TEXTURAL, MINERALOGICAL AND STRUCTURAL CONTROLS ON SOIL ORGANIC CARBON RETENTION IN THE BRAZILIAN CERRADOS DISSERTATION Presented in Partial Fulfillment of the Requirements for the Degree Doctor in Philosophy in the Graduate School of The Ohio State University By Yuri Lopes Zinn, M. S. The Ohio State University 2005 Dissertation Committee: Approved by Professor Rattan Lal, Adviser Professor Jerry Bigham Adviser Professor Warren Dick Soil Science Graduate Program Professor Andrew Ward ABSTRACT The retention of soil organic carbon (SOC) is commonly considered the result of climate, vegetation, internal drainage and management interactions. However, a sparse but considerable set of evidence in the literature suggests that soil texture, mineralogy and aggregation also affect SOC retention. The objective of this research was to qualitatively and quantitatively assess the controls that these three soil properties exert on SOC levels in the Brazilian Savanna (Cerrado) region, in tropical humid South America. Thus, three soils under similar climate and slope, but of contrasting texture and under native vegetation and Eucalyptus plantation, were sampled in triplicate to 1 m depth, and characterized by physical, chemical, mineralogical, Yoder wet sieving, and microscopic analyses. Total C and N concentration were determined in bulk soils, particle size separates (clay, silt, sand) and water-stable aggregates (WSA). A basic assumption was made that the SOC particle size is inherently associated to its retention mechanism: colloidal and soluble forms are retained by sorption to clays; particulate organic matter (POM, >20 μm) are retained outside (free-POM) or inside aggregates (occluded-POM), and silt-sized SOC has intermediate properties. The rationale is that these mechanisms are necessarily determined by soil texture, mineralogy and structure, which then control SOC retention. The three soils were highly weathered and composed mostly of quartz and kaolinite, with minor and variable contents of Fe-Al oxides, hydroxyl-interlayered vermiculite and illite. These soils were classified as clayey Haplustox, loamy Haplustox and sandy soils (one Quartzipsamment and two sandy Haplustoxes). In all soils, Eucalyptus plantation increased the C/N ratio and decreased aggregation and aggregate- occluded POM in the top 10 cm layer in relation to native vegetation, but bulk SOC ii concentrations and stocks were not affected. In the sandy soils only, the relative content of SOC in the sand fraction was enriched, whereas it decreased in the clay fraction. Soil texture and depth strongly affected bulk SOC concentrations, which could be credibly estimated as a function of clay+silt contents and depth, by means of a novel mathematical model (using data from samples under the two land uses). The specific surface area (SSA) of soil<2 mm under native Cerrado was modeled as a function of clay, silt and SOC contents but not depth. This suggests that SOC levels increase with higher SSA associated with higher clay contents. In a single soil profile, however, SSA decreases near the soil surface because of slightly lower clay contents but also because higher SOC levels promote clay flocculation and aggregation. The SOC concentration in particle size separates was inversely related to the proportion of that size fraction in soil (SOC dilution effect), which precluded its use in modeling SOC size partition. However, the calculation of relative amounts of total SOC in each fraction allowed for prediction of clay-sized SOC (as percent of total SOC), based on clay contents and depth. A quantitative assessment of clay mineralogy showed that, for the bulk soil, SOC concentrations were better correlated with contents of crystalline and amorphous Fe- oxides in surface layers and amorphous Al oxides in the subsoil, with higher coefficients of determination than those of SOC vs. clay+silt. These trends were even stronger when the clay-sized SOC pool was correlated to the same mineral phases. Aggregation, as indicated by the mean weight diameter (MWD) and percent of WSA>2 mm, was strongly correlated with clay+silt contents, but bulk SOC was poorly correlated with MWD and WSA>2 mm except for the 0-5 cm depth. The fraction of POM occluded inside aggregates was strongly affected by soil texture, varying from ca. 25% in the sandy soils to ca. 50% in the clayey Haplustox. The activity of soil fauna resulted in three types of zoogenic aggregates (fecal pellets, cocoons and aggrotubules), the latter more common and SOC-enriched in soils of loamy and coarser texture. Because texture directly affects the contents of Fe and Al oxides and the protection of POM within aggregates, SOC retention in comparable aerobic Cerrado soils is controlled, in decreasing order of importance, by: 