Two processes, particle size reduction and decomposition, are involved in these changes. Particle size reduction decreases the size of rocks and minerals without major changes in their composi tion. Decomposition results in major chemical changes; soluble components are released and new minerals may be synthesized. The decomposition of primary, and secondary minerals and soil material by humic, fulvic, mineral acids, and soil microorganisms will be discussed in the following sections. 180 A. Decomposition of Primary Minerals The decomposition of primary minerals (such as musco- vites, orthoclase, and biotite) are influenced by numerous weathering agents. These weathering agents include, mineral acids, humic substances, and simple organic acids. Researchers have used these agents to examine the weather ing mechanisms of primary minerals. Stahlberg (1960a, 1960b) used boiling IN HC1 to digest several types of primary minerals to determine release rates of nonexchange able K, Mg, and Ca. Stahlberg (1960a) reported that K released was highest from biotite and medium from phlogopite, whereas muscovite and microcline released very little. Stahlberg (1960b) also reported that augite and hornblende released large amounts of nonexchangeable Mg and minor amounts of Ca. He concluded that boiling IN HC1 decomposed primary minerals and caused the release of large amounts of nonexchangeable metals. Using much less harsh conditions than Stahlberg, Luce et al. (1972) used deion ized water adjusted to a wide range of pH with HNO3 or KOH, and incubated serpentine, forsterite, and enstatite at room temperature to examine the dissolution kinetics of Mg and Si. They reported that Mg was released in higher amounts and at faster rates than Si from the silicate minerals. The dissolution of several silicate primary minerals to determine the dissolution kinetics of silica and other 181 cations was also investigated by Siegel and Pfannkuch (1984) who dissolved labradorite, microcline, enstatite, augite, and forsterite, in deionized water at pH 4. They noted that saturation indices indicated that super-satura tion was achieved after about 200 hrs of incubation in the augite and forsterite experiments, whereas near saturation was only achieved in the enstatite experiments after about 700 hrs with respect to Fe, Mg, Ca, and Si. They reported that approximately 0.6 and 0.4 mMol/L of K and Mg were released from microcline and enstatite after 1500 hrs incubation at pH 4.0, respectively. They concluded that saturation indices and mass balance calculations suggested that after 700 hrs, the release of silica from forsterite and augite was controlled by the precipitation of a solid silica phase and that silica mass transfer from feldspars and enstatite was as silica acid. In summary, mineral acids or acidified water can cause the dissolution of primary minerals and release soluble ions. Wollast (1967) suggested that the rates of chemical dissolution of primary silicate minerals vary due to the formation of a crust on the mineral surface consisting of either an aluminosilicate weathering residue or a cation depleted zone. Both may limit diffusion and inhibit fur ther decomposition. A protective coating on silicate minerals has also been suggested by Krauskopf (1967). As silicates weather, some metal cations are preferentially 182 removed, leaving an "armor" protective layer which is different in composition from the mineral as a whole. This armor coat may influence the diffusion rates of other cations and make the dissolution of silicates extremely slow (Krauskopf, 1967). In contrast, several researchers have concluded that the dissolution of feldspars in acid solutions is controlled by chemical reactions at the feldspar-solution interface and not by diffusion, either through an aqueous solution or a continuous protective surface layer (Petrovic et al., 1976; Berner and Holdren, 1977, 1979). To observe if feldspars form a protective layer, Petrovic et al. (1976) suspended an alkali feldspar (sanidine) in aqueous electrolyte solutions adjusted from pH 4 to 8 at room temperature for 2 weeks. Shallow surfaces of the feldspar grains were then analyzed for K, Al, and Si by X-ray photo electron spectroscopy. They concluded that a continuous protective layer did not form on the grains and that disso lution was controlled by etches on the surface and defects in the crystal lattice. Using harsher conditions than Petrovic et al. (1976), Berner and Holdren (1977, 1979) suspended sodic plagioclase and potassium feldspar grains isolated from soils in a 5% HF plus 0.09N H2SO4 solution for periods ranging from 2 min to 2 hrs. The surfaces of the weathered grains were then examined by scanning elec tron microscopy. They reported that the surfaces of the 183 feldspar grains were severely etched by the acid solutions but a continuous protective layer was not formed. Both studies have indicated that the weathering of feldspar grains was controlled by the nature of the surface and not by the formation of a diffusion limiting protective layer. In addition to mineral acids, some thoughts and works have focused on the role of humic, fulvic, and simple organic acids in the weathering of primary minerals. Although some researchers (Krauskopf, 1967; Loughman, 1969) have questioned the magnitude of mineral weathering by humic substances, several reports of extensive mineral weathering by humic substance exists in the literature. The ability of humic acids to complex metals from primary minerals has been demonstrated by Kononova et al. (1964), Schalscha et al. (1967), Ong et al. (1970), Baker (1973), Singer and Navrot (1976), and Tan (1980). Schalscha et al. (1967) demonstrated that considerable quantities of Fe were removed from epidote, augite, biotite, and granodiorite by humic acids. Baker (1973) reported that 15,000 yg Cu and 3 000 yg of Pb were extracted by humic acids from chalcocite and galena, respectively. In contrast, CO2-equilibrated water extracted only 30 yg of Pb from galena and 200 yg of Cu from chalcocite. Singer and Navrot (1976) reported that humic acids extracted large amounts of Cu, Zn, Mn, Cr, Co, Ni, Al, Fe, Mg, and Ca from a mixture of primary minerals in a basalt. Tan (1980) agitated microcline, biotite, and 184 muscovite in humic acid solutions at pH 2.5 and 7.0 for 0 to 1000 hrs. He reported that after 1000 hrs of incubation at pH 7 humic acids dissolved a combine total of 5 mg of Si, Al, and K per g of mineral. Fulvic acids have also been shown to be effective in dissolving primary minerals (Schnitzer and Kodama, 1976; Tan, 1980; and Kodama et al., 1983). Schnitzer and Kodama (1976) suspended duplicate biotite, phlogopite and musco vite specimens in 0.2% (wt/vol) aqueous fulvic acid and dilute HCl solution for 710 hrs at room temperature. The fulvic acids were found to be considerably more efficient in dissolving Al, Fe, and Mg from the micas than the dilute HCl. Maximum amounts of metals extracted by the fulvic acids from 1 g of mineral were; Fe, 15.5 mg (from bio tite) ; Mg, 14.1 mg (from phlogopite); K, 12.8 mg (from biotite); Al, 9.0 mg (from biotite) and Si, 25.8 mg (from biotite). Tan (1980) reported that after 900 hrs incuba tion at pH 2.5 fulvic acids extracted 3.0 mg of Si and 1.4 mg of Al from 1 g of microcline. In a similar study, Kodama et al. (1983) agitated chamosite and biotite sus pended in 0.025 and 0.1% (wt/vol) fulvic acid solutions for several weeks at room temperature and assayed the solutions for Si, Al, Fe, Mg, and K. They noted that the dissolution of chamosite and biotite approached near-equilibrium condi tions for all elements except Si after 300 hrs of shaking. At termination, approximately 4 and 5% of the initial 185 weights of chamosite and biotite respectively, had been dissolved. They reported that molar Si:Al:Fe:Mg ratios in fulvic acid solutions in contact with the chamosite were (a) 2.0:1.9:3.0:1.0 for the 0.025% solution and (b) 2.1:2.5:3.7:1.0 for the 0.1% solution. The molar ratios for the initial mineral were 3.6:3.2:5.2:1:0. Effects of fulvic acid concentrations on the dissolution of biotite were similar to those observed for chamosite. Analysis of 0.025% fulvic acid solutions which were reacted with bio tite for 720 hrs gave molar Si:Al:Fe:Mg:K ratios of 3.7:2.2:2.8:3.0:1.0 as compared to 3.3:1.2:1.5:1.8:1.0 for untreated biotite. They also noted that the pH of both fulvic acid solutions increased during the incubation study. They concluded that the dissolution of chamosite and biotite was incongruent, and that octrahederal elements in both the chamosite and biotite were relatively easily dissolved by the fulvic acids, but the dissolution of interlayer K from biotite was not observed. Soil microorganisms secrete low-molecular weight simple organic acids, including formic, acetic, oxalic and butyric, as metabolic by-products. These acids are common ly formed during organic matter transformations. These simple organic acids may be important agents in the mobili zation and transport of metals and the weathering of rocks (Stevenson, 1982). The weathering of primary minerals by known strains of soil microorganisms has been investigated by Henderson and Duff (1963), Duff et al. (1963), Webley et al. (1963), Weed et al. (1969), and Agbim and Doxtader, (1975). In these studies, a pure primary mineral was suspended in a culture media containing known strains of bacteria or fungi. After incubation, the microbes were removed and the supernatants assayed for metals and simple organic acids. Henderson and Duff (1963) reported that several strains of fungi were active in causing the release of metallic ions from biotite, muscovite, phlogopite, and olivine. Chromatographic analysis of the supernatants revealed that more than half of the fungi produced citric acid, followed by smaller amounts of acetic, fumaric, and oxalic acids. In a similar study, Webley et al. (1963) reported that fungal strains most effective in dissolving silicate minerals were those which produced citric and oxalic acids. Another organic acid secreted by bacteria, 2-ketogluconic acid, was reported by Duff et al. (1963) to be very effective in dissolving resistant phosphate and silicate minerals. They reported a recovery of 17% organic phosphate in bacterial culture tubes which contained an insoluble dicalcium phosphate mineral. Many investigators have studied the dissolution of primary minerals in the laboratory with simple organic acids (Huang and Keller, 1970, 1971; Huang and Kiang, 1972; Boyle et al., 1974; and Manely and Evans, 1986). Research ers have used acetic, aspartic, citric, salicylic and 187 tartaric acids assuming that these acids simulated the action of humic, fulvic, and other simple organic acids naturally occurring in soils. In all of these studies, higher amounts of metal cations were released by minerals • in the organic acid solutions than in CC>2-equilibrated water. In summary, substantial quantities of cations are re leased by primary minerals because of dissolution by miner al, humic, fulvic and simple organic acids. Rates of primary mineral dissolution by acids may be controlled by the formation of an protective layer or by the nature of the mineral surface. B. Decomposition of Secondary Minerals Secondary minerals such as clays (Chernov, 1959; Barshad, 1960a, 1960b; Miller, 1965; Polzer and Hem, 1965; Miller, 1968; Barshad and Foscolos, 1970; Carstea et al., 1970; Bar-On and Shainberg, 1970; Shainberg 1973; Feigenbaum and Shainberg, 1975) and chlorites (Gilkes et al., 1973; Clemency and Lin, 1981; Lin and Clemency, 1981), are readily decomposed in acidic solutions. In an early study, Barshad (1960a) suspended Na-saturated montmorill- onite, vermiculite, and kaolinite in various acids and CO2- equilibrated distilled water and determined the composition of cations on the exchange sites and in solution following incubation. Higher contents of Ca and Mg were found in supernatants from clays suspended in acids than in CO2- equilibrated distilled water. It was noted that Mg was released in higher amounts than Al. Barshad (1960a) proposed a clay mineral dissolution mechanism whereby H ions readily enter the interior of the crystal lattice and displace Al, Mg, and possibly Fe. The displaced cations then migrate to exchange sites or into solution. Displacement of Mg, Al, and Fe from the crystal lattice by H ions will eventually lead to decomposition of the mineral (Barshad, 1960a). Similarly, Bar-On and Shainberg (1970) leached Na-montmorilIonite with distilled water for 1 week and recorded the Na, Mg, Al, and Si concentrations in the effluent and the composition of the clay at the end of the experiment. They found that all the exchangeable Na was replaced by Mg, Al and H ions, the exchange capacity decreased, and 15% of the clay had dissolved. Feigenbaum and Shainberg (1975) studied the rate of K, Al, Fe, and Mg release from Fithian illite in dilute salt and HCl solutions at a pH greater than 3. They reported that the quantities of Al, Fe, Mg, and K released after 4 weeks were 21.6, 14.3, 10.2, and 6.4 mMol/100 g, respectively. This amounted to 5.9, 22.8, 19.8, and 5.9% of the total contents of Al, Fe, Mg, and K, respectively. The fraction of Al and K released were of the same order of magnitude as that reported by Gilkes et al. (1973). Feigenbaum and Shainberg (1975) also noted that the percentages Fe and Mg released 189 from the clay was much higher than that of K and Al. It should be known that all of these studies showed preferential release of Mg from the clays. Preferential release of Mg can be explained by two mechanisms: isomorphous substitution and weak Mg-0 or Fe-0 bond in the Mg-O-Si or Mg-O-Al linkages (Feigenbaum and Shainberg, 1975). In the former, substitution of Mg and Fe for Al in the octrahederal sheet weakens the chemical stability of the clay (Shainberg et al., 1974) because the relatively big Mg and Fe ions do not fit comfortably into the octrahederal cavity. In the latter, the Mg-0 and Fe-0 bonds are weaker than the Al-0 bond and are preferentially attacked by protons resulting in their release. Shainberg et al. (1974) proposed a two step decomposi tion mechanism for montmorillonite by an acidified dilute salt solution. The decomposition reaction involves a rapid exchange between the H ion in solution and M, a cation held either by the exchange sites or in lattice positions. This step is illustrated below: M-clay + H+ = H-Clay + M+ (1) The second reaction is a slow decomposition one, in which the lattice cation, M, is released from the lattice into the exchange