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Long-term cropping systems effects on soil aggregate stability, corn grain yields, and yield stability

Dissertation
Author: Kulbhushan K. Grover
Abstract:
The growing awareness regarding the ecological and economic impacts of intensive agriculture has elevated interest in developing sustainable cropping systems that provide high crop productivity, reduce reliance on external inputs, and improve soil quality. Two studies which are described in the five chapters of this thesis, assessed long-term cropping systems effects on soil aggregate stability, corn grain yields, and yield stability in a 36-year-old crop rotation and fertility experiment, named the Hunter Rotation Experiment (HRE). In the first study, we evaluated long-term effects of ten different crops grown in four cropping systems of HRE on soil aggregate stability, estimated by water-stable aggregates (WSA), during four months of a growing season. Water-stable aggregates depend on temporary binding agents such as roots and fungi which are affected by crop management. Because annual plants support shorter live-root periods, we hypothesized that WSA would be higher in soils under perennials (PR) and double-cropped small grains (SG) than in soils under summer annuals in annual systems (SAa), and summer annuals in perennial and diverse systems (SAp). We also hypothesized that WSA would fluctuate more during a growing season in soils under summer annual crops than in soils under PR and SG. Further, the live-root period in a cropping system and soil microbial biomass carbon (SMBC) would predict WSA better than total soil organic carbon and tillage frequency. We sampled soils of 10 crops from four cropping systems managed under inorganic fertilizers in the HRE: two annual systems: two annual systems: continuous corn (Zea mays L.), corn-soybean [Glycine max (L) Merr.]; a perennial system: 4yr corn-4yr alfalfa ( Medicago sativa L.); and a diverse system: corn-oats (Avena sativa L.)/wheat (Triticum aestivum L.)-2yr red clover (Trifolium pratense L.)+ timothy (Phleum pratense L.) in spring, summer and autumn, 2005. We measured % WSA by slaking and wet-sieving the air-dried 1-2 mm aggregates in water. The WSA under PR, SG and SAp were respectively 106%, 107%, and 84% higher than under SAa. The WSA under SAp did not differ from the WSA under PR and SG, indicating a more pronounced cropping system effect than individual crop-effect. Water-stable aggregates varied more under the summer annual crops than under PR and SG from spring-autumn. Estimated live-root period predicted WSA best along with soil water content and SMBC (R 2 = 0.95). The second study evaluated average corn grain yields, yield trends and yield stability in the four cropping systems under three fertility regimes of the HRE during 1990-2005. We hypothesized that corn yields would be higher, more stable and increase more over time in: (i) perennial and diverse cropping systems compared to annual systems, and (ii) manure-fertility compared to inorganic-fertility regimes. Fertility regimes were inorganic, or manure based on crop N- or P-needs. Mean yields in 4C4A and COW2RT were 10-12% higher than CC and 7% higher in 4C4A than CS. Yields increases over time did not differ (0.28 Mg ha -1 yr-1 ) among all treatments. Coefficient of variation (CV) analysis, however, indicated that yield variability was significantly higher in CC (CV = 28%) than in 4C4A (CV = 21%) across fertility regimes. According to regression stability analysis, response of corn yields to the environmental conditions did not differ among four cropping systems within inorganic and P-based manure fertility. Under N-based manure fertility, however, yields diverged between CC and other systems in the poorest-yielding year but converged in the highest-yielding year. Response of corn yields to the environmental conditions did not differ between manure- and inorganic-fertility, but yields were 7% higher under manure-fertility in the poorest-yielding year. The results indicate that perennial and diverse cropping systems that rotate corn with perennials and winter double-cropped small grains, can promote higher soil aggregate stability than a corn-soybean or continuous corn system. Compared to CC, perennial and diverse systems are likely to produce higher yields across three fertility regimes, and more stable yields under N-based manure fertility.