1) texture, 2) mineralogy, and 3) structure (including pedogenic and faunal peds). iii DEDICATION This work is dedicated to my wife Alba who gave me, more than strength, a reason to go on. iv ACKNOWLEDGMENTS I am deeply indebted to the Capes Foundation within the Ministry of Education of Brazil, which gave the generous financial support that made this doctorate possible. I also thank the Graduate School of the Ohio State University for the award of a Presidential Fellowship, which allowed an essential extra time to complete this work. I thank Dr. Rattan Lal for many things, but especially for: accepting to supervise my doctoral studies; selecting an outstanding graduate committee; fast and efficient review of all submitted manuscripts; and the nomination to the Presidential Fellowship. The operational support that made possible the sampling travels was kindly provided by the Embrapa Cerrados, in the person of Dr. Dimas V.S. Resck, and to V & M Tubes Florestal Co., in the person of Dr. Hélder B. Andrade. My deep thanks are also due to Dr. Jerry M. Bigham and Franklin “Sandy” Jones, for their constant and fundamental support with the soil characterization, particle-size fractionation, mineralogy and micromorphology parts of this work. I must thank the following faculty, who provided very kind letters of recommendation for the Presidential Fellowship and other awards: within OSU, Drs. Jerry Bigham, Frank Calhoun, Warren Dick, Brian Slater (SNR), Andy Ward (FABE), Garry McKenzie (GS), and of course Dr. Rattan Lal. Outside OSU, I thank Drs. Wolfgang Zech (Univ. Bayreuth, Germany) and Dimas Resck (Embrapa Cerrados, Brazil). v A great number of people provided help with other fundamental phases of this work or my stau in the U.S., from which I specifically thank: Drs. Steve Saint-Martin and Bert Bishop (OARDC), and Antônio C. Gomes (Embrapa Cerrados), for help with the experimental design and statistical analyses; Mrs. Jesuíno S. Caldas and Wantuir C. Vieira (Embrapa Cerrados), for field and laboratory support in Brazil; Mrs. Yogendra Raut and Tony Karcher (SNR) for laboratory support; Mss. Pat Polczinsky and René Johnston for help with many SNR office issues; Dr. Neil Smeck for helping classify soils according to the U.S. Soil Taxonomy; and Dr. E. Bonnelo and his students of Dept. of Plant Pathology-OSU, for the use of their binocular microscope and digital camera. I cannot forget to thank my wife Alba who helped me many times, with data typing and relevant suggestions. Finally, I thank my parents, Jorge A. Zinn and Vera Lucia Lopes, for their support (personal and official), and all the other members of my family, in special to vó Maria Jacy and mana Lara, who supported me in their small but important ways. vi VITA November 29, 1970…………..… Born – Brasília-DF, Brazil 1994…………………………….. B.S. in Forestry, Federal University of Viçosa 1998………………………….…. M.S. Agronomy/Soil Science, University of Brasília 1997-2000……………………… Analyst of Science and Technology, Capes Foundation, Ministry of Education of Brazil 2001-2004……………………… Ph.D. student in Soil Science, the Ohio State University, with a grant from the Capes Foundation 2005-present…………………… Presidential Fellow, the Ohio State University PUBLICATIONS 1. Zinn, Y.L., D.V.S. Resck, and J.E. Silva. 2002. Soil organic carbon as affected by afforestation with Eucalyptus and Pinus in the Cerrado region of Brazil. For. Ecol. Manage. 166: 285-294. 2. Zinn, Y.L., R. Lal, and D.V.S. Resck. 2005. Changes in soil organic carbon stocks through agriculture in Brazil. Soil Till. Res. 84:28-40. 3. Zinn, Y.L.; R. Lal, and D.V.S. Resck. 2005. Texture and organic carbon relation described by a profile pedotransfer function in Brazilian Cerrado soils. Geoderma 127:168-173. FIELDS OF STUDY Major field: Soil Science vii TABLE OF CONTENTS Page Abstract…………………………………………………………………………….. ii Dedication…………………………………………………………………….…….. iv Acknowledgments………………………………………………………………….. v Vita…………………………………………………………………………………. vii List of Tables……………………………………………………………………….. x List of Figures………………………………………………………………………. xi 1. Introduction……………………………………………………………………… 1 2. Literature review………………………………………………………………… 3 2.1 Soil organic carbon (SOC)……………..……………………………………. 3 2.1.1 Textural controls on SOC storage and dynamics: proposed mechanisms………………………………………………………….……………... 4 2.1.2 Mechanisms of SOC storage: role of soil structure ……...…………… 8 2.1.3 Mechanisms of SOC storage: role of soil mineralogy.….……………. 12 2.2 The Brazilian Cerrados – environment, soils and land use………………….. 16 2.3 Hypotheses…………………………………………………………………... 19 3. Materials and Methods…………………………………………………….……. 20 3.1 Sampling areas……………………………………………………………….. 20 3.2 Experimental design and sampling…………………………………….…….. 23 3.3 Soil analyses………………………………………………………………….