complex of the clay. This is illustrated as: 190 H-clay ------> M-clay (2) The alow transformation of H-clay to M-clay is the overall regulating reaction and the M-clay is now available for rapid exchange as described in reaction 1. Barshad (1960b) proposed a similar acid dissolution mechanism. The ions released during acid dissolution can indicate preferential attack of either the tetrahederal or octrahed eral layers. In an early study, Brindley and Youell (1951) incubated a magnesian chlorite (penninite) mineral in dilute HCl and found that 100% of the Mg and Fe, but only 47% of the Al, was released by the mineral after 2 hrs of incubation. Based on XRD and cations extracted, they concluded that the octrahederal layer of the mineral was preferentially dissolved. The decomposition of secondary minerals in soils and sediments is often enhanced by the action of humic sub stances (Huang and Keller, 1970). Decomposition reactions are accelerated because humic, fulvic, and simple organic acids chelate metal ions. The chelated metals are easily removed in solution by percolating water. The resulting concentration gradient in solution causes the release of additional metal cations to attain equilibrium. Prolonged dissolution leads to mineral decomposition. Among soil humic substances, fulvic acids are regarded as being more important than humic acids in the decomposi tion of minerals (Kodama et al., 1983). Fulvic acids have 191 a greater propensity to chelate metals than humic acids because fulvic acids have a higher cation exchange capacity and are more water soluble. In an early study, Schnitzer and Skinner (1963) reported that 1 mole of fulvic acids dissolved 1 mole of Fe from goethite during a week of continuous leaching. Kodama and Schnitzer (1973) conducted a weathering study which involved shaking two chlorite samples, a Mg-leuchtenbergite and a Fe-thuringite with a 0.2% (wt/vol) (pH 2.5) aqueous fulvic acid and dilute HCl solutions. They found that after 312 hrs of incubation in the fulvic acid, 4 and 26% of the initial Mg- leuchtenbergite and Fe-thuringite were dissolved. In dilute HCl, only 2 and 6% of the Mg-leuchtenbergite and Fe- thuringite were dissolved, respectively. The amounts of Al, Fe, and Mg released by the Mg-leuchtenbergite and Fe- thuringite after 312 hrs in the fulvic acids solution were 4.2, 1.0 and 8.5, and 21.9, 61.1, and 6.0 mg per 1 g mineral, respectively. In comparison, the amounts of Al, Fe, and Mg released after 312 hrs incubation in dilute HCl for Mg-leuchtenbergite and Fe-thuringite were only 1.9, 0.5, and 4.4, and 6.0, 12.24, and 1.72 mg per 1 g of mineral, respectively. Humic acids have also been shown to promote the re lease of metal cations from minerals. Baker (1973) leached hematite with a 0.1% (wt/vol) aqueous humic acid solution (pH 3.4) for 24 hrs in a perfusion apparatus. 192 Hematite leached with humic acids released 340 pg of Fe as compared with 20 pg released by CO2-equilibrated water. The release of metal cations from secondary minerals by bacteria was demonstrated by Henderson and Duff (1963) who incubated vermiculite grains in culture tubes with known strains of bacteria. They reported that the bacteria secreted 2-ketogluconic acid which chelated approximately 14.5% of the initial amount of Mg. In summary, substantial amounts of cations are released by secondary minerals through acid dissolution by mineral, humic, fulvic and simple organic acids. Protons can readily enter the interior of the crystal lattice and displace Al, Mg and Fe eventually leading to the decomposi tion of the mineral. Several acid dissolution studies using secondary minerals showed that Mg was preferentially released from the octrahederal layer relative to Al. Preferential release of Mg was explained by two mechanisms: isomorphous substitution and a weak Mg-0 and Fe-0 bond in the Mg-O-Si or Mg-O-Al linkages. C. Decomposition of Soil Material Most studies investigating soil weathering by mineral acids, humic substances, and simple organic acids will utilize a pure and well characterized primary or secondary mineral. The use of soil is avoided because interpretation of dissolution data is complicated by the heterogenous 193 composition of soils. There are a few reports of metals released from soil material by soil microorganisms, mineral acids, simple organic acids, and fulvic acids. Duff et al. (1963) suspended several different soil samples in culture tubes of soil bacteria. In most cases, the soil bacteria were capable of solubilizing large amounts of Ca, Mg, Fe, and Al. Berthelin et al. (1974) placed soil samples into leaching tubes, some of which were sterilized, and nutrient broth was leached through the tubes to promote microbial activity. The leachates were analyzed for Fe, Al, and Mg. After six weeks very little was found in the sterile tubes, whereas large amounts were found in the nonsterile tubes. They concluded that soil microorganisms are capable of solubilizing Fe, Al, and Mg. Stahlberg (1960a, 1960b) boiled several soil size fractions in IN HCl and measured the amounts of nonexchangeable Ca, Mg, and K released. He reported that the release of nonexchangeable Ca and Mg increased with increasing clay contents. The average Mg release from the clay size fractions was approximately 3 to 6 times more than Ca. He also reported that the amount of K released from the clay fractions ranged from 12-31% of the initial amount. Pohlman and MC Coll (1986) leached some forest soil material with soluble organic acids and dilute HNO3 and analyzed the leachate for Al, Fe, Mn, and Mg. They concluded that organic acids solubilized more Al, Fe, Mg, and Mn than the dilute HNO3 acid. Schnitzer and 194 Skinner (1963) reported that a 0.04% (wt/vol) aqueous fulvic acid solution after 1 week of shaking removed 1.13 mg of Fe (9% of total Fe) and 2.64 mg of Al (6% of total Al) from 1 g of soil. It is apparent from such studies that although acids effectively weather soil, it is diffi cult to assign the soluble components to a particular source. 5.2.2 Sorption of Humic Substances by Minerals In several research papers, the binding of organic matter with minerals has been described as an adsorption mechanism because of a good correlation with an adsorption isotherm (Theng and Scharpeensel, 1975; Davis, 1982). In these studies, the organic matter was assumed to be totally adsorbed by the mineral phases with negligible loss to precipitation. This assumption may not be applicable in all experimental conditions. Sposito (1984) has suggested that "the adherence of experimental sorption data to an adsorption isotherm equation provides no evidence as to the actual mechanism of a sorption process in a soil." Conse quently, many so called adsorption reactions between soil organic matter and soil minerals may involve both adsorp tion and precipitation reactions. Sposito (1984) has suggested that the term "sorption" should be used to de scribe the loss of material to a solid phase to avoid the implication that either adsorption or precipitation is 195 occurring. In the following discussions, the term sorption will be used to describe the interaction between soil organic matter and soil mineral phases. There have been numerous studies conducted to investi gate the sorption of humic substances by minerals. For convenience, the following literature review has subdivided the discussion of the sorption of humic substances into nonexpanding and expanding mineral sections. A. Sorption bv Nonexpandinq Minerals Reactions of humic substances with nonexpanding miner als such as kaolinite (Evans and Russell, 1959; Rashid et al., 1972; and Kodama and Schnitzer, 1974) and muscovite (Kodama and Schnitzer, 1974) have been investigated. In an early study, Evans and Russell (1959) reported sorption rates of approximately 1.3, 2.8, and 3.4 g C as humic acids (0.03% wt/wt solution) per 100 g of K-, H-, and Ca-satu- rated kaolinite, respectively. Sorption rates for fulvic acids at the same concentrations were approximately 1, 2, and 3 g C per 100 g of K-, Ca-, and H-saturated kaolinite, respectively. Note that more humic than fulvic acids were sorbed and that maximum amounts of humic and fulvic acids sorption by kaolinite occurred when the exchange cations were Ca and H, respectively. The authors suggested that exchangeable Ca promoted sorption of humic acids by acting as a bridge linking the clay and humic acid polymers. For 196 fulvic acids, exchangeable H may increase sorption to clays by increasing the quantity of hydrogen bonding between the clay surfaces and the 0 containing functional groups. Summarizing some important points from Evans and Russell (1959) work: (1) sorption reactions were complete within 24 hrs and were unaffected by temperature, and (2) maximum sorption of humic and fulvic acids occurred at pH 4 or less with substantial decreases in sorption occurring with pH values from 4 to 7. Evans and Russell (1959) also showed that goethite, lepidocrocite, and gibbsite sorption was independent of the concentration of humic and fulvic acids with which it was equilibrated. For example, goethite sorbed 4.36 and 4.59 g of C as humic acids per 100 g mineral from a 0.0159 and 0.0318% C solutions, respectively. The sorption of fulvic acids by kaolinite was investi gated by Kodama and Schnitzer (1974) who reported that the maximum amount of fulvic acids sorbed at pH 3 by the less than 1 pm and 5 to 20ym size fractions of kaolinite were 73 and 37 mg/g clay, respectively. They also reported that the maximum amount of fulvic acids sorbed by muscovite at pH 2.5 and 6.0 were 49 and 27 mg/g clay, respectively. In both experiments, fulvic acid sorption amounts are similar and decrease with an increase in pH. Analysis of the minerals by XRD before and after sorption revealed no changes in the minerals c-axis spacing which indicates that 197 humic substances sorption occurred on the mineral surfaces and not in interlayer regions. In summary, humic and fulvic acids are rapidly sorbed by nonexpanding clay minerals with maximum sorption oc curring at low pH values. However, sorption of humic acids by kaolinite and muscovite is limited to the surfaces of the minerals. Also, maximum humic and fulvic acid sorption by kaolinite occurs when Ca and H are the saturating cations, respectively. Finally, sorption of humic and fulvic acids by sesquioxides was found to be independent of the concentration of humic substances. B. Sorption bv Expanding Minerals Reactions of humic substances with expanding minerals such as bentonite (Evans and Russell, 1959; Tan and McCreery, 1975), chlorite and illite (Rashid et al., 1972), montmorillonite (Schnitzer and Kodama, 1966; Theng and Scharpeensel, 1975) have been investigated. In an early study, Evans and Russell (1959) reported humic acid sorp tion of approximately 8.0, 5.0, and 1.5 g C (0.03% C wt/wt) per 100 g of Ca-, H-, and K-saturated bentonite clays, respectively. They also noted that the effects of pH and temperature, reaction completion time, and cation satura tion for humic acid sorption by bentonite clays were simi lar to those as reported above. There was no evidence for 198 humic acid sorption in the interlayer regions of the ben tonite. Tan and McCreery (1975) also reported that humic acids sorbed on the surfaces of bentonite and not the interlayer regions. Theng and Scharpeensel (1975) reported that there was less than 2 mg of humic acids sorbed per g of Na-saturated montmorillonite at pH 7. The amount of humic acid sorbed as reported by Theng and Scharpeensel (1975) for montmorillonite is lower than that reported by Evans and Russell (1959) for bentonite. Using minerals with less expansion than bentonite, Rashid et al. (1972) reported that chlorite and illite minerals sorbed 42 and 26 mg of humic acids per g of clay at pH 3.1, respectively. They noted a decrease in humic acid sorption with an increase in pH and no interlayer sorption of humic acids by illite and chlorite. The sorption of humic substances by clays has been reported to be influenced by the saturating cation. Theng and Scharpenseel (1975) reported that Ca-saturated benton ite clay sorbed more humic acid than Na-saturated bentonite clay. They concluded that Ca ions act as a bridge between the clay surface and the functional groups of humic acids. Lower humic acid sorption by clays saturated with mono valent cations has been attributed to low ionic potential (valency/ionic radius) and large hydration radius of the cation (Theng and Scharpenseel, 1975; Theng, 1976). As ionic potential of the cation increases, so does potential for humic acid sorption (Theng and Scharpenseel, 1975). An increase in ionic potential results in an increase in electrostatic interaction energy which favors electrostatic bonding between the cation and humic acid. Also, a cation with a small hydration radius can approach clay surfaces closer than a cation with a large hydration radius. The closer a cation can approach a charged surface, the stron ger the electrostatic bond which increases sorptive capaci ty. In addition, Theng and Scharpenseel (1975) reported that the affinity of humic acids for clays as measured by the logarithm of the isotherm slope, increases in the order: Na < K < Cs < Ba < Ca < Zn < Co < Cu < La < Al < Fe. In summary, the higher sorption of humic substances by clays saturated with polyvalent cations than monovalent cations is due to the cation's ionic potential, hydration radius and ability to form a clay cation bridge. Fulvic acids sorption by expanding minerals may occur at the surface (Evans and Russell, 1959) or in the inter layer regions of expanding clay minerals (Schnitzer and Kodama, 1977). In the latter, Schnitzer and Kodama (1966) were the first to report interlayer sorption of fulvic acids by montmorillonite. They noted that the d(001) spacing of air dry Na-montmorilIonite was 0.987 nm, but 1.76 nm following sorption of fulvic acids. They concluded that fulvic acids had penetrated the interlayer regions and caused an interlayer expansion of the montmorillonite. They also reported that interlayer sorption was pH depen dent, being greatest at low pH and not occurring above pH 5.0. They speculated that low pH promotes interlayer sorption because few functional groups on the fulvic acid polymer are ionized. Thus the fulvic acid polymer behaves like an uncharged molecule, and can penetrate the inter layer spaces and displaces water molecules in the clay interlayers. At pH values greater than 5, the functional groups ionize and impart a net negative charge on the polymer which is repelled