TABLE OF CONTENTS

List of Figures

viii

List of Tables

x

Acknowledgements

xii

CHAPTER 1

Review of Literature and Research O bjectives

1

Introduction

2

Soil quality

4

Soil aggregate stability

5

Crop yields

20

Objectives and hypotheses

27

References

29

CHAPTER 2

Water - stable Aggregates in Soils under Four Long - term Cropping Systems

46

Abstrac t

47

Introduction

49

Materials and Methods

51

Results

57

Discussion

60

Conclusions

64

References

65

CHAPTER 3

Corn Grain Yields and Yield Stability in Four Long - term Cropping Systems

82

Abstract

83

vii

Introd uction

85

Materials and Methods

88

Results

92

Discussion

95

Conclusions

101

References

102

CHAPTER 4

Overall Conclusions

123

CHAPTER 5

Perennial and Diverse Crop Rotations Improve Soil Quality and Corn Grain Yields

1 26

Introduction

127

Effects of crop rotations on soil quality

128

Effects of crop rotations on corn grain yields

131

viii

LIST OF FIGURES

CHAPTER 2

Figure

1

Water - stable aggregates (1 - 2 mm) in the soils under the 10 crops and four crop types of the Hunter Rotation Experiment, Rock Springs, PA in May, July, Aug. and Nov., 2005.

79

Figure 2

Water - stable aggregates (1 - 2 mm) in the soils of the corn crops in four cropping systems of the Hunter Rotation Experiment, Rock Springs, PA

in May, 2005.

81

CHAPTER 3

Figure 1

Mean corn grain yields for the four cropping systems of the Hunter Rotation Experiment, Rock Springs, PA from 1990 - 2005.

117

Figure 2

Linear regressions of cropping system treatment means on years from 1990 - 2005 in the Hunter Rotation Experiment at Rock Springs, PA.

118

Figure 3

Linear regressions of cropping system treatment means on the environment means fr om 1990 - 2005 under the a) inorganic fertility, b) N - based manure fertility, and c) P - based manure fertility regimes of the Hunter Rotation Experiment, Rock Springs, PA.

119

Figure 4

Corn grain yields in the four cropping systems in the poorest - yield ing year under the a) inorganic fertilizers, b) N - based manure, and c) P - based manure fertility regimes of the Hunter Rotation Experiment, Rock Springs, PA

from 1900 - 2005 .

121

CHAPTER 5

Figure 1

Soil aggregate stability as affected by crop types gr own in four crop rotations during the growing season in 2005 at Rock Springs, P A .

139

Figure 2

Year - to - to variability in corn grain yields during 1990 - 2005 at Rock Springs, P A .

140

ix

Figure 3

Stability of corn yields among four crop rotations during 19 90 - 2005 at Rock Springs , PA .

141

x

LIST OF TABLES

CHAPTER 2

Table 1

Cropping systems and crops studied for comparing soil water - stable aggregates (WSA) in spring, summer and autumn of 2005 in the Hunter Rotation Experiment, Rock Springs, PA.

74

Table 2a

Description of variables based on estimated live - root activity period and tillage frequency in the cropping systems and crops of the Hunter Rotation Experiment, Rock Springs, PA.

75

Table 2b

Estimates of cropping system proportion of time with live roots (CSPLR)

and cropping system proportion of time with tillage (CSPT)

for the four cropping systems and crops of the Hunter Rotation Experiment, Rock Springs, PA.

76

Table 3

Changes in Water - Stable Aggregates (WSA) of soils under the four crop types of the Hunter Rotation Experiment, Rock Springs, PA (Aug. - Nov., 2005).

77

Table 4

Relationship of estimated soil, live root, and tillage variables with percent stable aggregates.

78

CHAPTER 3

Table 1

Mixed - procedure ANOVA

of corn grain yields from 1990 - 2005 in the Hunter Rotation Experiment, Rock Springs, PA.

112

Table 2

Regression slopes of corn yields on environment means.

113

Table 3

Means and ANOVA of year - to - year variation, expressed as the CV, in corn grain yield s from 1990 - 2005 in the Hunter Rotation Experiment, Rock Springs, PA.

