THE INTERACTION OF CROP RESIDUE AND FERTILISER

MANAGEMENT AND THEIR EFFECT ON SOIL ORGANIC

MATTER, SOIL PHYSICAL PROPERTIES AND SUSTAINABILITY

By

Kenneth Guy Carlyle McMullen B.Rur.Sci (Hons) (UNE)

A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY OF THE UNIVERSITY OF NEW ENGLAND JUNE 2005 DECLARATION

I certify that the substance of this thesis has not already been submitted for any degree and is not currently being submitted for any other degree or qualification.

I certify that any help received in preparing this thesis, and all sources used, have been acknowledged.

Kenneth Guy Carlyle McMullen

ii ACKNOWLEDGEMENTS

This study was funded by the Grains Research and Development Corporation (GRDC). I am especially indebted to the dedication and persistence of my Principal Supervisor, Associate Professor Graeme Blair whose energy and enthusiasm was always obvious. Many thanks to Dr. Graeme Schwenke for his tolerant supervision and input in the planning and completion of this study. I also acknowledge Dr. Schwenke for the use of historical samples for the comparison of soil C pools with published nitrogen mineralisation measurements. Thanks to Ray Till for his skilled assistance in the use of 14C samples.

My thanks to Warwick Felton for access to his meticulously managed field trials. I am appreciative of the support from NSW Agriculture and the use of the Liverpool Plains Field Station and facilities at the Tamworth Centre for Crop Improvement. I would like to thank the Forsyth family of 'Gabo', Croppa Creek who hosted the long term tillage trial and allowed me access to take additional measurements. My gratitude to the landowners of the Liverpool Plains for allowing me access to take measurements from their properties. The knowledgeable and skilled technical assistance of Judi Kenny, Leanne Lisle and Nelly Blair was also invaluable.

I am most grateful to my family, especially my mother and sisters, for their continual support throughout my education. This thesis was only possible thanks to the never-ending support and encouragement of my wife, Patrice, who always remained positive, even if I was not. Her patience and understanding remained throughout this study although mine did not! And finally for Lucy, the final impetus though she did not know it, who makes everything seem worthwhile.

iii ABSTRACT

The cultivation and cropping of soils across the Australian grain growing regions has resulted in substantial losses of soil organic matter (SOM). These losses have been accompanied by even greater losses in the active or labile components of SOM. Soil organic matter is an important indicator of sustainable farming systems as it has a central role in all aspects of soil fertility.

The grain growing areas of northern New South Wales and southern Queensland in Australia are predominantly based on vertisol soils that have high water-holding capacities. This characteristic, along with initially high levels of fertility, has allowed reliable crop production on these soils in the past. Crop production systems in this area have increasingly used reduced tillage and the retention of crop stubble to increase soil water storage during the fallow period. Declining soil fertility has increasingly become evident but higher rates of N application have been successful in overcoming declines in grain yield and protein content.

Studying changes in the concentration of soil organic matter due to different management practices requires methods that measure responsive and meaningful soil C fractions against large background concentrations. Changes in labile soil C fractions were measured using oxidation with 333 mM KMn04. The relative amounts of labile and non- labile C (total C- labile C) were compared to a non-cropped reference soil and used to calculate a carbon management index (CMI).

This study focused on increasing the understanding of crop residue breakdown and the influence of tillage and rotation practices on soil carbon fractions and the physical fertility of vertisol soils from the northern grains region. In the first study, two long-term trials established in 1981 were used to assess changes in soil fertility under different tillage and rotation management practices. The trials were located on a commercial property, "Gabo" near Croppa Creek, and at the Liverpool Plains Field Station, Breeza managed by NSW Agriculture. The study of changes in soil C fractions and soil physical fertility at the long- term trial at Breeza was complemented by a survey of commercial farming paddocks located on the Liverpool Plains in northern NSW. All the soils in the survey were black vertisol soils similar to the long-term trial site with an uncropped reference site located nearby. The final component of this study was a glasshouse-experiment using dual-isotope labelling with 13C and 14C to measure changes in soil carbon (C) due to residue

iv decomposition and inputs from the growing crop. The influence of tillage and the type of crop residue were also investigated in this study.

The use of the KMn04 fractionation technique showed that cropping has resulted in large declines in the labile soil C fraction. Tillage management did not result in consistent changes in soil C fractions at the field trials. Tillage had no effect on soil C fractions at the Breeza site while it was shown that no-tillage practises at Croppa Ck resulted in a 21 and 14% higher concentration of labile C compared to stubble-mulching and stubble-burning, respectively.