by the negatively charged mont morillonite (Schnitzer and Kodama, 1966). In contrast, Evans and Russell (1959) reported that fulvic acids did not penetrate the interlayer regions of expanding clay saturated with either H-, Ca-, or K ions. Therefore, it is apparent from such studies that fulvic acids may be sorbed either on the surface or in the interlayer regions of expanding clays. The magnitude of fulvic acid sorption rates by ex panding clay minerals has been determined by several re searchers. Kodama and Schnitzer (1966) reported that the maximum amounts of fulvic acids sorbed by Na- montmorillonite were 77.5 and 40 mg per 100 g of clay at pH 2.5 and 6.0, respectively. These values are much lower than that reported by Evans and Russell (1959) who found that H-, Ca-, and K-saturated bentonite sorbed 7, 4.5, and 1.5 g of C per 100 g of mineral from a (0.03% wt/wt) fulvic 201 acid solution. In Evans and Russell's (1959) study, the amounts of fulvic acids sorbed by the bentonite clay were lower when compared to the amounts of humic acid sorbed. Although, the interlayer sorption of fulvic acids by expanding clay minerals has been documented in the labora tory (Schnitzer and Kodama, 1966) naturally occurring interlayer clay-humic complexes have been regarded as a pedogenic rarity (Theng et al., 1986). However, Kodama and Schnitzer (1971) and Theng et al. (1986) have both reported the presence of clay-organic complexes in the interlayer regions of naturally occurring expanding clays. Both of these researchers have suggested that interlayer sorption of humic substances is favored with smectite-type clays and promoted by a low soil pH. In summary, fulvic acids are sorbed at the surface (Evans and-Russell, 1959) and in the interlayer regions of expanding clay minerals (Schnitzer and Kodama, 1966). Most humic acid polymers, however, are excluded from interlayer penetration because of steric hindrances and size limita tions imposed by the basal spacing (Schnitzer and Kodama, 1977; Theng, 1982). Hence, large molecular-weight humic acid polymers are restricted to surface sorption. More humic acids are sorbed by expanding minerals than fulvic acids and sorption is favored with polyvalent exchange cations and promoted by a low pH. 202 5.2.3 Sorption Mechanism of Humic Substances by Minerals Several mechanisms are involved in the adsorption of organic compounds by minerals. Stevenson (1982) has iden tified the most significant, which include: (1) Van der Waals bonds, (2) electrostatic attraction, (3) hydrogen bonding, and (4) ligand exchange. These mechanism may occur jointly or independently depending on the organic species, reactive surface, surface acidity, cation on the exchange complex, and soil moisture content (Stevenson, 1982). A brief explanation of the four types of absorption mechanisms will be presented. A. Van der Waals Bonds Van der Waals bonds are short range attractive forces between molecules which result from fluctuations in the electrical charge density of individual atoms. A net attractive force is produced when an electrically positive fluctuation in one atom produces an electrically negative fluctuation in a neighboring atom (Stevenson, 1982). Although, Van der Waals forces operate between all mole cules, the energy of interaction is small when compared to a covalent or ionic bond, but the interactive forces are additive (Harter, 1977). Because of the additive effect, contribution of Van der Waals forces to the binding of organic molecules by minerals increases as molecular size 203 increases. These forces may dominate the adsorption pro cess for many nonpolar organic compounds (Stevenson, 1982). B. Electrostatic Bonding Positively charged organic molecules may be electro statically attracted to soil colloids through the process of protonation and cation exchange (Stevenson, 1982). Positively charged organic molecule can displace an inor ganic cation (M+) on the clay surface as follows: Clay -M+ + R-NH3+ ------►Clay -+NH3-R + M+ (3) As indicated, adsorption via cation exchange is usual ly proceeded by protonation, a process in which an organic molecule assumes a positive charge by accepting a proton. Protonation can occur when an uncharged molecule approaches an acid mineral surface (Harter, 1977) . Adsorption by this mechanism is related to both the basic character of the organic molecule, soil pH, the organic molecule length, and the type of cation on the exchange complex (Stevenson, 1982) . C. Hydrogen Bonding Hydrogen bonding (H-bonding) typically occurs between hydrogen and oxygen or nitrogen atoms. This attraction is due to hydrogen sharing unpaired electrons with either oxygen or nitrogen. The bond is weaker than ionic or covalent bonds, but stronger than Van der Waals forces of attraction. H-bonds are important in larger molecules or 204 polymers where additive bonds of this type may produce relatively stable complexes. The following illustrates H- bonds occurring between organic hydroxyl groups and clay surfaces: ROH O-Clay RCOOH O-Clay RCOO HO-Clay (4) H-bonding is assumed to be a major bonding mechanism in the adsorption of organic molecules to clay (Mortland, 1970). However, Farmer (1971) concluded that surface oxygens can only form weak hydrogen bonds with organic molecules. Although, H-bonding plays an important role in mineral-organic adsorption, the bonding is relatively weak. D. Liaand Exchange The anionic functional groups (C00-, and RO-) in humic substances may penetrate the coordination layer of iron and aluminum ions and become incorporated into the surface hydroxyl sheet (Greenland, 1971). This mode of bonding is referred to as ligand exchange (Hingston et al., 1967). Ligand exchange does provide one of the few mechanisms by which negatively charged organic molecules can be bonded to mineral surfaces (Harter, 1977). 5.2.4 Summary The following generalizations can be made about the interactions of humic and fulvic acids with minerals: (1) humic and fulvic acids can attack and degrade soil minerals by complexing and dissolving metals and transporting these 205 within soils and waters, (2) clay minerals weather primari ly by the dissolution of the octrahederal layer which results in preferential release of Mg, (3) humic acids are sorbed at the surface of minerals, whereas, fulvic acids are sorbed at both the surface and in the interlayer re gions, (4) greater quantities of humic acid are sorbed than fulvic acids, (5) sorption reactions between humic sub stances and minerals occur rapidly, (6) high sorption rates of humic substances are promoted by low pH, and with di- and trivalent cations on the exchange sites, and (7) sorption of humic substances to clay minerals may be ex plained by Van der Waals bonds, H-bonding, ligand exchange, and electrostatic bonding mechanisms. 5.3 Material and Methods 5.3.1 Separation of Total Clav Fraction Sorption of humic and fulvic acids and mineral disso lution studies were conducted on the total clay (< 2 ym) size fraction of the Xenia Bt2 horizon. Total clay frac-. tions were collected from samples which had been treated to remove organic matter and samples without this treatment. The former will be referred to as oxidized clays. Sorption studies were conducted with only the oxidized clays whereas dissolution studies were conducted with both untreated and oxidized clays. 206 A. Organic Matter Removal Oxidation of organic matter with H2O2 and removal of carbonates by acidification is usually done as a pretreat ment to facilitate isolation of sand, silt, and clay size fractions from mineral soils (Jackson, 1975). In this study, the pre-treatment was accomplished by placing two 50 g portions of 2 mm air-dried soil into separate 1000 ml glass beakers. Twenty five ml of IN NaOAc was added to each 50 g portion of soil and stirred into a slurry. The two slurries were heated on a hot plate at approximately 50° C for 30 min and stirred occasionally. During an addition al 2 hr period, while the slurries were maintained at 50°C, several 20 ml portions of 30% H2O2 were added to the slur ries. The samples were then allowed to digest overnight, at room temperature. B. Total Clay Fractionation 1. Oxidized Clavs The next step in separation of the total clay fraction was dispersion. Excess salts were removed from the oxi dized samples by repeated washings with deionized water until the soil remained dispersed after centrifugation for 5 min at 490 X g with an IEC model K centrifuge. The samples were then transferred to metal milkshake containers and 10 ml of IN sodium hexametaphosphate added. The sam- 207 pies were then mechanically stirred for 20 min. Three 50 g samples were treated in this manner in order to obtain sufficient clay. Following dispersion with sodium hexametaphosphate, the next procedure was sedimentation. The oxidized soil samples were transferred to separate 2.5 L glass bottles. The time required for a 2 u m particle to fall 10 cm in water at room temperature was calculated using Equation 3-2 of Jackson (1975). The bottles were then manually stirred and allowed to stand undisturbed for the required sedimen tation period. After the sedimentation time had elapsed, the clay fractions were siphoned into plastic jugs and flocculated with 1M Ca (NO3) 2M H 2O. Seven or 8 sedimenta tion periods were necessary to isolate the total clay fractions. The clay fractions were then Ca-saturated by three 100 ml washings using 1M Ca(N03>2*4H20 and excess Ca removed by repeated washings with deionized water until the samples remained dispersed after centrifugation. A small portion of the oxidized total clay fraction was Na-satu- rated with three 100 ml washings of 1M NaCl. Excess Na was removed by repeated washings with deionized water. The Ca and Na-saturated total clays and the silt + sand residues were transferred to plastic beakers and freeze-dried. The residual organic carbon not removed by oxidation with H2O2 was determined on oxidized clays. The OC content 208 was determined by dry combustion techniques as described in Section 1.3.5. 2. Untreated Total Clavs Total clay fractions with indigenous organic matter were collected from the Xenia Bt2 horizon in a manner similar to that used for oxidized clays. The sole differ ence was that organic matter was not removed from the soil prior to dispersion. The untreated total clays were Ca- saturated as described above. The OC content of the un treated total clay fraction was determined as outlined in Section 1.3.5. 5.3.2 Sorption of Humic Substances onto Oxidized Clay Fractions The optimum conditions for maximum sorption of humic substances were established in a series of preliminary studies. Two preliminary sorption studies were conducted to establish: pH, saturating cation, and clay:HA or FA (wt/wt) ratio necessary to attain maximum sorption. In the first preliminary sorption study, humic and fulvic acids from a Carlisle muck were used in order to conserve the humic substances extracted from pedons in this study. In the second preliminary sorption study, Rossmoyne and Dana Variant humic acids and Carlisle muck fulvic acids were used. 209 A. Preliminary Sorption Studies 1. Effect of p H and Saturating Cation The influence of pH and saturating cation on humic and fulvic acid sorption onto Ca or Na-saturated clays were investigated. Three humic and fulvic acid solutions were prepared. Eighty ml of deionized water were added to 50 mg of fulvic acids and stirred for 5 min. The pH of the three fulvic acids solutions were adjusted to 4, 5, and 6 with 0.IN NaOH. For the humic acids, 50 mg specimens were initially dissolved in 20 ml of 0.1N NaOH. After 5 min of stirring, 60 ml of deionized water were added and the pH adjusted to 6, 7, and 8 with 0.1N H2SO4. All humic and fulvic acids solutions were transferred to 100 ml volumet ric flasks, brought to volume with deionized water, and were then stored in plastic bottles. A 20:1 clay:HA or FA (wt/wt) ratio was arbitrarily chosen for the first preliminary sorption study. Two hundred-fifty mg of Ca or Na-saturated oxidized clays were mixed with 25 ml of humic or fulvic acids in 50 ml centri fuge tubes. The tubes were shaken slowly on a Eberbach shaker for 24 hrs at room temperature (24 + 2°C), and then centrifuged at 3488 X g for 30 min using a Beckman model J2-21 centrifuge. Supernatants were decanted and stored in plastic bottles. Humic and fulvic acids not sorbed to the clays were removed by washing the clays twice with 20 ml portions of deionized water. After each washing, the 210 samples were shaken for 5 min, and centrifuged for 30 min at 3488 X g. The supernatants were collected and stored in plastic bottles. The clays were transfered using about 20- 30 ml of deionized water into plastic beakers and freeze- dried. The quantity of humic or fulvic acids sorbed was determined by two separate methods. The first method involved determining the OC contents of the clays before and after sorption. This determination was done by the dry combustion technique described in Section 1.3.5. The quantities of humic or fulvic acids sorbed were determined by difference. The second method involved determining the DOC contents of all stock and washing solutions on a Dhormann Xertex C analyzer. This method has been described in section 5.3.3. The quantities of humic or fulvic acids sorbed were determined by difference between the DOC con tents of the initial stock and combined equilibration and washings solutions. 