114

Table 4

Amount of manure applied to continuous corn and corn - alfalfa cropping systems in N -

and P - based manure fertility treatments .

115

Table 5

Total monthly precipitation and p recipitation variability during the growing season from 1990 - 2005 at the Hunter Rotation Experiment, Rock Springs, PA.

116

CHAPTER 5

Table 1

Three fertility treatments in the long - term study at Rock Springs, P A .

134

xi

Table 2

Four crop rotations in

the long - term study at Rock Springs, P A .

135

Table 3

Soil quality characteristics as affected by crop rotations under inorganic fertilizers at Rock Springs , PA

(1996 - 2000 and 2005 - 2006).

136

Table 4

Effect of inorganic fertilizers and manure applica tions on soil quality under continuous corn at Rock Springs, P A (1996 - 2000).

137

Table 5

Average corn grain yields as affected by crop rotation over a 16 - year period (1990 - 2005) at Rock Springs, P A .

138

xii

Acknowledgements I owe sincere thanks to my major adviser, Dr. Heather Karsten, for her continued support, guidance, constructive criticism, and patience throughout the course of study. She has been very helpful with every aspect of the course of this study. I am also very grateful to my thesis committee members Dr. Greg Roth, Dr. Sjoerd Duiker, Dr. Douglas Archibald, Dr. Mary Ann Bruns, and Dr. Dave Eissenstat for their valuable suggestions and guidance from the proposal stage to the final dissertation writing stage. I would like to express my special thanks to Dr. Greg Roth, who took great personal interest in my success and taught me many valuable things. He provided me several opportunities to travel nationally and internationally to present my research. I greatly enjoyed my interaction with him. I am also thankful to the Penn State Agronomy Research Center farm crew, fellow students, and faculty who helped me in my field and lab studies, and statistical analyses. Rob helped me greatly in planning and implementing my field and lab work. Ephraim and Rupinder helped me with soil analyses and Amy helped in field work. Dr. Marvin Risius and Dr. Durland Shumway provided excellent help in statistical analyses of my complex research dataset. The funding provided by the Department of Crop and Soil Sciences, Penn State University during this study is also appreciated. Thanks are also due to all the friends in the US (old and new ones) and in India for their love and moral support. I will always cherish the great time that we shared together. Special mentions are Harjinder, Devinder, Jaspal, Sushil and Sukhpal. Thanks are also due to Vinod, Harinder, Ranvir, Rajan, Lakhvir, Gurjeet, and Swaran. Surinder bhaji deserve special thanks for all his great support and care.

xiii

It is difficult to explain in words how grateful I am to my wonderful family. I am indebted to my parents for their numerous sacrifices, unconditional support and love. Their strong belief in me is my source of strength and motivation. It is only because of their great dedication that I could achieve this position in my life. I dedicate this dissertation to them. I also owe special thanks to my sisters Meenu and Sharda, my brother Shashi and his wife Anju, and Ravi, in spite of being younger to me, have always been a source of motivation and inspiration for me. Special thanks are also due to my brother-in-law Rakesh, niece, Vishad, and their family for their continued love and support. My father- in-law; mother-in-law; brother-in-law, Parveen, and his wife Vishali bhabhiji, sister-in- law, Soni; and niece, Raghav also deserve special thanks for their love, and support. Last but not least, my better half, Kanchan, and the bond of our love, our daughter, Simran, are my true companions in this journey across seven. They never let me feel alone and have stood beside me in every thick and thin. I am indebted to Kanchan for her great love, support, care, and sacrifices. She showed great dedication and persistence and took excellent care of our daughter while also excelling in her graduate studies. It was only because of her determination and enthusiasm that we both could successfully finish our studies and graduate together. I am also amazingly pleased and thankful for the cooperation and understanding of my 5-yr old daughter, Simran, who often sacrificed her play time and spent weekends and evenings with us at work.

xiv

Finally, I am grateful to the Alimighty for everything, without whose desire, nothing really happens. Ja tu mere val hain ta kya muhchhanda.