Crop rotation and fallowing caused substantial losses of soil C. Long-fallowing caused a 160/0 decline in labile soil C and resulted in the CMI falling from 89 to 64. Prior to fallowing the carbon management index in a fababean-wheat rotation was 87 compared to 66 for continuous cereal. This improvement though was lost after the fallow period, with the CMI declining to 63 and 65 in both rotations, respectively.

Cropping caused a decline in soil physical fertility at the Breeza site. Infiltration through macropores fell by 63% in cropped areas compared to the reference site. Cropping also resulted in a 55%) decrease in water aggregate stability with a decline in the proportion of aggregates >250 IJm and an increase in those <125 IJm.

Seasonal variations in soil carbon fractions were studied using monthly samples taken from the long-term tillage trial at Breeza. The seasonal changes in soil C fractions were compared to nitrogen mineralisation data that had been determined in a previous project. There were some similarities between the fluctuations in soil Cfractions and nitrogen mineralisation but no significant relationship was found. Environmental factors, particularly rainfall, appeared to be the main drivers of these seasonal changes.

The soil survey of cropping paddocks on the Liverpool Plains showed substantial losses of labile soil C fractions compared to the uncropped reference sites. Cropping caused an average 540/0 decline in labile C compared to the reference sites resulting in an average CMI of 44. The loss of soil physical fertility in the cropped areas was reflected by a 69°!c> reduction in water stable aggregates and a 60-800/0 reduction in unsaturated hydraulic conductivity. Minimum-till tended to reduce the losses in infiltration, especially when large amounts of crop residues were present. Results from the survey and trial data showed a strong relationship between soil C fractions and soil physical fertility. Other studies that have investigated the role of organic matter and soil C in vertisol soils have found varied relationships between these aspects of soil fertility.

v The influence of plant residue quality and tillage management on soil C fractions was further investigated in a glasshouse experiment using a dual-isotope labelling technique. Residues of medic (Medicago truncatula), Flemingia (Flemingia macrophylla) and rice (Oryza sativa) labelled with 14C02 were either incorporated or allowed to decompose on the soil surface while growing wheat plants were labelled with 13C02. This allowed a measurement of the direct contribution of C from the growing crop through roots and root exudates to be determined while the residue-derived C could also be studied.

Residue type was found to be an important determinant of changes in soil C fractions. The incorporation of residues resulted in a 59% recovery of the C added in residues in the

soil compared to only 41 % when applied to the soil surface, as in no-till systems. The contribution from the growing crop in the form of both coarse and fine roots, and also from root exudates was equivalent to 2718 kg C/ha. This direct contribution from the roots plays an important role in improving the soil C concentration in no-till farming systems.

vi TABLE OF CONTENTS

DECLARATION ii

ACKNOWLEDGEMENTS iii

ABSTRACT iv

LIST OF FIGURES v

LIST OF TABLES vi

CHAPTER 1: INTRODUCTION ~~~~~~~~~~~~~~~~~~1

CHAPTER 2: LITERATURE REVIEW 4

2.1 Soil Organic Matter ~~~~~~~~~~~~~~~~~~~~~~4 2.1.1 Factors Influencing Soil Organic Matter 5

2.2 RoleofSoiIOrganicMatter~~~~~~~~~~~~~~~~~~~6 2.2.1 Chemical Fertility 7

2.2.2 Physical Fertility 8

2.2.3 Biological Fertility 10

2.3 Agricultural Practices and SOM~~~~~~~~~~~~~~~~~11 2.3.1 Cultivation 12

2.3.2 Crop Rotations and Residue Management ~~~~~~~~~~~13

2.3.3 Rhizodeposition 14

2.4 MeasurementofSOM~~~~~~~~~~~~~~~~~~~~~16 2.4.1 Chemical Fractionation 18 a. Total Organic C 18 b. Spectroscopic Techniques 18 c. Oxidation with Potassium Permanganate 19

2.4.2 Physical Fractionation 20

2.4.3 Isotopic Techniques 20 a. Labelling 20 b. Natural Abundance 21 c. 80mb C 22

2.5 SUMMARY 22

vii CHAPTER 3: INFLUENCE OF TILLAGE AND CROP RESIDUE MANAGEMENT ON SOIL CARBON FRACTIONS AND THE FERTILITY OF TWO VERTISOLS FROM NORTHERN NSW, AUSTRALIA. 24 3.1 INTRODUCTION 24