2. Effect of Clay:HA or FA Ratio The second preliminary sorption study was aimed at in vestigating the influence of the clay:humic substances ratios (wt/wt) on sorption rates. Humic acids extracted from Rossmoyne Ap and Dana Variant Ap and fulvic acids from Carlisle muck were used. All humic and fulvic acids were dissolved and adjusted to pH 6.0 as described in Section 5.3.2.A.1 and were added to Ca-saturated, oxidized clays in 250 ml centrifuge bottles. The clayrHA ratios employed were 5, 10, and 20:1 and the clay:FA ratios were 10 and 20:1. The suspensions were shaken on a Eberbach shaker for 24 hrs and centrifuged using a Beckman model J2-21 centri fuge at 3488 X g for 3 0 min. The supernatants were de canted and transfered to plastic bottles. Humic and fulvic acids not sorbed to the clays were removed by washing with two 100 ml portions of deionized water, shaking for 5 min, and centrifuging at 3488 X g for 30 min. The washings were decanted and transfered to plastic bottles. The clays with sorbed humic or fulvic acids were quantitatively trans ferred to plastic beakers with 20-30 ml of deionized water and freeze-dried. The sorption studies were repeated three times. The quantities of humic and fulvic acids sorbed to the clays at different clay/humic or fulvic acid ratios were determined as described in section 5.3.2.A.I. B. Sorption of Humic and Fulvic Acids to Clavs Used in Dissolution Studies The sorption of humic and fulvic acids to the clays in the dissolution experiments utilized conditions established as optimal during the preliminary studies. The optimum conditions were Ca-saturation, pH 6.0, and a clay/humic substances ratio of 10 or 20:1. Two separate sorption studies were conducted. In the first study, humic acids extracted from Dana Variant Ap and Rossmoyne Ap and fulvic 212 acids extracted from Dana Variant A were utilized. The appropriate quantities of clay and humic substances were used to prepare the following systems: 1. Dana Variant humic acids with ratios of 10 and 2 0 :1 , 2. Rossmoyne humic acids with a ratio of 20:1, and 3. Dana Variant fulvic acids with a ratio of 20:1. Throughout this discussion, sorption of humic acids by clays at a ratio of 10:1 will be referred to as (10:1). In the second study, the same humic acids were used, however; fulvic acids extracted from a Carlisle muck were substi tuted for Dana Variant fulvic acids. All clay:humic sub stances ratios were 20:1. Humic and fulvic acids not sorbed were removed in the same manner as described in Section 5.3.2.2. Following the sorption studies, the clays with sorbed humic or fulvic acids were freeze-dried and stored in glass vials. The quantities of humic and fulvic acids sorbed were determined as outlined in Section 5.3.2.2. 5.3.3 Dissolved Organic Carbon The concentration of DOC in all humic and fulvic acid stock and washings solutions were measured using a Dohrmann Xertex C analyzer. The humic acid solutions were usually diluted to 25:1 whereas, the fulvic acid solutions were diluted to 10:1 prior to injection. A sample injection volume of 200 pi and a 400 yg C/ml as potassium acid phthalate standard were used. All samples were eluted with 2% (wt/wt) potassium persulfate solution. 5.3.4 Dissolution of Clays in Sulfuric Acids In an attempt to identify the influence of humic and fulvic acids on clay mineral dissolution in acid media, two studies were conducted. A. Dissolution of Clavs in 0.1N H2SO4 Samples of the following Ca-saturated clays were incubated in 0.1N H2SO4 for 1320 hrs: 1. oxidized clays, 2. untreated clays, 3. oxidized clays equilibrated with Dana Variant humic acids at a clay:HA ratio of 20:1, 4. oxidized clays equilibrated with Dana Variant humic acids at a clay:HA ratio of 10:1, 5. oxidized clays equilibrated with Rossmoyne humic acids at a clay:HA ratio of 20:1, 6. and oxidized clays equilibrated with Dana Variant fulvic acids at a clay:FA ratio of 20:1. The samples 1.8 g of each of the above clays, were placed into Union Carbide dialysis tubing with 25 ml of 0.IN H2SO4. Each treatment was replicated. The dialysis tubing was sealed and suspended in 337.5 ml of 0.1N H2SO4. The samples were then incubated at room temperature (24.5°C + 1.4). A control consisting of dialysis tubing filled with 25 ml of 0.1N H2SO4 was set up in the same manner. Ten ml aliquots of dialyzate were removed at 24, 48, 72, 214 96, 144, 192, 288, 408, 504, 600, 696, 792, 912, 1008, 1128, 1200 and 1320 hrs. The aliquots were stored in plastic bottles at 5°C. Each time aliquots were collected, the dialysis bags were carefully agitated and pH values of the dialyzate were measured with an Orion pH meter, model 301, using a combination electrode. After 1320 hrs, the clays were quantitatively trans fered to 100 ml plastic beakers and freeze-dried. After freeze-drying, the clay weights were determined and com pared to initial weights. The percent weight loss was determined. The aliquots of dialyzate were analyzed for Si, Al, Fe, Ca, and Mg. The concentrations of Si was determined colorimetrically using the blue silicomolybdous acid method of Hallmark et al. (1982). Concentrations of Al and Fe were also determined colorimetrically using the ferron-o- phenanthroline method as described in Skougstad et al. (1979). A Beckman Du-20 spectrophotometer was used for all colorimetric measurements. Mg was determined by atomic absorption and K by flame emission using a Varian Techtron AA6 instrument. Instru ment parameters used were similar to Table 1. Fungal growth was not observed during the incubations up to 1300 hrs. But, after 1320 hrs, fungal colonies were observed in one oxidized clay system and in one oxidized 215 clay equilibrated with fulvic acid system. The study was terminated at 1320 hrs because of fungal growth. B. Dissolution of Clay in 0.005N H3SO4 In the second dissolution study, the following Ca- saturated clays were incubated in 0.005N H2S04 for 1176 hrs: 1. oxidized clays, 2. untreated clays, 3. oxidized clays equilibrated with Dana Variant humic acids at a clay:HA ratio of 20:1, 4. oxidized clays equilibrated with Rossmoyne humic acids at a clay:HA ratio of 20:1, 5. and oxidized clays equilibrated with Dana Variant fulvic acids at a clay:FA ratio of 20:1. Preliminary observations revealed that fungal growth was much more prevalent in 0.005N H2SO4 than the stronger acid. Fungal colonies were observed attached to the surface of the dialysis bags which were destroyed by the fungi. To inhibit fungal growth, cycloheximide, at a concentration of 25 to 100 yg/ml, was recommended by Dr. Jerry Sims. A screening test was conducted involving the addition of 0, 25, 50, 75, or 100 yg/ml cycloheximide to plastic beakers which contained dialysis bags filled with 200 mg of clays suspended in 37.5 ml of 0.005N H2SO4. Each beaker was inoculated with fungal colonies from a previous infested sample and incubated for 14 days at room temperature (24°C). Each treatment was inspected periodically for 216 fungal hyphae or colonies. After 14 days of incubation, fungal growth was not inhibited by any of the treatments. In an attempt to eliminate fungal growth with the lower acid strength, the use of dialysis tubing was discon tinued. The clays were simply incubated with the acid in the centrifuge tubes. A preliminary test was conducted involving the incubation of 1.06 g of Ca or Na-saturated oxidized and untreated clays in 250 ml centrifuge tubes containing 200 ml of 0.005N H2SO4. Each treatment was replicated twice. The tubes were loosely capped and incu bated for 19 days at room temperature (24°C). Periodical ly, each tube was inspected for visual evidence of fungal growth. After 19 days of incubation, there was no visual evidence of fungal hyphae or colonies. It appeared that fungal growth was promoted by the cellulose in the dialysis tubing which served as a food source. In order to avoid fungal growth the second dissolution study was conducted without the use of dialysis tubing. In this study, 1.33 g of clays, described earlier, were added to 250 ml centrifuge tubes with 250 ml of 0.005N H2SO4 . The tubes were sealed and shaken slowly for approximately 5 sec. After shaking, the caps on each tube were partially unscrewed to allow gas to exchange with the atmosphere. A control which consisted of 250 ml of 0.005N H2SO4 was also incubated in a similar manner. All suspensions were incu bated at room temperature (24°C ± 0.7). Each treatment and 217 controls were replicated twice. To insure that no clays were removed when aliquots were withdrawn, all suspensions were centrifuged at 1962 X g for 30 min prior to sampling. Ten ml aliquots were removed from all samples at 42, 111, 184, 278, 386, 477, 570, 663, 783, 936, and 1176 hrs. The aliquots were placed in plastic bottles and stored at 5°C. At the time of each sampling, pH was recorded with an Orion model 301 pH meter using a combination electrode. After aliquot withdrawal, the tubes were shaken for approximately 5 sec and incubated further. After 1176 hrs of incubation, the clays were quantitatively transfered to 100 ml plastic beakers and freeze-dried. After freeze-drying, total clay weights were determined and compared to initial amounts in order to calculate the percent weight loss. All sample aliquots were analyzed for Mg and K in a manner similar to that described in the first dissolution study. 5.3.5 Total Chemical Analysis The total Fe, Mg, and K contents of the Xenia Bt2 Ca- saturated clay fraction were determined using a modifica tion of the procedure developed by Bernas (1968). Fifty to 75 mg of oven dried (12 hrs at 110°C) clays were weighed to the nearest 0.0001 g and placed in a Teflon decomposition vessel. One-half ml aqua regia (a 3:1 vol. mixture of conc. HCl to conc. HNO3) was added as a wetting agent. Next, 5 ml of 48% HF acid were added with a Nalgene pipet and the decomposition vessel was placed inside a Parr No. 4745 Acid Digestion Bomb. The bomb was heated in a 110°C oven for 1 hr. After cooling to ambient temperature, the digested sample was quantitatively transferred to a plastic beaker containing 60 ml of 5.6% (wt/wt) boric acid solu tion. The sample was stirred with a Teflon stirring rod to hasten dissolution of any precipitate and then transferred to a 100-ml volumetric flask and adjusted to volume with deionized water. The solutions were transferred to 125 ml polyethylene bottles for storage. Concentrations of Fe, Mg, and K were determined by AAS or FES using dilution factors of 10:1. Standards which had the same matrix as the dilutions ranged from 0 to 5 Mg/ml. Instrument set tings are given in Table 1. 5.4 Results and Discussion To evaluate the role of humic substances in mineral weathering, the total clay (TC) fraction from the Xenia Bt2 horizon was equilibrated with humic and fulvic acids and then incubated in dilute H2SO4 for several weeks. Prelimi nary sorption studies established conditions for optimum sorption of humic substances by the clays. Two dissolution studies were conducted, one using 0.1N and the other with 0.005N H2SO4. During the incubation period, aliquots were periodically removed and analyzed for Mg, K, Al, Si, and Fe. The results from the sorption studies and two 219 dissolution studies will be presented in the following section. 5.4.1 Comparison of Sorption Amounts Determined by Xertex and Drv Combustion Methods The amounts of humic and fulvic acids sorbed by the Xenia TC fraction were measured by two methods. In one method, the DOC content was determined in the humic and fulvic acid solutions both before and after sorption using a Xertex C analyzer. The DOC was determined for all washings and added to that determined after sorption. The other method involves determining the OC content of the clays before and after sorption using a dry combustion determination. In both methods, sorption is determined by difference. Initially the sorption of humic substances by the clays was determined by analysis, before and after equilibration, of the solutions with the Xertex C analyzer. In order to determine if consistent sorptions could be attained, the sorptions were replicated. As shown in Table 24, the results were unfortunately quite discouraging as sorption varied considerably between replicates. Before it was concluded that the sorptions were in fact quite variable, the possibility of analytical error was investigated. Several techniques were attempted to ensure reliability of the analysis with the Xertex C Analyzer. These included: several injections of each 220 Table 24. Sorption"*" of humic acids (HA) and fulvic acids (FA) by clays as determined by C measured by Xertex C analysis of solutions and dry combustion analysis of clays.t ______Sorption______Dry Treatment______Replic.______Xertex______Combustion mg OC/g clay ----- Ox. clay + A 22.0 18.0 Mollisol HA B 8.6 12.5 C 8.0 12.7 D 8.4 13.3 E 5.2 11.3 F 5.4 12.0 Ox. clay + A 26.7 18.1 Alfisol HA B 14.0 16.2 C 9.8 13.7 D 11.3 13.9 E 7.5 16.6 F 6.5 11.2 Ox. clay + A 4.1 7.1 Histisol FA B 1.9 8.1 "•■Sorption as determined by differences in DOC contents of solutions (Xertex) and in C content of clays (dry combustion) before and after sorption. ^Sorption study employed a clay:HA or FA ratio of 20:1 and a pH of 6.0. replicate at different dilution ratios, a new C standard each day, baseline drift corrections, and fresh persulfate eluent. None of these resulted in more consistent results It was noted that all washing solutions containing unsorbed humic and fulvic acids precipitated a few hours to a few days after termination of the sorptions. The precipitation was rapid enough to yield a clear colored supernatant within 24 hrs. Calcium and Mg were suspected of causing some precipitation of humic and fulvic acids during the sorptions. Calcium and Mg ion concentrations to analysis, the samples were treated with small portions of 3 0% H2O2 to remove humic substances. The quantities of Ca and Mg ions in humic and fulvic acid stock solutions were negligible; however, the Ca and Mg ion concentrations of washing solutions did vary. Linear regression analyses (Figure 44) indicated that sorption was dependent on the concentration of Ca and Mg ions in the washing solutions. The coefficients of determinations of the linear regression equations between sorption amounts and Mol of Ca and Mg ions is presented in Table 25. With one exception, there is a good correlation between Ca and Mg concentration and sorption. This suggests that sorption as determined by the Xertex C analyzer is influenced by the Ca and Mg concentrations in the washing solutions. As Ca and Mg concentration increase in the washing solutions, sorption also increases. In order to determine if the actual sorption or analytical determination of sorption was affected by variations in Ca and Mg concentrations, an alternate analytical method was employed— direct OC analysis of clays before and after sorption. The results of the latter 222 Y = 0.085X+ 7.8 r2 = 0.694 O 30 O) li w O 5 20 - X O) E 10 - 60 120 180240 300 360 uMol Ca+Mg Figure 44. Regression plot of sorption vs yMol of Ca + Mg ions in washing solutions for the clay + Alfisol HA sample. Table 25. Regression coefficient of determinations of sorption'*' vs yMol of Ca + Mg ions in washing solutions for several samples. Ratio Clay: Prediction Coeff. Samoles HA or FA ecruation d e t . — % — Clay + Mollisol HA 20:1 Y = 0 .062(X)+7.64 0.523 Clay + Mollisol HA 10:1 Y=0.123(X)—6.34 0.942 Clay + Alfisol HA 20:1 Y=0.085(X)+7.84 0.694 Clay + Histisol FA 20:1 Y=0.032(X)+0.76 0.970 ■'"Sorption as determined by Xertex C analysis. 223 analyses (Table 24) showed that sorption of humic substances was consistent among replicates and that sorption determined in this manner was higher than that determined using the Xertec C analyzer. Thus it was concluded that sorption was reproducible but that the sorption must be quantitified using dry combustion of the clays. 