1

CHAPTER 1 REVIEW OF LITERATURE AND RESEARCH OBJECTIVES

2

INTRODUCTION Industrial agriculture based on the principles of specialization and intensification has achieved great success for the past several decades (Kirschenmann, 2002; Kirschenmann, 2007). The increased crop production with technological advancement, however, has come at an environmental cost. For instance, conventional cropping systems involving only annual row crops grown with intensive tillage often degrade soil quality (Varvel, 1994; Karlen et al., 2006). Similarly, reliance on external inputs such as synthetic fertilizers and pesticides has environmental concerns due to leakage to non- intended places such as surface and ground waters (Barbash et al., 2001; Domagalski et al., 2008), and high consumption of energy (Pimentel et al., 2005a). Agriculture faces the challenge to produce enough food for growing populations at an acceptable environmental cost (Robertson and Swinton, 2005; Miller, 2008). The growing awareness regarding the ecological and economic impacts of intensive agriculture has elevated interest in developing sustainable cropping systems that provide high crop productivity, reduce reliance on external inputs, and improve soil quality (e.g. Pimentel et al., 2005b; Smith et al., 2007; Posner et al., 2008). A large body of research has studied cropping systems effects on soil quality or crop yields, while relatively few studies have studied both together. Understanding how long-term effects of cropping systems on both soil quality and crop yields could help identify and design sustainable cropping systems that provide high and stable yields along with maintaining soil quality. Two studies that assess cropping systems effects on soil aggregate stability and corn yields and yield stability are described in the five chapters of this thesis. The first

3

chapter reviews the current literature pertaining to soil aggregate stability and crop productivity. Specifically, the first part of this chapter includes a review of research on the effects of crop rotations on soil aggregate stability and the possible mechanisms/factors contributing to the differences among cropping systems. The second part of the first chapter reviews research on crop yields and yield stability. Chapter two is the manuscript describing the long-term effects of cropping systems on soil aggregate stability that has been submitted to the Soil Science Society of America Journal and is under review. Chapter three is the manuscript describing long- term effects of cropping systems on corn grain yields and yield stability, and will be submitted to the Agronomy Journal. Chapter four describes the general conclusions from the two studies on soil aggregate stability and corn yields and yield stability. Chapter five is an extension publication describing results from previous and this research on various soil quality indicators and corn grain yields and yield stability conducted at the same long-term cropping systems study. This publication is intended for extension educators and the farming community to help farmers identify and design sustainable, productive and profitable cropping systems.

4

Soil Quality Soil quality is a broad term indicating overall capacity of a soil to support optimum crop growth and high crop yields and maintain environmental quality (Karlen et al., 1997). It includes physical, chemical and biological indicators. Some examples of the indicators of soil quality are soil aggregate stability, water infiltration, erodibility, soil porosity, pH, CEC, fertility, and microbial and faunal activities. Crop rotations that include perennials as compared to only annual crops, have been reported to increase various indicators of soil quality such as soil aggregate stability, soil organic carbon, microbial biomass C, and water infiltration; and lower bulk density (Tisdall and Oades, 1982; Chaney and Swift 1984; Angers and Mehuys, 1989; Drury et al., 1991; Haynes et al., 1991; Angers et al., 1993; Katsvairo et al., 2002; Karlen et al., 2006; Pikul et al., 2006; Pikul et al. 2007). Cropping systems effects on soil carbon Explanations of how agronomic practices affect soil C include the quantity and quality of crop residues of different crops in cropping systems. Various studies have found that the quantity of crop residues returned to the soil was important in maintaining soil C. Havlin et al. (1990) found that compared to continuous soybean, the rotation regimes with a high frequency of sorghum (i.e. sorghum-soybean and sorghum-sorghum) resulted in 10 and 24% more soil organic C, respectively, under conventional tillage; and 30 and 39% more soil organic C, respectively, under no-till conditions. On the other hand, Drinkwater et al (1998), concluded from their 15-year farming-systems comparative study, that it was the crop type and diversity, not the quantity of the residues added by different cropping systems that affected soil C. A more