3.2 MATERIALS AND METHODS 25 3.2.1 Description of Field Sites 25 3.2.2 Treatments and Experimental Design 25 3.2.3 In-situ Unsaturated Hydraulic Conductivity 27 3.2.4 Wet Aggregate Stability 27 3.2.5 Total Carbon and Nitrogen 28 3.2.6 Labile Carbon and the Carbon Management Index 28 3.2.7 Statistical Analyses 31

3.3 RESULTS 32 3.3.1 Effect of crop rotation and tillage practices on soil carbon fractions of a black vertisol from LPFS, Breeza NSW 32 3.3.2 Changes in the physical fertility of a black vertisol at the LPFS, Breeza NSW. 34 3.3.3 Relationships between soil C fractions and physical fertility of black vertisols from the Liverpool Plains, NSW 35 3.3.4 Seasonal changes in soil C fractions in a vertisol from long term trial located at Liverpool Plains Field Station, Breeza NSW, Australia 39 3.3.5 Relationship between soil carbon fractions and nitrogen mineralisation in a black vertisol from the LPFS, Breeza NSW 43 3.3.6 Changes in Soil Carbon Fractions in a grey vertisol from Croppa Creek due to tillage and crop rotation. 45 3.3.7 Changes in physical fertility after a sorghum crop in a grey vertisol from Croppa Creek. 47

3.4 DISCUSSION 49 3.4.1 Soil C Fractions 49 3.4.2 Physical Fertility 52

3.5 CONCLUSIONS 53

viii CHAPTER 4:_SURVEY OF SOIL ORGANIC MATTER AND SOIL PHYSICAL PROPERTIES ON CROPPED AND VIRGIN VERTISOL SOILS OF THE LIVERPOOL PLAINS, NORTHERN NSW, AUSTRALIA. 55 4.1 INTRODUCTION 55

4.2 MATERIALS AND METHODS 56 4.2.1 Site Selection 56 4.2.2 Hydraulic Conductivity 58 4.2.3 Wet Aggregate Stability 58 4.2.4 Total Carbon and Nitrogen 59 4.2.5 Labile Carbon and the Carbon Management Index 59 4.2.6 Statistical Analysis 59

4.3 RESULTS 60 4.3.1 Changes in Soil C Fractions with Cropping 60 4.3.2 Changes in Soil Physical Measurements with Cropping 61 4.3.3 Relationship between Soil C Fractions, Water Sta~le Aggregates and Unsaturated hydraulic conductivity 66

4.4 DISCUSSION 67 4.4.1 Soil C Changes with Cropping 67 4.4.2 Changes in Water Stable Aggregates with Cropping 69 4.4.3 Unsaturated Hydraulic Conductivity 71 4.4.4 Relationship Between Aggregation and Soil C Fractions 71

4.5 CONCLUSIONS 72

CHAPTER 5: DETERMINING CROP AND RESIDUE DERIVED C USING DUAL CARBON ISOTOPE LABELLING 73 5.1 INTRODUCTION 73

5.2 MATERIALS AND METHODS 74 5.2.1 Soil Collection 74 5.2.2 Residue Application 74 5.2.3 Pot Design 75 5.2.4 Basal nutrient and nitrogen applications 77 5.2.5 Labelling of plant material with 13C02. 77 5.2.6 Air and Soil Temperature 77 5.2.7 13C02 Labelling Procedure 80 5.2.8 Respiration Trap Management 81 5.2.9 Sample Processing 81

ix 5.2.10 Soil and PlantAnalysis~~~~~~~~~~~~~~~~~~~82 5.2.11 Wet Aggregate Stability 85

5.3 RESULTS 86 5.3.1 Shoot Weight 86 5.3.2 Plant N derived from residues 87 5.3.3 Soil C derived from residue breakdown 88 5.3.4 Carbon derived from the growing wheat crop 90 5.3.5 Changes in Soil C Fractions 91 5.3.6 Water Stable Aggregates (WSA) 92

5.4 DISCUSSION 94 5.4.1 Changes in soil C fractions. 94 5.4.2 Contributions of residue-derived C to soil C determined using 14C labelled plant material. 95 5.4.3 Direct contribution of growing plants to soil C. 99 5.4.4 Water Stable Aggregation 100