5.4.2 Sorption of Humic Substances Preliminary sorption studies of humic and fulvic acids by the Xenia Bt2 horizon clay fraction were conducted to determine the optimum pH, saturating cation, and clay:HA or FA ratio for maximum sorption. Humic and fulvic acids from a Carlisle muck instead of humic and fulvic acids from the pedons under investigation were used because of the low supply of the latter. The sorption of humic and fulvic acids as a function of pH and saturating cation are presented in Table 26. Although there seems to be little affect of pH on sorption of fulvic acids, sorption of humic substances is greatest at pH 6.0. Evans and Russell (1959) reported that sorption of both humic and fulvic acids by nonexpanding and expanding clay minerals was influenced by pH. Sorption amounts were strongly influenced by the saturating cations. More sorption of humic and fulvic acids occurs when the clays are Ca- than Na-saturated. Little to no fulvic acids are sorbed into Na-saturated 224 Table 26. Sorption+ of Carlisle humic (HA) and fulvic (FA) acids by oxidized Na- or Ca-saturated Xeniji Bt2 horizon clay fraction as a function of pH.* Clavs OH S o m t i o n mg OC/g clay Na-sat'd 6 5.0 Clay + HA 7 6.0 8 3.9 Ca-sat1d 6 18.9 Clay + HA 7 12.8 8 14.7 Na-sat'd 4 0.6 Clay + FA 5 0.0 6 0.0 Ca-sat'd 4 6.8 Clay + FA 5 6.1 6 6.7 Sorption as determined by difference in C content of clay before and after sorption. +Ratio of clay:HA or FA was 20:1 in all treatments. clays. Greater sorption by Ca- than Na-saturated clays is consistent with reports of Evans and Russell (1959) and Theng and Scharpenseel (1975). The increased sorption of humic substances by Ca-saturated relative to Na-saturated total clays is probably due to the formation of a clay-Ca bridge between the functional groups of humic polymers and clay surfaces. The sorption of humic and fulvic acids by Na-saturated clays is minimal because of low ionic potential and large hydration radius of the Na ions (Theng and Scharpenseel, 1975). It is interesting to note that more humic than fulvic acids are sorbed. Higher sorption 225 amounts of humic acids than fulvic acids by Ca-saturated total clays may be due to (1) humic acids possessing more acidic functional groups, or (2) fulvic acids having a confirmation unfavorable for forming the clay-cation bridge. Because maximum sorption of humic and fulvic acids occurred when the total clays were Ca-saturated, and since maximum sorption of humic acids occurred at pH 6, all subsequent sorptions were conducted with Ca-saturated total clays and at pH 6. The influence of clay:HA or FA ratios on sorption by Ca-saturated clays at pH 6 is presented in Table 27. The amount of humic and fulvic acids sorbed seems to be independent of the amount with which it is equilibrated. Consequently, the 20:1 clay:HA or FA ratio was arbitrarily selected for subsequent sorption experiments. Humic acid sorption by sesquioxides minerals was also found to be independent of the concentration of humic acids (Evans and Russell, 1959). In most cases, 11 to 13 mg C were sorbed from humic acid solutions by one g of clay and 6.5 to 8.5 mg C from the fulvic acids solutions. These data suggest that the clay fraction has a given capacity to sorb humic substances; and that capacity is attained irrespective of the amount with which it is equilibrated. Similar amounts of Mollisol and Alfisol humic acids were sorbed. This condition may be explained if selective absorption of certain organic structures occurred. This is not 226 Table 27. Sorption^ of humic (HA) and fulvic (FA) acids by oxidized Ca-saturated Xenia Bt2 horizon clay as a function of the clay:HA or FA ratios. Humic Ratio Ratio Substances ReD. 20:1 10:1 5:1 20:1 10:1 5:1 — mg OC/g iclay — % sorbed Mollisol HA A 13.0 46 B 11.3 13.1 12.7 40 24 11 C 12.0 29.6 13.0 42 46 9 Alfisol HA A 13.7 ------50 ------B 16.7 11.1 14.9 59 20 14 C 11.2 16.8 13.2 40 29 13 Histisol FA A 8.5 7.2 --- 27 17 --- B 7.7 6.9 --- 29 19 --- ^Sorption as determined by dry combustion analysis by measuring the difference in C content of clays before and after sorption. surprising because the Alfisols and Mollisol humic acids were shown to be similar with 13C NMR. The sorption of humic acids is greater than fulvic acids which is in agreement with that found by Evans and Russell (1959). More humic acid than fulvic acid sorption also was found in the preliminary sorption studies. The percent of the humic and fulvic acid sorbed by the clay fraction increases as the amount of humic components equilibrated with the clay fraction decreases. Direct comparisons of amounts of humic substances sorbed by the Xenia clay fraction with that reported in the literature must be interpretated with caution because most of these studies have used pure mineral and different system pHs. Similar humic acid sorption amounts were reported by Evans and Russell (1959) who equilibrated K-, H-, and Ca-saturated kaolinite with humic acids and by Rashid et al. (1972) who equilibrated humic acids with the less than 4 Pm size fraction of chlorite and illite at pH 3.1. However, the humic acid sorptions are much less than that reported by Evans and Russell (1959) who equilibrated humic acid with either H-, K-, or Ca-saturated bentonite clays. On the other hand, fulvic acids sorption by Xenia clay fraction is less than that reported by Evans and Russell (1959) who equilibrated fulvic acids with either H-, K-, or Ca-saturated bentonite and kaolinite clays and by Kodama and Schnitzer (1974) who equilibrated fulvic acids with kaolinite at pH 3. As previously mentioned, the preliminary humic component sorption studies, just discussed, were conducted to determine the optimum pH, saturating cation, and clay:HA or FA ratio for maximum sorption. These conditions (at pH of 6.0, with Ca-saturated clays, and a 20:1 clay:HA or FA ratio) were then used to prepare the clay with sorbed humic components for the dissolution studies. In Table 28, data for the sorption of humic and fulvic acids by the Xenia Bt2 horizon clay fraction used in the dissolution studies are presented. Generally, sorption of the humic and fulvic acids by clays used were consistent with the preliminary 228 Table 28. Sorption'*' of humic (HA) and fulvic (FA) acids by oxidized Ca-saturated Xenia Bt2 horizon clay fractions used in the 0.1 and 0.005N H2SO4 dissolution studies.* Humic Sorption Substances mcr OC/a clav % sorbed 0.IN H 2SO4 Mollisol HA 16.4 58 Mollisol HA (10:1) 13.1 24 Alfisol HA 18.1 63 Mollisol FA 5.3§ —— 0.005N H ?SO>] Mollisol HA 12.6 44 Alfisol HA 16.2 58 Histisol FA 7.8 32 4* t t Sorption amounts as determined by dry combustion analysis by measuring the difference in C content of clay fraction before and after sorption. ^Sorption experiment was preformed with all HA and FA solutions at pH 6 and a clay:HA or FA ratio of 20:1, except a Mollisol which was at 10:1. ^Sorption amounts determined by Xertex C analyzer. sorption studies, although there may be slightly greater sorption of humic acids (Table 27). The amount of Mollisol fulvic acids sorbed was estimated by the Xertex C analyzer because of sample spillage during the dry combustion analysis. 5.4.3 Clay Dissolution in Dilute Sulfuric Acid During the review of the literature, it became evident that humic substances generally were found to facilitate weathering in most studies, however most of these studies involved primary minerals. As revealed in the literature and in the sorption studies just discussed, clay minerals sorb humic substances. In this study, it is hypothesized that sorbed humic substances will, in fact, retard clay mineral dissolution. To test this hypothesis, clays treated with H2O2 to remove indigenous organic matter, clays with indigenous organic matter, and clays which had been treated with H2O2 to remove indigenous organic matter and then equilibrated with various humic substances in order to promote these sorptions, were all subjected to weathering in dilute acid. The sorbed humic substances included both humic and fulvic acids as well as two different humic acids: one extracted from a Mollisol and the other extracted from an Alfisol. The latter was included to determine if any differences in mineral weathering could be expected between Alfisols and Mollisols which could be attributed to the role of humic acids. A clay equilibrated with twice the concentration of humic acid (noted as Mollisol 10:1) was also included because it was thought at the time the dissolution study was set up that this clay would sorb more humic acid, but this turned out to be incorrect. This system was therefore not included when the second (0.005N H2SO4) study was set up. 230 A. Clav Dissolution in 0.1N H2SO4 The quantities of Mg, K, Al, Si and Fe released by the clays in the dissolution studies is presented in Figures 45 to 49, respectively. An equilibrium state with respect to Mg, K, Al, Si, and Fe was generally attained between 400 and 600 hrs. The time for attainment of equilibrium is in agreement with that reported by Kodama et al. (1983). Minor fluctuations in ion concentrations following attainment of equilibrium is believed to be due to analytical variations. The decrease in Si concentrations after 1000 hrs may be due to Si precipitating as a gel or other siliceous solid phase (Siegel and Pfannkuch, 1984). The quantity of cations released expressed as mole ratios, percentage of total cation released, and mMol/100 g clay in the 0.1N H2SO4 dissolution study are presented in Table 29. Overall, Mg is released in the highest amounts, followed by K, Al, and then Si and Fe. Fifty-six to 67% of the total Mg is released by the clays after 1300 hrs incubation in 0.1N H2SO4. Because Mg occurs mainly in the octrahederal layer, the high release of Mg suggests that there is preferential dissolution of the octrahederal layer. Preferential dissolution of octrahederal layers is in agreement with reports by Brindley and Youell (1951). The high release of Mg relative to the other cations in the octrahederal layer (Al and Fe) also suggests that there is selective dissolution of Mg. Similar observations were mMol Mg/kg clay Legend 160 — Oxidized clay f - I Untreated clay 120 • Clay + Moll. HA - A - Clay +Moll. HA(10:1) Clay + Alt. HA Clay + Moll. FA 0 200 400 600 800 1000 1200 1400 Incubation Hours Figure 45. Plot of total mMol Mg/kg clay released from Xenia Bt2 horizon clay fraction after incubation in 0.IN f^SO^ at various incubation hrs. mMol K/kg clay Legend Oxidized clay —0— Untreated clay Clay + Moll. HA - A - Clay +Moll. HA(10:1) Clay + Alt. HA Clay + Moll. FA 0 200 400 600 800 1000 1200 1400 Incubation Hours Figure 46. Plot of total mMol K/kg clay released from Xenia Bt2 horizon clay fraction after incubation in 0.1N H 2 SO 4 at various incubation hrs. ! mMol Al/kg clay Legend 30 ------Oxidized clay “ 0 — Untreated clay 20 Clay + Moll. HA - A - Clay +Moll. HA(10:1) -■ # - Clay + Alf. HA 10 Clay + Moll. FA 0 0 200 400 600 800 1000 1200 1400 Incubation Hours Figure 47. Plot of total mMol Al/kg clay released from Xenia Bt2 horizon clay fraction after incubation in 0.1N H 2 SO 4 at various incubation hrs. 233 1■> mMol Si/kg clay Legend — ■ Oxidized clay 16 Untreated clay 12 Clay + Moll. HA - A - Clay +Moll. HA(10:1) 8 Clay + Alt. HA Clay + Moll. FA 4 0 200 400 600 800 1000 1200 1400 Incubation Hours Figure 48. Plot of .total mMol Si/kg clay released from Xenia Bt2 horizon clay fraction after incubation in 0.1N H 2 SO4 at various incubation hrs. 234 ! mMol Fe/kg clay Legend Oxidized clay --0 “ Untreated Clay 12 Clay + Moll. HA - A - Clay -4-Moll. HA(10:1) ■ V -•••- Clay + Alt. HA 6 Clay + Moll. FA 0 0 200 400 600 800 1000 1200 1400 Incubation Hours Figure 49. Plot of total mMol Fe/kg clay released from Xenia Bt2 horizon clay fraction after incubation in 0.1N H 2 SO 4 at various incubation hrs. 235 I Table 29. Cations released expressed as mole ratios', percentage of total cation releasedt and mMol/100 g clay after incubation in 0.1N H2S04.§ Cations released bv clavs Clavs Ma K Al Si Fe Ma K Fe Ma K Al Si Fe — moles rat]LOS --- —— — — — — % — — — — — mMol/100 g clay — — — Oxidized 5.2 1.0 0.8 0.4 0.5 65.3 5.9 0.8 14.7 2.8 2.3 1.2 1.4 Untreated 5.3 1.0 0.8 0.4 0.5 66.9 5.9 0.8 15.1 2.8 2.3 1.2 1.3 Ox. clay + 4.1 1.0 0.7 0.4 0.4 56.3 6.4 0.6 12.7 3.1 2.2 1.2 1.2 Mollisol HA Ox. clay + 4.7 1.0 0.8 0.4 0.4 59.6 6.0 0.7 13.5 2.9 2.2 1.2 1.2 Mollisol HA at 10:1 Ox. clay + 4.8 1.0 0.8 0.5 0.4 56.1 5.6 0.7 12.6 2.7 2.2 1.3 1.2 Alfisol HA Ox. clay + 5.0 1.0 0.9 0.4 0.5 61.8 5.8 0.8 14.0 2.8 2.4 1.2 1.4 Mollisol FA + Mole ratios arbitrarily based on K. fPercent of the total wt loss calculated by: ma of cation in solution x 100. total mg in clay § All values are averages of two replicates. to U> OS made by Barshad (1960a, 1960b), Bar-On and Shainberg (1970), and Feigenbaum and Shainberg (1975). Preferential release of Mg from silicate clays has been attributed to isomorphous substitution of Mg and Fe for Al in the octrahederal sheet which weakens the clay structure and the Mg-0 and Fe-0 bonds are weaker than the Al-0 bonds and are preferentially attacked by protons (Shainberg et al., 1974). Less than 7% of the total K, which resides in the interlayer positions, is released. Similar K releases have been reported by Feigenbaum and Shainberg (1975) for the dissolution of Fithian illite in acidified dilute salt solutions. The quantities of Al and Fe released in this study are much lower than reported by Feigenbaum and Shainberg (1975). The low release of Si by the clays suggests that dissolution of the tetrahederal layer is minimal. No differences in the amounts of most cations released could be detected among the various clays used in the 0.IN H2SO4 dissolution study. One explanation may be that the acid strength is so strong that any differences among clays attributed to organic matter are masked by the intense weathering. This possibility is supported by the extent of clay dissolution as shown in Table 30. Twenty-one to 24% of the clay is dissolved after 1320 hrs incubation in 0.1N H2SO4. It is anticipated that the influence of the humic 238 Table 30. Percentage of Xenia clay fraction dissolved after 1320 hrs incubation in 0.1N H2SO4. Clay Clavs Dissolved ---% ---- Oxidized 22.5 Untreated 23.7 Ox. clay + 21.2 Mollisol HA Ox. clay + 21.8 Mollisol HA (1 0 :1) Ox. clay + 22.9 Alfisol HA Ox. clay + 23.4 Mollisol FA ^ All values are averages of replicates. substance on clay weathering is probably quite small in comparison with clay destruction of such a magnitude. However, the Mg data (Table 29) does possibly suggest that clays with sorbed humic acids release somewhat less Mg (56 to 60% for clays with sorbed humic acids as compared to 61 to 67% for the other clays). This would tend to support the hypothesis that sorbed humic substances inhibit the weathering of clay minerals. The pH values for each treatment at the time of sampling is presented in Figure 50. The pH of the dialyzate remained constant throughout the entire study. The 0.1N H2SO4 system is well buffered. ! pH Legend 2.4 Oxidized clay Untreated clay 2 h Clay + Moll. HA - A - Clay +Moll. HA(10:1) Clay + Alf. HA Clay + Moll. FA - A r - Control 0 200 400 600 800 1000 1200 1400 Incubation Hours Figure 50. pH of dialyzate liquite for each treatment from 0.1N H2SO4 dissolution study. 