Published in Soil Sci. Soc. Am. J. 69:141-147 (2005).
© 2005 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
Division S-3—Soil Biology & Biochemistry
Soil Carbon and Nitrogen Storage in Aggregates from Different Tillage and Crop Regimes
Alan L. Wright* and
Frank M. Hons
Dep. of Soil and Crop Sciences, Texas A&M Univ., 2474 TAMU, College Station, TX 77843-2474
* Corresponding Author (awright{at}ag.tamu.edu).
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ABSTRACT
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No-tillage (NT) has the potential to enhance C and N sequestration in agricultural soils of the southern USA, but results may vary with crop species. The objectives of this study were to investigate the impacts of NT, conventional tillage (CT), and crop species on soil organic carbon (SOC) and nitrogen (SON) sequestration and distribution within aggregate-size fractions in a central Texas soil after 20 yr of management. No-tillage increased SOC over CT at the 0- to 5-cm depth by 97, 47, and 72%, and SON by 117, 56, and 44% for continuous grain sorghum [Sorghum bicolor (L.) Moench], wheat (Triticum aestivum L.), and soybean [Glycine max (L.) Merr.], respectively. Crop species had significant impacts on SOC and SON sequestration. On average, the wheat monoculture had greater SOC (9.23 Mg C ha–1) at the 0- to 5-cm depth than sorghum (6.75 Mg C ha–1) and soybean (7.05 Mg C ha–1). No-tillage increased the proportion of >2-mm and 250-µm to 2-mm macroaggregate fractions in soil compared with CT. At the 0- to 5-cm depth, NT increased SOC compared with CT by 158% in macroaggregate fractions, but only 40% in <250-µm fractions. No-tillage increased SON compared with CT by 300, 94, 41, and 39% for >2-mm, 250-µm to 2-mm, 53- to 250-µm, and <53-µm fractions, respectively. Long-term impacts of NT included a greater proportion of macroaggregates and increased C and N sequestration, but impacts were dependent on crop species and varied with soil depth.
Abbreviations: CT, conventional tillage • NT, no-tillage • SOC, soil organic carbon • SOM, soil organic matter • SON, soil organic nitrogen
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INTRODUCTION
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IN RECENT YEARS, increasing attention has focused on the use of agricultural soils for the mitigation of elevated atmospheric CO2 levels through the sequestration of soil C. Ancillary benefits of C sequestration include improved soil structure, fertility, and moisture status. Carbon sequestration potential may be enhanced by adoption of reduced tillage, type of residue management, and altered cropping sequences.
Impacts of tillage on soil organic matter (SOM) dynamics have been well documented, but results vary due to soil type, cropping systems, residue management, and climate (Paustian et al., 1997). Tillage promotes SOM loss through crop residue incorporation into soil, physical breakdown of residues, and disruption of macroaggregates (Paustian et al., 2000; Six et al., 2000a, 2000b). Cultivation alters the distribution and stability of aggregates and reduces SOM (Six et al., 2002). No-tillage promotes soil aggregation through enhanced binding of soil particles as a result of greater SOM content (Jastrow, 1996; Paustian et al., 2000; Six et al., 2002). Macroaggregates often form around particles of undecomposed SOM, providing protection from decomposition (Gupta and Germida, 1988; Gregorich et al., 1989; Six et al., 2002). Microaggregates are more stable than macroaggregates, and thus tillage is more disruptive of large aggregates than smaller aggregates, making SOM from large aggregates more susceptible to mineralization (Cambardella and Elliott, 1993; Six et al., 2002). Since tillage often increases the proportion of microaggregates to macroaggregates (Six et al., 2000a), there may be less crop-derived SOM in CT than NT soils. Thus, C and N sequestration may be enhanced by increasing the proportion of macroaggregates in soil through the utilization of NT.
Most impacts of NT on C sequestration have been observed in surface soils, near the rooting zone and crop residues (Paustian et al., 1997). The greatest increases in SOM are often observed in intensive cropping systems, where multiple crops are grown yearly (Franzluebbers et al., 1995b; Ortega et al., 2002; Wright and Hons, 2004). Crop species play important roles in C sequestration because their residues vary in quantity and quality (e.g., lignin, phenolic content, C/N), which affect their turnover rates in soil (Lynch and Bragg, 1985; Ghidey and Alberts, 1993; Chesire and Chapman, 1996; Martens, 2000b).
The SOM levels in the USA are often related to climatic patterns, generally increasing from south to north due to cooler temperatures and lower decomposition rates, and from west to east due to lower water deficits (Kern and Johnson, 1993; Paustian et al., 1997). The greatest amount of research on C sequestration in agricultural soils has been conducted in cooler, northern climates. However, since native SOM levels are often lower in the southern than the northern USA, it is expected that C and N sequestration potential will be lower in the south. Long-term impacts of tillage and cropping sequences on the distribution of SOM among aggregate-size fractions have not been widely reported for the southern USA. Research regarding the effects of tillage on SOM dynamics and distribution among aggregate-size fractions for various crop species will help to determine the potential of these soils for C sequestration. Specific objectives of this study were to determine the long-term impacts of tillage and crop species on SOC and SON sequestration and distribution within aggregate-size fractions in a central Texas soil 20 yr after treatment imposition.