5

diverse manure-based system had significantly higher soil organic carbon than a conventional annual cropping system after 15 years of the study, in spite of the fact that both systems had added similar quantities of organic residues. Pikul et al. (2007) also observed a 36% increase in fine particulate organic matter and total soil organic matter content with a 5-yr diverse crop rotation as compared to corn monoculture. Other researchers, however, have found that quantity as well as quality of the crop residues added to the soil influences the soil C (Varvel, 1994). At low rates of N fertilizer, a diverse rotation of corn-oat+ clover-grain sorghum-soybean (C-OCL—SG- SB) and C-SB-SG-OC had significantly more soil C than the C-C and SG-SG monocultures. However, at economically highest rates of nitrogen fertilizer, the C-C and SG-SG monocultures had similar or more soil carbon as compared to the diverse rotations due to increased residues. By contrast, in another long-term study, continuous corn had similar total soil C as a diverse (corn-oats/wheat/2 yr red clover) and a perennial (4 yr corn-4 yr alfalfa) rotation (Bucher, 2002).

Soil aggregate stability Soil aggregates are soil particles joined together more strongly than their surrounding particles (Martin et al, 1955). Aggregate stability is the ability of soil particles to resist the disruptive forces of wind and water (Kemper and Rosenau, 1986). Stability of soil aggregates is an important indicator of soil quality as it plays a significant role in improving soil organic carbon storage, pore size distribution, water infiltration, aeration and plant root growth; and reducing soil erosion (Prove et al., 1990; Angers and Caron, 1998; Barthes and Roose, 2002; Lado et al., 2004).

6

Soil aggregate formation and stabilization Various theories have been proposed in the literature about how aggregates are formed and stabilized in soil. Edwards and Bremner (1967) proposed the microaggregate theory according to which microaggregates (<250 µm) consist of clay-polyvalent metal- organic matter complexes (Cl-P-OM) which are further joined with similar complexes (Cl-P-OM) to form macroaggregates [(Cl-P-OM) x ] y . The authors suggested that microaggregates are more stable than macroaggregates. Tisdall and Oades (1982) proposed the aggregate hierarchy concept according to which different binding agents act at different hierarchical stages of aggregation. Primary soil particles and smaller microaggregates (<20 µm) are joined together into stable microaggregates (20-250 µm) by persistent binding agents such as humidified organic matter and polyvalent metal cation complexes, oxides and alluminosilicates. These microaggregates unite together to form macroaggregates (>250 µm) via the function of temporary agents such as roots and fungal hyphae, and transient binding agents such as polysaccharides. They further stated that microaggregates are less affected by management than macroaggregates. Oades (1984) proposed an opposing order of aggregate formation hierarchy and postulated that macroaggregates form first in soil and that microaggregates form around the roots and fungal hyphae contained within macroaggregates. Results from several studies have substantiated this modified theory (e.g. Elliott, 1986; Golchin et al., 1994; Angers et al., 1997; Gale et al., 2000a,b). In another study, Oades and Water (1991) concluded that aggregate hierarchy exists only in soils where organic matter is the

7

dominating binding agent and does not exist in oxide-rich soils where oxides are the major binding agents. Dexter (1988) offered the ‗principle of porosity exclusion‘; according to which larger aggregates have higher porosity than smaller aggregates because they contain pores between the smaller dense aggregates. The larger aggregates are weaker than the smaller aggregates because the former contain larger pores which act as planes of weakness. Further, Kay (1990) proposed that various binding agents act at different aggregate stages depending upon their size and thus physical accessibility to different pore sizes. The small persistent binding agents such as humic material will thus stabilize microaggregates by accessing smaller pores, while roots and fungal hyphae can access larger pores only and will thus stabilize macroaggregates. It has been reported that the aggregate hierarchy occurs mainly in soils in which organic material is the main binding agent (e.g. Mollisols dominated by 2:1 minerals) but not in soils dominated by oxides (e.g. Oxisols with 1:1 type clay mineralogy) where oxides or electrostatic interactions are the main binding agents (Oades and Waters, 1991; Six et al., 2000; Denef and Six, 2005). Soil aggregates have been classified into two categories: macro (>250 µm) and microaggregates (<250 µm) and this classification holds true for practical consideration (Edwards and Bremer, 1967; Tisdall and Oades, 1982). Microaggregates are relatively stable and less sensitive to management practices (Edwards and Bremer, 1967). The stability of macroaggregates may depend upon the management practices such as crop rotation and tillage, and thus soil organic matter level, except in highly weathered soils dominated by 1:1 clay minerals, in which Al- and Fe-oxides unite particles into