239 240 B. Clav dissolution in 0.005N H 2 SO 4 In the previous dissolution study, it was concluded that the concentration of H2SO4 employed masked any influence of indigenous and sorbed humic substances on clay weathering as measured by the release of cations. Therefore, a second dissolution study was initiated in which the H2SO4 acid strength was reduced to 0.005N. In order to expedite the analysis, only Mg and K concentra tions were monitored since they were found to be the primary cations released in the first study. Due to a lack of sufficient fulvic acids extracted from the Mollisols, Histisol fulvic acids were substituted. Preliminary studies with 0.005N H2SO4 encountered problems with fungal growth on the cellulose dialysis bags which destroyed the integrity of the bags. The introduction of fungicide proved to be ineffective. To overcome this problem, dialysis bags which served as as a cellulose food source for the fungi were eliminated. The clays were incubated directly in acid in 250 ml centrifuge tubes. Because the clays readily flocculated in the 0.005N H2SO4, aliquots of the solution could easily be collected without any loss of clay by centrifugation prior to sampling. The quantities of Mg and K ions released by the clays during incubation in 0.005N H2SO4 are shown in Figures 51 and 52, respectively. The most striking result is that clay treated with peroxide to remove indigenous organic I mMol Mg/kg clay Legend 60 — Oxidized clay — Clay + Moll. HA 40 Clay + Alt. HA 30 Clay + Hist. FA 20 10 0 0 200 400 600 800 1000 1200 Incubation Hours Figure 51. Plot of total mMol Mg/kg clay released from Xenia Bt2 horizon clay fraction after incubation in 0.005N H 2 SO4 at various incubation hrs. 241 I mMol K/kg clay Legend 8 ----- — Oxidized clay Untreated clay 6 Clay + Moll. HA Clay + Alt. HA 4 Clay + Hist. FA 2 0 0 200 400 600 800 1000 1200 Incubation Hours Figure 52. Plot of total mMol K/kg clay released from Xenia Bt2 horizon clay fraction after incubation in 0.005N H 2 SO4 at various incubation hrs. 242 243 matter released much more Mg than any of the other clays with indigenous or sorbed humic substances. Similar amounts of K are released from all clays. All clays reached equilibrium with respect to Mg at about 400 hrs, which is similar to that observed in the 0.1N study. After 400 hrs of incubation, the Mg concentration decline slowly with time for the oxidized clay. For all clays, the maximum amount of K was attained at 42 hrs and remained constant thereafter. The quantities of Mg and K expressed as mole ratios, % of the total cation released, and as mMol/100 g clay released from all clays during incubation in 0.005N H2SO4 are presented in Table 31. There is approximately 3 times more Mg released from oxidized clay than from clays with indigenous or sorbed humic substances. Similar quantities of Mg are dissolved from clays with indigenous or sorbed humic substances. These data suggest that clay dissolution, as measured by Mg release, is reduced by the presence of either indigenous or sorbed humic substances. Two possible mechanisms may explain reduced Mg release from clays with indigenous or sorbed humic substances. First, the functional groups of the humic substances may act as a buffer by accepting protons. In this way, the protons available to attack the clay lattice are reduced. Second, the indigenous or sorbed humic substances are primarily attached to the clay surfaces through a cation 244 Table 31. Cations released expressed gs mole ratios^, percentage of total cations*, and mMol/100 g total clay for all clays after incubation in 0.005N H2S04.§ Cations released bv clavs Clavs Ma K M ct K M ct K mole ratios — % — mMol/100 g clay Oxidized 10.2 1.0 18.1 2.5 4.0 0.3 Untreated 2.6 1.0 5.9 2.6 1.3 0.4 Ox. clay + 2.0 1.0 5.5 3.1 1.2 0.5 Mollisol HA Ox. clay + 2.4 1.0 5.3 2.7 1.2 0.4 Alfisol HA Ox. clay + 3.0 1.0 5.3 2.3 1.2 0.3 Histisol FA + iMole ratios arbitrarily based on K. ^Percent of the total cation in clay, calculated by: ma of cation in solution x 100 total mg in clay § All values are averages of two replicates. bridge. The sorbed organic coating may act as a physical barrier which restricts the movement of protons and other cations. It is possible that both of these mechanisms may contribute to the decrease in cations dissolved from the clays by acid attack. The quantities of K released from the clays is quite low, 0.3-0.5 mMol per 100 g clay. There is 2 to 13 times more Mg released than K. The small quantities of K 245 released may be a function of acid strength. Approx imately 10 times more K and Mg are released by clays in 0.1N than in 0.005N H2SO4. The protons in the more dilute acid may not be able to diffuse into the interlayers to release K ions which results in low concentrations of K ions in the external solution. Similar results were reported by Feigenbaum and Shainberg (1975) who observed an increase in K ion release from Fithian illite with an increase in acid strength. Overall, the quantities of Mg and K released in the 0.005N H2SO4 study are lower than that reported by Feigenbaum and Shainberg (1975) who incubated Fithian illite in 0.001N HCl for 840 hrs. The percentage of the clays dissolved in 0.005N H2SO4 is presented in Table 32. The dissolution of clays with sorbed humic substances was notably less than that of clays with indigenous organic matter or those treated with peroxide to remove organic matter. Clays with sorbed humic substances have less than 1% clay dissolved. In addition, more clay with indigenous humic substances dissolved than those with sorbed humic substances. This suggests that clays with sorbed humic substances are less weathered than clays with indigenous organic matter. However, clays with sorbed humic substances or organic matter have similar quantities of Mg and K released. The pH of the dialyzates at the time of sampling is presented in Figure 53. The pH increased in all of the 246 Table 32. Weight percentage of clay dissolved after 1176 hrs incubation in 0.005N H2SO4. Total clay Clavs dissolved^- Oxidized 5.8 Untreated 2.6 Ox. clay + 0.5 Mollisol HA Ox. clay + 0.2 Alfisol HA Ox. clay + 0.5 Histisol FA ^Average values of two replicates. dialyzates as a function of time. The increase in pH in all treatments is probably due to the release of base cations and sorption of protons by the clays. The largest pH increase occurred with the clays treated with peroxide to remove organic matter, followed by clays with sorbed humic and fulvic acids, then clays with indigenous organic matter. Assuming that the increase in pH is a reflection of the amount of cations dissolved, this suggests that indigenous humic substances may be more effective than sorbed humic substances in protecting the clays from weathering. This conclusion is contrary to our conclusion based on weight loss of clays just discussed. This finding is a minor point because the assumption was not verified, ! pH Legend — Oxidized Clay --0 " Untreated Clay Clay + Moll. HA Clay + Alf. HA Clay + Hist. FA -e-er - - A r - Control 200 400 600 800 1000 1200 Incubation Hours Figure 53. pH of dialyzate liquid for each treatment from 0.005N H2SO4 dissolution study. 247 248 however, the clay loss was smaller in clays with sorbed humic substances than clay with indigenous organic matter. 5.5 Summary Sorption of fulvic acids by clays was independent of pH in the range 4 to 6, whereas humic acid sorption was greatest at pH 6. Both were also independent of the ratio of clay:HA or FA, but were influenced by the type of saturating cation. Ca-saturated clays sorbed more humic and fulvic acids than the Na-saturated clays. The sorption of humic acids was greater than the sorption of fulvic acids by Ca-saturated clays at pH 6. Ca-saturated clays with and without indigenous organic matter and with sorbed humic acids were weathered in 0.IN and 0.005N H2SO4. In both dissolution studies, all clays attained equilibrium with respect to Mg, K, Al, Si, and Fe release between 400 and 600 hrs. In both studies, Mg was preferentially released by the clays. Assuming that Mg occurs mainly in the octrahederal layer, the high release of Mg suggests that there was preferential dissolution of the octrahederal layers. No differences were found with respect to Mg, K, Al, Si, and Fe released from the different clays in the 0.1N H2SO4 study. Approximately, 21 to 24% of the clays were dissolved in the 0.1N H2SO4 study. These data suggests that the 0.1N H2SO4 may have been so strong that any influence of indigenous or sorbed humic 249 substances on cation release may have been masked. By using a less concentrated acid (0.005N H2SO4), differences were noted in degree of weathering among the clays. Approximately 3 times more Mg was dissolved from clay treated with peroxide to remove organic matter than clays with either indigenous or sorbed humic substances. Similar amounts of K were released by all clays. Using the 0.005N H2SO4 more clays treated with peroxide were dissolved than clays with indigenous or sorbed humic substances. Data for of Mg release and clay dissolution in 0.005N H2SO4 both suggest that weathering is reduced by the presence of indigenous or sorbed humic substances. Although no differences were evident in Mg and K release between clays with indigenous or sorbed humic substances, more clays were dissolved with indigenous than sorbed humic substances. Magnesium and K release from clays was found to be a function of acid strength. Approximately, 10 times more Mg and K were released from clays incubated in 0.1N than in 0.005N H2S04. In conclusion, there was no evidence of any influence of indigenous or sorbed humic substances in protecting the clays from mineral dissolution in 0.1N H2SO4. However, when the dissolution was conducted in 0.005N H2SO4, indigenous and sorbed humic substances did reduce the rate of Mg release and clay dissolution. No weathering differences could be detected between clays with sorbed 250 humic vs fulvic acids or between sources (Mollisol vs Alfisol) of humic substances. CHAPTER VI SUMMARY AND CONCLUSIONS In southwestern Ohio, moderately-well drained Mollisols and Alfisols occur in close proximity in many landscapes. These soils share similar landscape positions and parent materials but differ in organic matter content of surface horizons with Mollisols having higher amounts. A pair of contiguous Mollisols and Alfisols on the Wisconsinan (Dana and Xenia) and Illinoian (Dana Variant and Rossmoyne) surfaces of Warren County, Ohio were studied in order to investigate possible chemical and structural differences of the organic matter and to evaluate the role of humic substances in the weathering of soil clays. Specific objectives were (1) to characterize alkali extracted humic and fulvic acids, (2) to characterize concentrated soil organic matter, and (3) to examine the role of humic and fulvic acids in the weathering of soil clays. The Dana and Xenia series are deep, moderately-well drained soils formed in Peoria Loess and the underlying calcareous Wisconsinan glacial till. The Dana and Xenia pedons contain 2.49 and 1.48% organic carbon, respectively, in the surface horizons. Both pedons have a silt loam 251 252 epipedon and clay loam, silty clay loam, and loam textures in the underlying horizons. The Dana and Xenia pedons have a solum thickness of 96 and 89 cm, respectively, and abun dant clay films on ped faces in the B horizons. The Rossmoyne and Dana Variant pedons consist of deep, moderately-well drained soils formed in a mantle of Peoria Loess and underlying Illinoian glacial till. The Dana Variant and the Rossmoyne pedons contain 1.96 and 1.10% organic carbon, respectively, in the surface horizons. Both pedons have a silt loam epipedon and silty clay loam and silty clay textures in the B horizons. The Dana Variant and Rossmoyne pedons have a solum thickness of 152 and 188 cm, respectively. All pedons have a near neutral pH throughout the solum, except the Dana and Xenia surface horizons which are fairly acidic. Particle size analysis indicated that the upper hori zons of the Alfisols (Xenia and Rossmoyne) contain less fine and total clay contents than the associated Mollisols (Dana and Dana Variant). The clay contents in the argillic horizons of the Alfisols are, however, similar to the clay contents in the argillic horizons of the Mollisols. It is speculated that the higher fine and total clay contents in the surface horizons of Mollisols than Alfisols may be related to higher organic matter content; however because the influence of organic matter on particle size distribution was not an objective of this study; no attempt has been made to test this hypothesis. The clay mineralogy of the horizons derived from loess was dominated by vermiculite, smectite, and clay mica with low amounts of kaolinite. The clay mineralogy of both tills is dominated by clay mica and vermiculite with low amounts of smectite and kaolinite. There is higher smectite (includes any com ponent expandable to greater than 1.4 nm with glycerol) content and less Al-interlayering in the smectite and vermiculite clays in the Mollisol surficial horizons than in those of the Alfisols. The higher smectite content in Mollisols may also be related to the higher organic matter of these soils. It is speculated that the low smectite of the Alfisols may be due to increased rates of mineral weathering attributable to lower organic matter contents. Soils on the Wisconsinan surface contain more Al- interlayered vermiculite than soils on the Illinoian surface. The organic matter in the surface horizons of all four pedons was (1) extracted with 0.1N NaOH and separated into humic and fulvic acids and (2) concentrated in a clay size fraction using sonification and sedimentation techniques. In the former, more humic substances were extracted from the Mollisols than the Alfisols with the majority being humic acids. Only minimal quantities of fulvic acids were extracted from all four pedons. Wet chemical, infrared and 13c CPMAS NMR spectroscopy showed that chemical and structural properties of humic acids from the Alfisols and Mollisols differed somewhat. Although, both Alfisol and Mollisol humic acids were dominated by alkyl substituted aromatic structures, followed by aliphatic, carbohydrate, and carboxylic acids groups and minor amounts of aldehyde and ketone groups, 13C NMR showed that Mollisol humic acids contain a higher aromatic content than those of the Alfisols. The low aromatic character of the Alfisol humic acids was supported by the wide H/C ratios and higher E4/E6 absorption ratios than Mollisol humic acids. Phenolic-OH groups were detected in Alfisol humic acids using 13C NMR; however, none was detected in Mollisol humic acids. 