5.5 CONCLUSIONS 101

CHAPTER 6: GENERAL DISCUSSION 103 6.1 Future Research 106

REFERENCES 107

x LIST OF FIGURES Figure 1.1 Broad soil groups of the northern grains region, NSW, Australia. 2 Figure 2.1 Simplified carbon cycle (Source: Cheng and Kimble 1998) 4 Figure 2.2 Functions attributed to SOM and the interrelationships between these functions (Baldock and Skjemstad 1999) 7 Figure 2.3 Components of organic matter including the active, or labile, fraction (from Gregorich et al. 1997) 17 Figure 3.1 Relationship between labile C and unsaturated hydraulic conductivity at 10 mm tension from different samplings at Breeza (**regressions are significant at P<0.01 with the 95%) confidence levels shown). 37 Figure 3.2 Relationship between total C and unsaturated hydraulic conductivity at 10 mm tension from different samplings at Breeza (**regressions are significant at P<0.01 with the 95%) confidence intervals shown). 37 Figure 3.3 Relationship between labile C (g C Ikg) and mean weight diameter (mm) from different samplings at Breeza (**regressions are significant at P<0.01 level with the 950/0 confidence intervals shown). 38 Figure 3.4 Relationship between total C (g C Ikg) and mean weight diameter (mm) from different samplings at Breeza (**regressions are significant at P<0.01 level with the 95% confidence intervals shown). 38 Figure 3.5 Daily rainfall (bars) and soil temperature (line) from December 1997 to December 1998 at Liverpool Plains Field Station, Breeza NSW (Drop bars indicate sampling times with the total rainfall (mm) between samplings shown between the bars). 39 Figure 3.6 The effect of tillage and time of sampling on soil C fractions of a vertisol soil from Liverpool Plains Field Station, Breeza, NSW Australia from December 1997 to December 1998 (5% LSD is shown). 40 Figure 3.7 The effect of crop rotation and time of sampling on soil C fractions of a vertisol soil from Liverpool Plains Field Station, Breeza, NSW Australia from December 1997 to December 1998 (5% LSD is shown). 42 Figure 3.8 Soil nitrogen mineralisation and C fractions from December 1997- December 1998 at Liverpool Plains Field Station, Breeza NSW, Australia (standard errors are shown). 43 Figure 3.9 Effect of tillage on unsaturated hydraulic conductivity (mm/hr) of grey vertisol soil at Croppa Creek. 48 Figure 4.1 Location (*) of commercial properties surveyed in northern NSW, Australia. _ 58 Figure 4.2 Distribution of WSA from cropped and virgin vertisol soils after wet sieving. _ 63 Figure 4.3 Hydraulic conductivity (Kunsat) of cropped and virgin vertisol soils at various tensions (with 5% LSD shown as error bars). 65

xi Figure 4.4 Linear relationship between the mean weight diameter and CNL and CL(g C/kg soil) and of vertisol soils from the Liverpool Plains, NSW. 66 Figure 4.5 Linear relationship between labile (Cd and non-labile C (CNd and unsaturated hydraulic conductivity at 10 mm tension from survey of vertisol soils from the Liverpool Plains, northern NSW. 67 Figure 5.1 Pot design to study soil C dynamics by separating contributions from the growing plant labelled with 13C02 and plant residues previously labelled with 14C02. 76 Figure 5.2 Daily maximum and minimum soil temperature (OC) within the pots used for dual C radioisotope study of soil C dynamics. 78 Figure 5.3 Daily maximum and minimum air temperature within the labelling tent used to label wheat plants with 13C02 to study the contributions of growing plants to soil C dynamics. 78 Figure 5.4 Schematic diagram of labelling chamber used label plants with 13C02 to determine the contribution of the growing crop to soil C. 79 Figure 5.5 Picture of labelling chamber during pulse labelling of wheat plants with 13C02 to determine the contribution of growing plants to soil C dynamics. 80 Figure 5.6 Soil core before removal from pot showing surface applied Flemingia residue (left), after sectioning (middle) and finally fully processed prior to drying (right). 81 Figure 5.7 Modified digestion tubes for determination of 14C from soil and plant material to study soil C dynamics. 84 Figure 5.8 Effect of residue on shoot dry weight production and shoot total N of wheat (T. aestivum cv. Sunbrook) (Error bar represents LSD at 50/0) 86 Figure 5.9 Effect of residue placement on dry matter production (g OM/pot) and the shoot N concentration (%» of wheat (T. aestivum cv. Sunbrook). (Error bar represents