8

macroaggregates (Six et al 2000). Aggregate stability as discussed in this review will refer to stability of aggregates to wetting disintegrative forces of water, thus described as water stable aggregates (WSA) [Amezketa, 1999]. Cropping systems effects on soil aggregate stability Cropping systems significantly influence aggregate stability of soils. Perennial and diverse cropping systems have been found to improve aggregate stability as compared to annual row crop based cropping systems (e.g. Rachman et al., 2003). Peters et al. (1997) reported the highest WSA in soils of a farming system in which green manure, hay or small grain crops were present in winter in addition to summer crops. These systems maintained live plant cover (and hence live roots) for a longer period than a corn-soybean system that did not include living plants during the winter months. Similarly, inclusion of cover crops in the rotation can enhance soil organic matter, biological activity and thus soil aggregate stability by providing live roots during an otherwise fallow period. Winter cover crops of rye, oats and their combination significantly enhanced fungal hyphal length and aggregate stability as compared to a winter fallow (Kabir and Koide, 2002). Similarly, in another study, cover crops of fall rye and annual ryegrass resulted in significantly higher aggregate stability measured as mean weight diameter of aggregates as compared to a bare fallow treatment (Liu et al., 2005). Villamil et al., (2006) also reported up to 17% increase in WSA by including winter cover crops in a no-till corn-soybean system. Cropping systems affect soil aggregate stability due to various factors such as type of crops, sequence of crops, intensity of cropping, crop and soil management techniques (Tisdall and Oades, 1980; Elliott, 1986; Karlen et al, 2006; Villamil et al.,

9

2006; Pikul et al., 2007). These factors can affect soil aggregate stability directly or indirectly by modifying other related soil characteristics. Crop effects Crops differ in their ability to influence aggregate stability and related characteristics of soil, depending upon the quantity and quality of residues left on the soil, and their rooting patterns and activities (Martens, 2000; Power et al., 1998). Perennial crops have been found to enhance soil aggregation as compared to annual crops (Tisdall and Oades, 1980; Stone and Buttery, 1989; Rachman et al., 2003), which appears to be due to higher root contributions and associated microbial activity. In a study comparing annual and perennial crops, Drury et al. (1991) recorded significantly higher soil microbial biomass C and wet aggregate stability in soils under perennial crops of reed canary grass and alfalfa than under annual crops of corn and soybean at different sampling dates from June to September. The authors attributed the increases in microbial activity and WAS under perennials to their increased root exudates and C inputs. Perfect et al. (1990) compared WSA under six perennial forages established two years previously and conventional and zero-till corn. They sampled at different periods during a growing season. They found a significant increase in WSA under all the forages as compared to the corn treatments. In another study, Stone and Buttery (1989) compared the effect of many grass and legume forages in a growth chamber study. They observed highest root growth and aggregate stability under reed canary grass after 80 days of growth. Perennial crops also result in less disturbed ecosystems due to reduced frequency of cultivation, thus directly reducing mechanical disturbance of soil (Elliott, 1986) and