13C NMR also showed that humic acids from Alfisols contain more lignin and lignin-like products than Mollisol humic acids. Fulvic acids were found to contain mostly aliphatic, carbohydrate, and carboxylic acid structures, followed by aromatic groups, and minor amounts of aldehyde and ketone groups. The aromatic structures of the fulvic acids are largely alkyl substituted with minor amounts of phenolic-OH structures. The proportions of nonaromatic and aromatic structures is similar for Alfisol and Mollisol fulvic acids, however fulvic acids from soils on the Wisconsinan surface are higher in carbohydrates than those from soils on the Illinoian surface. Organic matter content was concentrated 2 to 4 times the organic matter content of whole soil using sonification and sedimentation techniques to isolate a fine fraction. The organic carbon enriched fine fraction was 77 to 89% clay of which approximately 50% was less than 0.2 Mm. 13C NMR spectra of the organic carbon enriched fine fraction are of low quality due to a low signal to noise ratio and a low intensity; however, peaks indicative of aliphatic, carbohydrate and carboxylic acid structures are evident for Dana Variant and Xenia. Spectra for Dana is similar except there is little evidence for aliphatic structures. There is little evidence of aromatic structures in the organic matter of the Dana Variant pedon, but some evidence exists for aromatic structures in the organic matter of the Dana and Xenia pedons. Some evidence for carbonyl character exists in the fine fraction of all three samples. Spectral resolution of the fine fraction was not enhanced significantly with an increase in organic carbon content. The proportion of aromatic and nonaromatic structures is similar for the Dana and Xenia organic matter enriched fine fraction, however little evidence of aromatic character exists in the Dana Variant fine fraction. To evaluate the role of humic substances in clay mineral weathering, humic and fulvic acids from Alfisols, Mollisols and Histisols were sorbed to the clay (< 2 ym) size fraction of Xenia Bt2 horizon and used in a 0.1N and 0.005N H2SO4 dissolution studies. Sorption of fulvic acids by clays was independent of pH in the range 4 to 6, but humic acid sorption was greatest at pH 6. Ca-saturated clays sorbed more humic and fulvic acids than Na-saturated clays. Both humic and fulvic acid sorption was also inde pendent of the clay:HA or FA ratio. Ca-saturated clays with and without indigenous organic matter and with sorbed humic or fulvic acids were weathered in 0.1N and 0.005N H2SO4. In both dissolution studies, all clays attained a steady state with respect to Mg, K, Al, Si, and Fe release between 400 and 600 hrs. In both studies, Mg was preferen tially released by the clays. Assuming that Mg occurs mainly in the octrahederal layer, the high release of Mg suggests that there was preferential dissolution of the octrahederal layers. No differences were found with re spect to Mg, K, Al, Si, and Fe released from the different clays in the 0.1N H2SO4 study. Approximately, 21 to 24% of the clays were dissolved in the 0.1N H2SO4. These data suggest that 0.1N H2SO4 may have been so strong that any influence of indigenous or sorbed humic substances on cation release may have been masked by the intense weather ing. By using a less concentrated acid (0.005N H2SO4), differences were noted in the degree of weathering among the clays. Approximately 3 times more Mg was dissolved from the clay treated with peroxide to remove organic matter than clays with either indigenous or sorbed humic substances. Similar amounts of K (0.3 to 0.5 mMol per 100 g clay) were released by all clays. Using 0.005N 257 H2SO4 , a larger portion (5.8%) of the clay treated with peroxide was dissolved than of clays with indigenous (2 .6%) or sorbed humic substances (< 1%). Data for Mg release and clay dissolution in 0.005N H2SO4 both suggest that weathering is reduced by the presence of indigenous or sorbed humic substances. No weathering differences could be detected between clays with sorbed humic vs fulvic acids or between sources (Mollisol vs Alfisol) of humic substances. Both Mg and K release from clays were found to be a function of acid strength. Approximately, 10 times more Mg and K were released in the 0.1N than the 0.005N H2SO4. In conclusion, the organic matter of Mollisols contain more aromatic structures whereas, organic matter of Alfisols contain more phenolic-OH substances, lignin and lignin-like products than Mollisols. Indigenous and humic substances sorbed to clays which were incubated in 0.005N H2SO4 were found to reduce the amount of Mg released and clay dissolved when compared to clays with organic matter removed. However, no weathering differences could be detected between clays with sorbed humic substances extracted from a Mollisol or an Alfisol. Therefore, there is evidence to suggest that organic matter and/or humic substances retards clay mineral weathering in soils. Thus the lower smectite and higher Al interlayered 2:1 clays present in the Alfisols relative to the Mollisols may be 258 due to more intense weathering as a result of lower organic matter content. APPENDIX A Pedon Description 259 260 Ohio State University Soil Characterization Laboratory SOIL SERIES: DANA [TAXADJUNCT1 COUNTY: WARREN SITE: Wfr-69 PEDON CLASSIFICATION: FINE-SILTY, MIXED, HESIC, TYPIC HAPLUDOLL LOCATION: 0TTERBE1N FARN 1 MI N JNCT OH 63 t 741 300 YDS E OF 741 NN1/4 SEC.24 T.3 R.* PHYSI06RAPHY: ground aoraine ELEVATION: 855 FT. TOPOGRAPHY: nearly level JSLOPE: 1 ASPECT: W DRAINAGE: aoderately well VEGETATION: cultivated field COLLECTORS: SMECK/ NOVAK/ BURRAS/ JAYNES DATE: 3/26/85 PARENT MATERIALS: late wisconsinan (peorian) loess, wisconsinan glacial till, shale and liaestone. HORIZON DEPTH Ap 0-15 centiaeters; 10YR3/2—silt loan; aoderate aediua granular structure; friable; few fine roots; abrupt saooth boundary. A 15-33 centiaeters; 10YR3/1—silt loan; strong aediua angular blocky structure; friable; few fine roots; clear saooth boundary. % 33-43 centiaeters; 10YR2.5/1-silty clay loaa; aoderate aediua subangular blocky parting to strong fine subangular blocky structure; friable; few fine roots; cocaon 10YR4/4 channel fillings in the aatrix; clear saooth boundary. Btl 43-56 centiaeters; 10YR4/4-silty clay loaa; few fine faint 10YR5/6 aottles; weak aediua prisaatic parting to Noderate fine subangular blocky structure; friable; few fine roots; thick continuous 10YR3/1 organic coatings in channels; aediua continuous 10YR3/2 argillans on faces; gradual saooth boundary. Bt2 56-69 centiaeters; 10YR5/4-silty clay loaa; few fine faint 10YR5/6 aottles; weak aediua prisaatic parting to weak aediua subangular blocky structure; friable; few fine roots; few distinct 10YR3/2 organic coatings in channels; aediua patchy 10YR4/2 argillans on faces; gradual saooth boundary. Bt3 69-84 centiaeters; 10YR5/4-silty clay loaa; coaaon fine faint 10YR5/6 aottles; weak aediua subangular blocky structure; friable; few fine roots; thick patchy 10YR4/2 argillans on vertical faces; clear saooth boundary. 2BC 84-97 centiaeters; 2.5Y5/4-silty clay loaM; coaaon fine faint 10YR5/6 aottles; weak aediua subangular blocky structure; fira; few fine roots; thick very patchy 10YR4/2 argillans on vertical faces; 5* coarse fragaents; slight effervescence; clear saooth boundary. 2C 97-122 centiaeters; 2.5Y6/4-loaa; aassive; fira; 15% coarse fragaents; strong effervescence; abrupt saooth boundary. 3Cr 122-152 centiaeters; 2.5Y5/6-silty clay; aassive; very fira; strong effervescence. NOTE: AB has patches of 10YR4/4 froa aixing. 3Cr is interbedded shale and liaestone (Ordovician). Ohio State University Soil Characterization Laboratory 261 SOIL SERIES: XENIA COUNTY: WARREN SITE: WA-70 PEDON CLASSIFICATION: FINE-SILTY, MIXED, MESIC, AQUIC HAPLUDALF LOCATION: 0TTER8EIN FARM 190 YDS SSE OF HA-69 NW 1/4 SEC. 24 T.3 R.4 PHYSIOGRAPHY: ground aoraine ELEVATION: 675 FT T0P06RAPHY: nearly level *SLOPE: 1 ASPECT: NW DRAINAGE: aoderately nell VEGETATION: cultivated field COLLECTORS: NOVAK/ SMECK/ JAYNES DATE: 3/26/85 PARENT MATERIALS: late wisconsinan (peorian) loess, Nisconsinan glacial till. HORIZON DEPTH Ap 0-20 centiaeters; 10YR4/2-silt loaa; weak aediua platy parting to weak fine granular structure; friable; few fine roots; clear saooth boundary. AB 20-36 centiaeters; 10YR4/2-silt loaa; weak aediua platy parting to weak fine subangular blocky structure; friable; few fine roots; coaaon 10YR4/4 channel fillings in the aatrix; clear saooth boundary. Btl 36-46 centiaeters; 10YR4/4~silty clay loan; few fine faint 10YR5/6 aottles; aoderate fine subangular blocky structure; friable; few fine roots; aediua continuous 10YR4/3 argillans on faces; few 10YR4/2 organic coatings in channels; coaaon 10YR2/1 concretions in the aatrix; gradual saooth boundary. Bt2 48-61 centiaeters; 10YR4/4-silty clay loan; few fine faint 10YR5/6 and few fine faint 10YR5/2 aottles; weak aediua prisaatic parting to aoderate aediua subangular blocky structure; friable; few fine roots; aediua continuous 10YR4/3 argillans on faces; coanon 10YR2/1 concretions in the aatrix; gradual saooth boundary. Bt3 61-76 centiaeters; 10YR4/4-silty clay loan;aany aediua faint 10YR5/6 aottles; weak coarse prisaatic parting to weak aediua subangular blocky structure; friable; few fine roots; thin patchy 10YR4/3 argillans on faces; few 10YR2/1 concretions in the aatrix; clear saooth boundary. 2BC 76-69 centiaeters; 10YR4/4-clay loaa; coaaon fine faint 10YR5/6 aottles; weak coarse prisaatic parting to weak coarse subangularblocky structure; friable; few fine roots; thin patchy 10YR4/2 argillans on vertical faces; 8* coarse fragaents; clear saooth boundary. 2C1 89-104 centiaeters; 2.5Y5/4-clay loaa; aassive; fira; thin continuous 5Y5/2 calcans in cleavages; thin continuous 10YR5/6 iron-rich zones subcutaneously; 15* coarse fragaents; strong effervescence; gradual saooth boundary. 2C2 104-124 centiaeters; 2.5Y5/4-loaa; aassive; fira; thin continuous 5Y5/2 calcans in cleavages; thin continuous 10YR5/6 iron-rich zones subcutaneously; 15* coarse fragaents; strong effervescence; gradual saooth boundary. 2C3 124-152 centiaeters; 2.5Y5/4-loaa; aassive; fira; thin continuous 5Y5/2 calcans in cleavages; thin continuous 10YR5/6 iron-rich zones subcutaneously; 15* coarse fragaents; strong effervescence. Ohio State University Soil Characterization Laboratory 2 6 2 SOIL SERIES! ROSSMOYNE COUNTY: WARREN SITE: WA-71 PEDON CLASSIFICATION: FINE-SILTY, NIXED, MESIC, AQU1C FRA6IUDALF LOCATION: 1500 FT E OF WARD-KOBEL RD S 700 FT S OF WILMINSTON RD LISTON BURTON FARM SEC. T. R. PHYSIOGRAPHY: ground aoraine ELEVATION: 955 FT TOPOGRAPHY: nearly level XSUJPE: 3 ASPECT: S DRAINAGE: aoderately well VEGETATION: cultivated field COLLECTORS: SMECK/ NOVAK/ JAYNES DATE: 4/15/85 PARENT MATERIALS: late wisconsinan (peorian) loess, illinoian glacial till. HORIZON DEPTH Apl 0-20 centiaeters; 10YR4/2-silt loaa; weak aediua granular structure; fira; fe* fine roots; clear saooth boundary. Ap2 20-36 centiaeters; 10YR4/2—si It loaa; aoderate aediua granular structure; friable; fe* fine roots; abrupt saooth boundary. Btl 36-51 centiaeters; 10YR4/4-silty clay loaa; fe* fine faint 10YR5/6 aottles; aoderate fine subangular blocky structure; friable; fe* fine roots; thick continuous 10YR5/3 si1tans on faces; thick patchy 10YR4/4 argillans on faces; clear snooth boundary. Bt2 51-69 centiaeters; 10YR4/6-silty clay loaa; fe* nediun faint 10YR5/6 and fe* fine faint 10YR5/2 aottles; weak aedira prisaatic parting to Moderate aediua subangular blocky structure; fira; fe* fine roots; few 10YR2/1 ferro-aangans on faces; thick continuous 10YR5/3 siltans on vertical faces; aediua patchy 10YR4/3 argillans on faces; gradual snooth boundary. Bt3 69-86 centiaeters; 10YR5/6-light silty clay loaa; fe* aediua faint 10YR5/2 aottles; weak coarse prisaatic parting to aoderate aediua subangular blocky structure; friable; fe* fine roots; coaaon 10YR2/1 ferro-aangans on faces; thick continuous 10YR5/3 siltans on vertical faces; aediua patchy 10YR4/4 argillans on faces; clear wavy boundary. 2Btxi 86-112 centiaeters; 10YR5/6-light clay loaa; weak very coarse prisaatic parting to weak coarse subangular blocky structure; fira; few fine roots; thick continuous 2.5Y5/2 argillans on vertical faces; thick continuous 10YR5/3 siltans on vertical faces; coanon 10YR2/1 ferro-aangans on faces; 2% coarse fragaents; gradual saooth boundary. 2Btx2 112-137 centiaeters; 10YR5/6-clay loaa; weak very coarse prisaatic parting to weak coarse subangular blocky structure; fira; few fine roots; thick continuous 2.5Y5/2 argillans on vertical faces; thick continuous 10YR5/3 siltans on vertical faces; c ow on 10YR2/1 ferro-aangans on faces; 2% coarse fragaents; clear saooth boundary. 2Btl 137-155 centiaeters; 10YR5/6-clay loaa; aoderate very coarse prisaatic structure; fira; aediua continuous 10YR5/2 argillans on vertical faces; aany 10YR2/1 ferro-aangans on faces; 2* coarse fragaents; clear saooth boundary. 2Bt2 155-168 centiaeters; 10YR5/3-silty clay loaa; coaaon coarse proainent 7.5YR5/8 aottles; strong very coarse prisaatic parting to aoderate coarse angular blocky structure; very fira; nediun continuous 10YR5/2 argillans on vertical faces. 263 2Bt3 168-188 centiaeters; 10YR4/3—silty clay loan; few fine faint 10YR5/2 mottles; weak aediua subangular blocky structure; friable. NOTEi CORE SAMPLES COLLECTED BELOW 86 CENTIMETERS. HIGH BASE STATUS AND OCCURRENCE OF CARBONATES ATTRIBUTED TO ITS LOCATION ON FOOTSLOPE OF LIMESTONE HI6H(SPRING HILL). 