% LSD at 5 ) 87 Figure 5.10 Differences in the apparent fungal populations of surface applied Flemingia (left) and Rice (right) after 80 days. 97

xii LIST OF TABLES Table 2.1 Functional classification of soil pores and associated particles (Oades 1984) _ 8 Table 2.2 Aggregate hierarchy and bonding mechanisms. 9 Table 2.3 Description of products derived from plant roots (derived from Meharg 1994) _ 15 Table 3.1 Treatments sampled from Croppa Creek long-term trial and the crops for the preceding and years of this study (W = Wheat, F = Fallow, FB = Fababean)_ 26 Table 3.2 Treatments sampled from long-term trial located at LPFS, Breeza NSW and the crops for the preceding years of this study (W=Wheat, B=Barley, F=Fallow, FB=Fababean). 26 Table 3.3 Effect of fallowing and cropping on soil C fractions of a black vertisol from Liverpool Plains Field Station, Breeza. 32 Table 3.4 Effect of crop rotation and fallowing on soil C fractions of a black vertisol from the Liverpool Plains Field Station, Breeza. 33 Table 3.5 Effect of crop rotations and fallowing on the soil C fractions of a black vertisol relative to a nearby uncropped reference site. 33 Table 3.6 Effect of previous crop rotation and fallowing on hydraulic conductivity (Kunsat) at 10 and 20 mm tensions and relative to a reference site. 34 Table 3.7 Influence of cropping on the mean weight diameter and the proportion of WSA >250 IJm and <125 IJm from a black vertisol at the LPFS, Breeza NSW. 35 Table 3.8 Linear relationships between soil C fractions and N mineralisation at the Liverpool Plains Field Station, Breeza NSW of different crop management treatments. 44 Table 3.9 Effect of tillage on soil C fractions in the surface soil (0-5 cm) of a grey vertisol from Croppa Creek in 1999 after 21 years of no-tillage 45 Table 3.10 Effect of tillage on soil C fractions in the surface soil (0-10 cm) of a grey vertisol from Croppa Creek in 2000 after an opportunity sorghum crop 46 Table 3.11 Effect of sorghum production on soil C fractions (0-10 cm) and sustainability at Croppa Creek 1999-2000. 46 Table 3.12 Effect of tillage on mean weight diameter (MWD) and the proportion of WSA >250 IJm and <125 IJm of a grey vertisol at Croppa Creek, NSW in 2000. __ 47 Table 4.1 Crop rotation (showing most recent crop last) and tillage management histories of the survey sites. 57 Table 4.2 Effect of cropping on soil C fractions (g/kg) and sustainability indicators 60 Table 4.3 Effect of cropping on WSA and unsaturated hydraulic conductivity of vertisol soils from the Liverpool Plains, NSW. 61 Table 4.4 Linear relationships between unsaturated hydraulic conductivity at 10 mm tension (Kunsat[10]), mean weight diameter (MWD), proportion WSA >250 IJm , proportion of WSA <125 IJm and soil C fractions. 66

xiii Table 5.1 Properties of three plant residues of differing quality to study soil C dynamics in a black vertisol. 75 Table 5.2 Summary of treatments to study the effect of tillage and residue placement on soil C dynamics in a black vertisol from the LPFS Breeza, Australia. 75 Table 5.3 Timing and amount of Na213C02 used to pulse label wheat plants. 80 Table 5.4 Effect of residue type and placement and previous tillage history on the proportion of wheat top N derived from residues 87 Table 5.5 Effect of residue type on the recovery of residue N in wheat tops 88 Table 5.6 Effect of residue type, placement and previous tillage history on the quantity of plant N derived from applied residues. 88 Table 5.7 The effect of residue placement, residue type and previous soil management on soil C derived from the growing plant, derived from residues and the residue C lost (mg C/pot after normalisation of added residue C to 2500 mg C/pot) 90 3 Table 5.8 Total carbon, nitrogen and stable isotopes CC and 15N) in roots and tops from wheat plants labelled with 13C02 and 15N labelled plant residues (± s.e.). 91 Table 5.9 Contributions to soil C from residue breakdown and plant growth (mg C/pot and

kg C/ha and as a % of total soil C) determined simultaneously using dual C isotope labelling (± s.e.). 91

Table 5.10 Effect of management history and residue placement on total (Cr ), labile (C L) and non-labile carbon (CNd and the Carbon Management Index (CMI) 92

Table 5.11 Effect of residue type on total (Cr ), labile (Cd and non-labile carbon (CNd and the Carbon Management Index (CMI). 92 Table 5.12 Mean weight diameter (MWD), percentage of WSA >250 ~m and <125 ~m for vertisol soil with different tillage history and residue placement. 92 Table 5.13 Mean weight diameter (MWD), percentage of WSA >250 ~m and <125 ~m for different residue types and placement of the vertisol soil. 93

xiv