10

benefiting microorganisms such as mycorrhizae fungi which further promote soil aggregation. For example, Jastrow (1987) found that restoration of land under cultivated corn to prairies caused significant increase in percent stable macroaggregates associated with increased root and fungal growth as compared to the land retained under cultivated corn. Tisdall and Oades (1980) studied stability of macroaggregates as affected by various 50-year crop rotations including fallow, annual crops of wheat, pastures, and native virgin land. They observed that cultivation significantly reduced macroaggregate stability as compared to the virgin land. They concluded that macroaggregation can be restored by growing crops with extensive root systems and minimum cultivation such as pastures of rye grass. Thus crop rotations influence soil aggregation because of the crop related factors and their associated management practices. It has been found that particulate organic matter (POM), which consists of plant residues, is abundant in water stable macroaggregates, and that this POM may be central in macroaggregate formation and stabilization (Golchin et al., 1994). Thus an input of organic residues is an important factor for soil aggregation and stabilization. Plant roots Plant roots promote soil aggregate stability by acting as temporary binding agents via various processes such as physical enmeshment of soil particles, penetration and anchorage of soil, release of soil-binding exudates, supply of organic residues, and change in soil water regime (Tisdall and Oades, 1982; Miller and Jastrow, 1990; Chantigny et al., 1997; Angers and Caron, 1998; Jastrow et al., 1998). Aggregate stability

11

is greater in rhizosphere soil than in bulk soil (Caravaca et al., 2002). Reid and Goss (1981) proposed that root growth may be the major factor controlling magnitude of aggregate stability under arable crops. Roots often promote formation and stabilization of macroaggregates by enmeshing soil particles. Miller and Jastrow (1990) observed that root length, the lengths of roots colonized by mycorrhizal fungi, and hyphal lengths of mycorrhizal fungi were all highly correlated with soil aggregate stability measured as geometric mean diameter of water- stable aggregates in a restored prairie soil. In another study from the same experiment, Jastrow et al. (1998) further observed that the restoration of macroaggregate structure was driven by the direct and indirect effects of roots and fungal hyphae. Roots supply organic C to the soil through normal growth and senescence of root segments and hairs. They also release organic materials and thus enhance the microbial activity in soil which furthers the release of organic binding agents quantified and described as hot water-extractable carbohydrates (Lynch and Whipps, 1991). Root mucilage may also promote stability of aggregates by increasing bond strength and reducing wetting rate (Czarnes et al., 2000). Baldock and Kay (1987) observed that roots promote stable aggregates through exudation of material such as polysaccharides. Decomposing roots also promote aggregate stability (Puget and Drinkwater, 2001; Gale et al., 2000a,b). Root derived carbon promotes WSA because of its continuous supply and high retention as occluded particulate organic matter (POM)-carbon in soil aggregates (Gale et al. 2000a, b; Puget and Drinkwater, 2001). Gale et al (2000a) found that root derived intra-aggregate particulate organic matter (POM) was more important than surface residue C in stabilization of small macroaggregates (250-2000 µm) under

12

simulated no-till conditions. They emphasized the importance of plant roots and root exudates in the formation of stable macroaggregates in relatively undisturbed systems like no-till. The amount of organic C supplied by roots depends upon their mass and length (Shamoot et al., 1968); roots with greatest mass often contribute to the greatest increases in soil aggregation (Stone and Buttery, 1989). Root penetration also induces loosening and fragmentation of soil; it decreases proportion of relatively unstable macroaggregates and increases the proportion of relatively stable microaggregates (Materechera et al., 1994). Roots can also increase aggregate stability via water uptake from soil causing a localized drying of soil and adsorption of root exudates on soil particles (Reid and Goss, 1982). Soil microorganisms Soil microorganisms such as bacteria and fungi promote soil aggregation by acting as temporary binding agents (Tisdall and Oades, 1982; Gupta and Germida, 1988; Jastrow et al., 1998). Soil microbial biomass has been found positively correlated with WSA (Gupta and Germida, 1988; Carter, 1992). Drury et al. (1991) found higher soil microbial biomass C (MBC) and WSA under reed canarygrass and alfalfa than under continuous corn and soybean at different sampling dates from June to September. They found that WSA was significantly related with the MBC at all the sampling dates. Bacteria mainly play a role in the stabilization of microaggregates by producing mucliages (Oades, 1993), while fungi may be more important for stabilizing macroaggregates (Tisdall and Oades, 1982; Schutter and Dick, 2002). Fungal hyphae enhance soil aggregate stability through physical enmeshment effects and release of mucilages (Tisdall and Oades, 1982; Haynes and Beare, 1995;