264 Ohio State University Soil Characterization Laboratory SOIL SERIESs DANA VARIANT COUNTY: WARREN SITE: WA-72 PEDON CLASSIFICATION! FINE-SILTY, MIXED, MESIC, TYP1C HAPLUDOLL LOCATION! 400 FT WSW OF SITE WA-71 LISTON BURTON FARM SEC. T. R. PHYSI06RAPHY: ground aoraine ELEVATION: 950 FT TOPOGRAPHY! nearly level KSLOPEi 3 ASPECT! S DRAINAGE! aoderately •tell VEGETATION! cultivated field COLLECTORS! SffiCK/NOVAK/JAYNES DATE: 4/15/85 PARENT MATERIALS: late Nisconsinan (peorian) loess, illinoian glacial till. HORIZON DEPTH Ap 0-25 centiaeters; 10YR3/2-silt loaa; Neak coarse angular blocky structure; fira; few fine roots; 0% coarse fragaents; abrupt saooth boundary. A £5-43 centiaeters; 10YR3/1—silt loan; weak aediua subangular blocky parting to aoderate aediua granular structure; friable; few fine roots; 0% coarse fragaents; clear saooth boundary. AB 43-58 centiaeters; 10YR4/4-silty clay loaa; aoderate aediua subangular blocky structure; friable; f a t fine roots; aediua continuous 10YR4/2 argillans on faces; 0% coarse fragaents; clear Navy boundary. Btl 58-81 centiaeters; 10YR5/4-silty clay loaa; aoderate fine prisaatic parting to aoderate aediua subangular blocky structure; friable; few fine roots; aediua continuous 10YR4/4 argillans on faces; OK coarse fragaents; gradual saooth boundary. Bt£ 81-108 centiaeters; 10YR5/&-silt loaa; aoderate fine prisaatic parting to weak aediua angular blocky structure; friable; fee fine roots; aediua continuous 10YR5/4 argillans on faces; IK coarse fragaents; gradual saooth boundary. 2Bt3 102-122 centiaeters; 10YR5/6-silty clay loaa; Meak aediua prisaatic parting to weak aediua subangular blocky structure; friable; fe* fine roots; thick continuous 10YR5/4 argillans on faces; IK coarse fragaents; gradual saooth boundary. 2Bt4 128-140 centiaeters; 10YR5/6-silty clay loaa; Neak aediua subangular blocky structure; friable; fe* fine roots; thick continuous 10YR5/4 argillans on vertical faces; IK coarse fragaents; clear saooth boundary. 2Bt5 140-152 centiaeters; 10YR5/6-clay loaa; Neak coarse subangular blocky structure; fira; fe* fine roots; thin very patchy I0YR3/4 argillans on faces; 3K coarse fragaents. NOTE: 3RD HORIZON 10YR3/1 T0UN6ED. COULD NOT GO DEEPER THAN 152ca BECAUSE OF HATER IN PIT. THE OCCURRENCE OF A DANA-LIKE PEDON ON THE ILLINOIAN SURFACE MAY BE ATTRIBUTED TO THE LOCAL INFLUENCE OF A NEARBY LIMESTONE HIGHtSPRINB HILL). RECHARGE OF THE PEDON BY LIME RICH HATER MAY ACCOUNT FOR THE HIGH BASE STATUS AND OCCURRENCE OF CARBONATES. APPENDIX B Characterization Data 265 266 Table 33. Characterization data for Dana Taxadjunct silt loam (WA-69). SOIL SERIES! DANA ITAXADJUNCTJ COUNTYi WARREN SITE) HA-69 DATE! 3/26/83 OSU LAB. NUMBERS: 2S33S - 25343 CO. PARTICLESIZE DISTRIBUTION (*(£■>)------DEPTH HORIZON FRAB. ------SAND------SlLT(u»)------CLAY(ua)— TEXT. )£m VC C H F VF TOTAL 30-20 20-5 3-2 TOTAL 2-.2 (.2 TOTAL CLASS Cl ...... --1 0- 13 Bp 0.0 l.l 1.0 0.7 1.3 2.3 6.4 29.7 31.7 7.3 66.7 11.6 13.3 24.9 S1L 15- 33 A 0.0 O.B 0.9 0.7 1.4 2.4 6.2 26.0 30.6 6. 0 66.6 113 13.7 27.2 SICL 33- 43 AB 0.0 1.2 1.0 0.6 1.2 2.3 6.3 21.B 33.1 6.5 63.4 12.B 13.3 213 SICL 43- 56 Btl 0.0 1.7 1.2 0.6 1.1 2.0 '6.6 19.3 37.0 9.3 66.0 16.7 10.7 27.4SICL 36- 69 Bt2 0.0 0.9 0.6 0.4 O.B 2.4 3.3 23.6 36.6 6.6 67.2 110 9.5 27.3 SICL 69- 64 Bt3 0.1 0.9 1.0 0.6 1.7 3.6 6.2 25.0 33.7 7.1 67.6 112 6.S 24.0 SIL 84- 97 2BC 3.1 l.S 2.6 2.3 6.0 3.9 16.6 16.0 27.3 6.6 33.9 17.1 10.2 27.3 SICL 97-122 2C 117 4.4 4.6 17 6.6 7.0 26.3 9.6 24.2 10.3 44.3 17.6 9.6 27.4 CL 122-132 3Cr 0.7 0.6 0.3 0.3 0.7 0.6 2.9 1.6 23.4 17.1 42.3 36.2 16.6 54.6 SIC III .01N 0R6. CAL- DOLO- CARB- ----EXTRACTABLE CATIONS------BASE DEPTH WATER CaC12 C CITE HITE ONATE H C) Kg K Na SUM SAT. ca — Pit % - ■Eq.X— -■eq/lOOg— t 0- IS IB 13 149 13 13.9 4.0 0.62 0.04 219 71 13- 33 6.2 5.6 2.46 ' 7.7 1B.0 2.9 0.36 0.04 29.0 73 33- 43 13 IB 116 7.1 17.7 3.3 0.36 0.06 28.7 73 43- 56 17 13 0.98 16 16.9 3.1 0.41O.OB 28.1 60 56- 69 7.0 6.4 0.42 0.1 1.5 1.7 13 13.1 4.6 0.34 0.07 21.4 63 69- 64 7.2 17 0.30 0.3 1.7 14 3.0 11.6 3.B 0.25 0.06 117 B4 64- 97 7.3 IB 0.27 0.2 I t 16 1.9 11.7 3.6 0.18 0.06 17.4 89 97-122 7.7 7.2 6.0 112 20.3 122-132 7.9 7.4 3.7 4.3 8.4 TIE PARTICLE SIZE FAMILY CONTROL SECTION HAS CONSIDERED TO BE BETWEEN 23 AND 102 CENTIMETERS. 1E1EHTED BVERABEi 73-. 1HI WHOLE SOIL 7.8 12 MICRON FINE EARTH 26. B 267 Table 34. Characterization data for Xenia silt loam (WA- 70) . SOIL SERIES: XEHIB COUNTY: WARREN SITE: MA-70 DATE: 3/26/85 OSU LAS. NUMBERS: £5544 - 25552 CO. PARTICLESIZE DISTRIBUTION (*(&■>------DEPTH HORIZON FRAG. ------SIM)------SILTIu*) —CUtYCua)— TEXT. >2»i VC C H F VF TOTAL 50-20 20-5 5-2 TOTAL 2-.2 1.2 TOTAL CLASS C l ------*■ 0- 20 Bp 0.0 O.B 1.7 1.8 2 . 2 3.1 9.6 25.1 40.7 8.6 74.4 12.2 3.8 16.0 SIL 20- 36 AB 0.2 1.6 2.0 1.4 2 . 2 3.0 10.2 24.4 38.4 9.2 72.0 11.9 5.9 17.8 S1L 36- 48 Btl 0.0 1.4 1.5 0.9 1.6 2.9 8.3 19.1 35.0 7.6 61.7 19.0 11.0 30.0 SICL 48- 61 Bt2 0.0 0.8 1.2 0.8 1.8 3.6 8.2 22.0 32.9 7.7 62.6 19.110.1 29.2 SICL 61- 76 Bt3 0.2 1.1 1.5 1.5 3.4 4.8 12.3 24.4 31.5 7.3 63.2 16.0 8.5 24.5 SIL 76- B9 ESC 4.7 3.1 5.5 5.6 12.8 8.1 35.1 10.7 19.4 7.9 38.0 17.69.3 26.9 L 89-104 2CI 43.5 6.1 5.6 4.2 9.7 7.6 33.2 10.5 21.3 10.6 42.4 16.5 7.9 24.4 L 104-124 2C2 22.2 5.3 5.7 4.0 8.9 7.0 30.9 13.5 21.8 10.5 45.B 16.0 7.3 23.3 L 124-152 2C3 24.2 4.4 5.1 3.8 8.67.2 29.1 11.9 22.8 11.4 46.1 17.6 7.2 24.8 L III .om ORS. CAL- DOLO- CARB------EXTRACTABLE CATIONS------BASE DEPTH HATER CaC12 C-. CITE HITE ONATE H Cl Mg K Na SUN SAT. Cl ---- P»— t Eq.F— nq/lOOg— t 0- 20 5.9 5.4 1.48 5.6 '8.4 2.7 0.45 0.03 17.2 67 20- 36 6.6 5.9 1.22 5.2 9,3 3.3 0.25 0.04 18.1 71 36- 48 6.4 5.6 1.01 5.8 13.2 5.0 0.39 0.06 24.5 76 48- 61 6.6 6.1 0.69 4.9 13.6 5.5 0.39 0.07 24.5 BO 61- 76 7.0 6.3 0.34 0.0 1.3 1.3 2.6 11.3 4.9 0.30 0.06 19.2 86 76- 89 7.7 7.0 0.9 4.1 5.3 89-104 8.17.4 12.9 17.6 32.0 104-124 8.2 7.6 19.9 17. B 39.2 124-152 8.3 7.7 19.7 19.6 41.0 THE PARTICLE SIZE FAMILY CONTROL SECTION HAS CONSIDERED TO BE BETWEEN 36 AND B6 CENTIMETERS. IC16HTED AVERAGE: 75-.IMM WHOLE SOIL i= 10.9 (2 MICRON FINE EARTH 27.5 268 Table 35. Characterization data for Rossmoyne silt loam (WA-71). SOIL SERIES: ROSSMOYNE COUNTY: HARREN SITE: HA-71 DATE: 4/15/85 OSU UIB. NUMBERS: 25533 - 25568 CO. DEPTH HORIZON FRAG. —CLAYlua)— TEXT. )2m C M F VF TOTAL 50-20 20-5 5-2 TOTAL 2-.2 (.2 TOTAL CLASSVC c* 0- 20 Apl 0.2 1.9 1.9 1.3 2.1 2.8 10.0 25.0 38.5 10.0 73.5 12.7 3.8 16.5 SIL 20- 36 Ap2 0.2 1.4 1.9 1.3 l.B 2.2 8.6 24.6 44.1 6.8 75.5 12.1 3.8 15.9 SIL 36- 31 Btl 0.0 0.3 0.7 0.4 0.7 1.1 3.2 20.4 36.4 10.4 67.2 22.3 7.3 29.6 SICL 51- 69 Bt2 0.0 0.1 0.5 0.3 0.7 1.4 3.0 23.9 36.3 8.8 69.019.8 B. 2 28.0SICL 69- 66 Bt3 0.0 0.6 1.0 0.6 1.2 2.4 5.8 26.9 34.3 8.3 69.5 17.0 7.7 24.7 SIL 86-112 2Btxl 0.1 1.5 2.3 l.B 4.8 5.5 15.9 16.8 25.1 9.3 51.2 23.7 9.2 32.9 SICL 112-137 2Btx2 0.1 0.8 1.6 1.3 4.1 6.8 14.6 15.1 28.4 6.3 50.0 24.9 10.3 33.4 SICL 137-155 2Btl 0.4 1.5 1.7 1.6 6.6 7.B19.2 14.2 23.5 8.1 45.8 24.6 10.4 35.0 SICL 155-168 2812 0.2 0.5 1.1 1.2 3.1 3.1 9.0 13.0 33.0 7.7 33.7 27.3 10.0 37.3 SICL 168-188 2Bt3 0.1 1.1 2.2 2.2 4.7 4.3 14.3 8.5 25.1 10.4 44.0 32.4 9.1 41.5 SIC 188-208 2Bt4 0.0 0.5 1.7 1.8 5.2 3.2 14.4 9.1 25.1 9.1 43.3 32.4 9.9 42.3 SIC 208-229 2Bt5 0.0 0.6 1.5 1.8 3.1 5.1 14.1 9.7 28.1 4.4 42.2 33.5 10.2 43.7 SIC 229-254 2B16 0.0 0.6 1.6 1.8 5.6 5.3 15.1 8.4 20.2 11.7 40.3 35.1 9.3 44.6 SIC 254-272 2817 0.0 0.5 1.7 2.3 6.8 6.0 17.3 7.8 22.8 12.4 43.030.4 9.3 39.7 SICL 272-310 2BtB 0.6 0.6 2.2 2.8 8.1 6.8 20.3 9.4 19.110.4 38.9 31.8 8.8 40.6 C 310-328 2Bt9 1.0 1.0 2.9 3.7 11.2 8.8 27.6 7.3 15.09.4 31.9 30.5 10.0 40.5 C 1:1 .DIM DR6. CAL- DCLO- CARB------EXTRACTABLE CATIONS------BASE DEPTH HATER C*C12 C CITCMITE ONATE H b I] K Na SIM SAT. n -----pH— * 'EM------leq/lOOg— * 0- 20 7.5 7.1 1.10 0.6 1.6 2.3 1.7 10.3 3.6 0.35 0.03 16.0 B9 20- 36 7.3 6.9 0.93 0.6 0.8 1.4 2.1 10.3 3.1 0.26 0.07 15.8 87 36- 51 7.1 6.5 0.42 0.0 0.9- 1.0 3.5 12.9 4.9 0.36 0.11 21.8 84 51- 69 7.1 6.4 0.37 0.1 O.B 1.0 3.4 12.1 5.1 0.32 0.12 21.0 84 69- 86 7.0 6.4 0.18 0.3 0.6 1.0 3.1 10.3 5.0 0.29 0.13 18.8 84 86-112 6.8 6.3 0.13 0.1 1.1 1.3 3.3 11.6 6.3 0.26 0.13 21.6 85 112-137 6.6 6.2 0.11 0.7 0.8 1.5 3.3 12.2 6.4 0.26 0.14 22.5 84 137-155 7.1 6.3 0.15 0.6 0.9 1.5 3.5 12.5 6.1 0.24 0.14 22.5 84 155-168 7.0 6.3 0.16 0.4 1.0 1.4 2.8 14.4 7.4 0.26 0.16 25.0 89 168-188 7.0 6.4 0.20 0.5 1.1 1.6 3.4 15.6 7.6 0.32 0.17 27.1 87 188-208 6.9 6.4 0.26 0.2 1.3 1.6 4.1 15.1 7.8 0.34 0.19 27.5 85 208-229 7.0 6.4 0.39 0.2 1.0 1.3 4.2 15.6 7.8 0.37 0.19 28.2 85 229-254 6.9 6.6 0.40 0.6 0.7 1.3 4.4 17.1 8.8 0.38 0.21 30.9 86 254-272 6.9 6.3 0.44 0.4 1.3 1.9 4.1 17.4 8.7 0.41 0.21 30.8 87 272-310 7.0 6.4 0.49 0.7 1.0 1.7 4.0 17.0 7.9 0.38 0.18 29.5 86 310-328 7.0 6.3 0.35 0.3 1.1 1.6 3.2 15.8 7.3 0.41 0.19 26.9 88 THE PARTICLE SIZE FAMILY CONTROL SECTION MAS CONSIDERED TO BE BETTIEEN 36 AND 86 CENTIMETERS. WEIGHTED AVERAGE: 75-. 1MH WHOLE SOIL t* 2.4 (2 MICRON FINE EARTH t= 27. A 269 Table 36. Characterization data for Dana Variant silt loam (WA-72). SOIL SERIES! DANA VARIANT COUNTY! WARREN SITE! HO-72 DATE! 4/15/85 OSU LAB. NUMBERS: 25569 - 25576 CO. PARTICLESIZE DISTRIBUTION t*(2«a>------DEPTH HORIZON FRAG.------SAM)------SILT(ua)------CLAYIta)— TEXT. >2ai VC C M F VF TOTAL 50-20 20-5 5-2 TOTAL 2-.2 (.2 TOTAL CLASS ca -----% 0- 25 Ap 0.0 0.1 0.4 0.5 1.1 1.4 3.5 20.9 33.5 9.6 64.0 15.1 17.4 32.5 SICL 25- 43 A 0.2 0.4 0.7 0.5 1.2 1.5 4.3 19.8 32.7 9.4 61.9 15.7 18.1 33.8 SICL *3- 58 AB 0.0 0.6 0.9 0.6 1.2 1.6 4.9 15.9 36.9 12.1 64.920.9 9.3 30.2 SICL 58- 61 Btl 0.0 0.1 0.5 0.3 0.9 1.6 3.4 18.7 41.4 6.7 66.819.7 10.1 29.8S1CL 81-102 Bt2 0.0 0.2 0.4 0.4 1.4 2.4 4.B 21.1 37.3 9.0 67.4 18.4 9.4 27.8 SICL 102-122 2B13 0.2 0.6 0.9 0.9 5.3 6.1 13.8 17.5 27.3 9.2 54.0 21.1 11.1 32.2 SICL 122-140 2St4 0.6 1.3 1.7 1.5 5.5 6.0 16.0 15.5 26.9 10.3 52.7 20.7 10.6 31.3 SICL 140-152 2Bt5 1.3 3.6 4.6 3.5 10.0 9.9 31.6 12.8 18.7 8.2 39.719.9 8.8 28.7a 111 .OIH ORS. CO.- DOLO- CAR8------EXTRACTABU CATIONS------BASE DEPTH WATER CoC12 C CITE MITE ONATE n Cl Kg K Na SUM SAT. * P . rf C* -----yn Cqt% * 0- 25 6.6 6.1 1.96 6.3 ia.9 5.3 0.68 0.07 31.5 79 25- 43 6.6 6.2 2.10 6.1 21.5 5.3 0.41 0.09 33.4 82 43- 58 6,8 6.4 1.24 4.5 19.0 4.7 0.41 0.09 28.7 84 58- 81 7.0 6.6 0.16 0.3 1.4 1.8 3.2 14.5 4.2 0.36 0.09 22.4 66 81-102 7.3 6.9 0.16 0.6 1.3 1.9 2.3 13.3 3.70.32 0.08 19.7 BS 102-122 7.4 6.9 0.14 0.6 1.4 2.1 2.5 13.9 3.7 0.30 0.09 20.5 88 122-140 7.4 7.0 0.16 0.6 1.5 2.2 2.1 13.4 3.6 0.28 0.08 19.5 89 140-152 7,5 7.0 0.10 0.7 1.9 2.7 2.0 13.3 4.0 0.25 0.05 19.6 90 THE PARTICLE SIZE FAMILY CONTROL SECTION WAS CONSIDERED TO BE BETWEEN 25 AND 102 CENTIIETERS. WEIGHTED AVERAGE: 75-. 1* WHOLE SOIL f= 2.5 12 MICRON FINE EARTH 30.3 APPENDIX C Data from Dissolution Study 270 Table 37. Total mMol Mg/kg clay released from Xenia Bt2 horizon clay fraction after incubation in 0.1N H 2 SO 4 at various incubation hrs.+ Incubation Hours Treatments 24 4? 72 192 408 504 600 696 792 912 1008 1128 120C 1320 mMol Mg/kg clay Oxidized 22 38 48 60 88 116 132 126 125 132 133 124 148 147 150 147 Clay Untreated 24 45 52 61 93 113 125 121 121 128 129 122 147 152 152 151 Clay Clay + 23 41 53 64 95 115 127 119 123 127 127 119 140 130 130 127 Mollisol HA Clay + 24 41 53 66 94 120 128 128 129 134 132 124 147 140 135 135 Mollisol HA (10:1) Clay + 17 41 49 62 89 112 121 116 120 120 124 117 137 131 127 126 Alfisol HA Clay + 25 44 59 69 99 122 134 128 131 136 135 126 145 141 139 140 Mollisol FA +Average of 2 replicates. ! Table 38. Total nMol K/kg clay released from Xenia Bt2 horizon clay fraction after incubation in 0.1N H2SO4 at various incubation hrs.* ______Incubation Hours______ ------mMol K/kg clay ------ Oxidized 8 11 13 14 16 20 23 21 22 24 25 25 26 25 26 28 Clay Untreated 9 10 13 13 15 20 23 21 21 24 26 26 26 25 25 28 Clay Clay + 11 13 15 17 19 23 25 24 26 27 28 28 30 28 29 31 Mollisol HA Clay + 11 13 15 16 18 25 25 25 25 26 27 26 27 28 26 29 Mollisol HA (10:1) Clay + 7 10 12 15 16 20 21 20 21 23 25 25 26 23 24 27 Alfisol HA Clay + 11 12 15 16 17 23 25 23 24 26 28 27 28 27 26 28 Mollisol FA +Average of 2 replicates. 272 I Table 39. Total mMol Al/kg clay released from Xenia Bt2 horizon clay fraction after incubation in 0.1N H2SO4 at various incubation hrs. ______Incubation Hours______Treatments 24— 49 72.,P6 192__ 288 408 504 900 696 792 912__ 1008 1128 1200 1320 ------mMol Al/kg clay ------ Oxidized 6 8 10 11 13 17 19 19 21 21 22 21 21 24 23 23 Clay Untreated 6 9 10 11 14 16 18 19 20 21 21 21 21 23 23 23 Clay Clay + 7 8 10 11 13 17 18 19 20 21 20 20 20 23 22 22 Mollisol HA Clay + 7 8 10 11 13 16 18 18 20 20 20 20 21 23 23 22 Mollisol HA (10:1) Clay + 5 8 10 11 13 16 17 18 19 21 20 21 20 23 23 22 Alfisol HA Clay + 6 9 11 11 14 17 19 20 21 22 22 22 21 24 24 24 Mollisol FA +Average of 2 replicates. 273 Table 40. Total mMol Si/kg clay released from Xenia Bt2 horizon clay fraction after incubation in 0.1N H 2 SO 4 at various incubation h r s . t Incubation Hours Treatments 24 48 2 96 1?2 29? 408 504 600 696 792 912 1008 1128 1200 1320 ------mMol Si/kg clay Oxidized 2 3 5 7 10 12 14 14 14 16 16 16 15 13 12 Clay Untreated 2 3 5 8 10 11 13 13 14 14 15 16 16 14 12 Clay Clay + 2 3 5 8 10 12 13 13 13 15 15 15 15 13 12 Mollisol HA Clay + 2 3 5 7 9 11 12 13 13 15 15 15 15 13 12 Mollisol HA (10:1) Clay + 2 3 5 7 9 11 13 13 13 14 15 i 14 15 12 13 Alfisol HA Clay + 2 3 5 8 10 12 14 14 14 15 15 16 16 14 13 Mollisol FA Average of 2 replicates. Table 41. Total mMol Fe/kg clay released from Xenia Bt2 horizon clay fraction after incubation in 0.1N H 2 SO 4 at various incubation hrs. Incubation Hours Jreatments 24 48 72 ?6 192 288 409 504 500 696 792 912 1008 1128 1200 1320 mMol Fe/kg clay Oxidized 1 2 2 3 6 7 8 9 9 9 10 11 11 12 12 14 Clay Untreated 1 2 3 3 6 7 9 9 9 9 10 11 11 12 12 13 Clay Clay + 1 1 2 3 5 7 8 9 8 9 9 10 10 11 11 11 Mollisol HA Clay + 1 1 2 3 6 7 8 9 9 9 9 10 10 11 12 12 Mollisol HA (10:1) Clay + 1 1 2 3 5 7 8 9 8 8 9 10 10 11 11 12 Alfisol HA Clay + 1 2 3 4 7 8 9 10 10 10 10 11 11 13 12 14 Mollisol FA +Average of 2 replicates. Table 42. Total mMol Mg/kg clay released from Xenia Bt2 horizon total clay fraction after incubation in 0.005N H 2 SO 4 at various incubation hrs.+ Incubation Hours Treatments 42 111 184 278 386 477 5701 663 783 936 1176 mMol Mg/kg clay --- Oxidized 24 57 55 54 55 52 50 49 47 46 41 Clay Untreated 1 5 10 11 13 16 13 14 14 14 13 Clay Clay + 1 4 9 10 14 16 13 12 12 13 12 Mollisol HA Clay + 1 4 8 10 12 12 12 12 12 12 12 Alfisol HA Clay + 1 4 8 10 13 13 12 12 12 12 12 Histisol FA ^ Average of 2 replicates. Table 43. 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