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Abstract: The growing awareness regarding the ecological and economic impacts of intensive agriculture has elevated interest in developing sustainable cropping systems that provide high crop productivity, reduce reliance on external inputs, and improve soil quality. Two studies which are described in the five chapters of this thesis, assessed long-term cropping systems effects on soil aggregate stability, corn grain yields, and yield stability in a 36-year-old crop rotation and fertility experiment, named the Hunter Rotation Experiment (HRE). In the first study, we evaluated long-term effects of ten different crops grown in four cropping systems of HRE on soil aggregate stability, estimated by water-stable aggregates (WSA), during four months of a growing season. Water-stable aggregates depend on temporary binding agents such as roots and fungi which are affected by crop management. Because annual plants support shorter live-root periods, we hypothesized that WSA would be higher in soils under perennials (PR) and double-cropped small grains (SG) than in soils under summer annuals in annual systems (SAa), and summer annuals in perennial and diverse systems (SAp). We also hypothesized that WSA would fluctuate more during a growing season in soils under summer annual crops than in soils under PR and SG. Further, the live-root period in a cropping system and soil microbial biomass carbon (SMBC) would predict WSA better than total soil organic carbon and tillage frequency. We sampled soils of 10 crops from four cropping systems managed under inorganic fertilizers in the HRE: two annual systems: two annual systems: continuous corn (Zea mays L.), corn-soybean [Glycine max (L) Merr.]; a perennial system: 4yr corn-4yr alfalfa ( Medicago sativa L.); and a diverse system: corn-oats (Avena sativa L.)/wheat (Triticum aestivum L.)-2yr red clover (Trifolium pratense L.)+ timothy (Phleum pratense L.) in spring, summer and autumn, 2005. We measured % WSA by slaking and wet-sieving the air-dried 1-2 mm aggregates in water. The WSA under PR, SG and SAp were respectively 106%, 107%, and 84% higher than under SAa. The WSA under SAp did not differ from the WSA under PR and SG, indicating a more pronounced cropping system effect than individual crop-effect. Water-stable aggregates varied more under the summer annual crops than under PR and SG from spring-autumn. Estimated live-root period predicted WSA best along with soil water content and SMBC (R 2 = 0.95). The second study evaluated average corn grain yields, yield trends and yield stability in the four cropping systems under three fertility regimes of the HRE during 1990-2005. We hypothesized that corn yields would be higher, more stable and increase more over time in: (i) perennial and diverse cropping systems compared to annual systems, and (ii) manure-fertility compared to inorganic-fertility regimes. Fertility regimes were inorganic, or manure based on crop N- or P-needs. Mean yields in 4C4A and COW2RT were 10-12% higher than CC and 7% higher in 4C4A than CS. Yields increases over time did not differ (0.28 Mg ha -1 yr-1 ) among all treatments. Coefficient of variation (CV) analysis, however, indicated that yield variability was significantly higher in CC (CV = 28%) than in 4C4A (CV = 21%) across fertility regimes. According to regression stability analysis, response of corn yields to the environmental conditions did not differ among four cropping systems within inorganic and P-based manure fertility. Under N-based manure fertility, however, yields diverged between CC and other systems in the poorest-yielding year but converged in the highest-yielding year. Response of corn yields to the environmental conditions did not differ between manure- and inorganic-fertility, but yields were 7% higher under manure-fertility in the poorest-yielding year. The results indicate that perennial and diverse cropping systems that rotate corn with perennials and winter double-cropped small grains, can promote higher soil aggregate stability than a corn-soybean or continuous corn system. Compared to CC, perennial and diverse systems are likely to produce higher yields across three fertility regimes, and more stable yields under N-based manure fertility.