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MATERIALS AND METHODS
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Site Description
A long-term field experiment was initiated in 1982 along the Brazos River floodplain in central Texas (30°32' N, 94°26' W). Average annual rainfall is approximately 980 mm and annual temperature is 20°C. The soil is a Weswood silty clay loam (fine-silty, mixed, superactive, thermic Udifluventic Haplustepts) with pH 8, having 115 g sand kg–1, 452 g silt kg–1, 310 g clay kg–1, and 94 g CaCO3 kg–1. The experimental design was a split-split plot within a randomized complete block. Tillage treatment served as the main plot, crop species was the split plot, and N rate was the split-split plot, although only the N rate resulting in optimal yield for each crop species was reported. Field plots measured 4-m wide by 12.2-m long and treatments were replicated four times.
Three crop species were grown annually under CT and NT. Crop species were continuous grain sorghum [Sorghum bicolor (L.) Moench], wheat (Triticum aestivum L.), and soybean [Glycine max (L.) Merr.]. Conventional tillage in sorghum and soybean consisted of disking to a depth of 10 to 15 cm after harvest, followed by chiseling to 20 cm, a second disking, and ridging before winter. Conventional-tillage sorghum and soybean also received one to three in-season cultivations annually. Sorghum stalks were shredded for both CT and NT treatments. Wheat under CT was disked three to four times to a depth of 10 to 15 cm after harvest. For NT, minimal soil disturbance occurred except for banded fertilizer application and planting. Sorghum was planted in 1-m-wide rows in March and harvested in August, wheat was planted in 0.18-m-wide rows in November and harvested in May, and soybean was planted in 1-m-wide rows in June and harvested in October. Herbicides were applied for weed control during the growing season and fallow. Nitrogen, as NH4NO3, was banded preplant at 90 kg N ha–1 for sorghum. Wheat received 68 kg N ha–1, with half surface broadcast shortly after emergence, and half in late February. Soybean received 15 kg P ha–1 as triple superphosphate, which was banded preplant with no added N. Soil tests indicated that all nutrients except N were adequately available.
Soil Sampling and Analysis
Soil samples were taken in May 2002 after wheat harvest, August 2002 after sorghum harvest, and November 2002 after soybean harvest using a 2.5-cm-diam. probe, with 25 cores taken per plot to a depth of 15 cm. Soil cores were sectioned into 0- to 5- and 5- to 15-cm depth intervals. Samples from respective depths in each plot were combined, dried at 50°C for 7 d, and passed through a 4.75-mm sieve. Additional triplicate samples were taken using an 8-cm sampler (Blake and Hartge, 1986) for measurement of bulk density (Table 1). Visible pieces of crop residues and roots were removed from soil samples because these large particles were not considered part of SOM.
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Table 1. Average soil bulk density, grain yield, crop residue production, and stover N content of sorghum, wheat, and soybean monocultures under conventional tillage (CT) and no-tillage (NT).
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Soil aggregate-size fractionation was achieved by a wet-sieving method (Elliott and Cambardella, 1991; Cambardella and Elliott, 1994). Following capillary wetting of 90 g of oven-dried soil to field capacity, samples were immersed in water on a nest of sieves (2 mm, 250 µm, and 53 µm) and vertically shaken 3 cm for 50 times during a 2-min period. Water-stable aggregates retained on sieves were backwashed into preweighed containers, oven dried at 50°C for 3 d, and weighed. Soil that passed through the 53-µm sieve was not collected, but contents of this fraction were determined by calculation of the difference between whole soil and the sum of the three aggregate-size fractions (>2 mm, 250 µm–2 mm, 53–250 µm). Aggregate-size fractions included macroaggregates (>2 mm), small macroaggregates (250 µm–2 mm), microaggregates (53–250 µm), and silt + clay associated particles (<53 µm). Subsamples of aggregate-size fractions were ground to pass a 0.5-mm sieve and analyzed for SOC and SON. Whole soil samples that did not undergo aggregate-size fractionation were also analyzed. Total SOC and SON storage were calculated using bulk density, aggregate-size distribution, and SOC and SON concentrations.
The SOC was determined using the modified Mebius method (Nelson and Sommers, 1982). Approximately 0.5 g of soil was digested with 5 mL of 1 N K2Cr2O7 and 10 mL of concentrated H2SO4 at 150°C for 30 min, followed by titration with standardized FeSO4. The SON was quantified using a Kjeldahl digestion procedure (Gallaher et al., 1976), followed by NH4–N determination (Technicon Industrial Systems, 1977).
Statistical Analysis
Data were analyzed using CoStat (Cohort Software, CoStat, Monterey, CA). A one-way ANOVA model was used for individual treatment comparisons at P < 0.05, with separation of means by the LSD. The determination of differences in SOC and SON between aggregate fractions was performed using a three-way ANOVA with factors being crop species, tillage regime, and the proportion of aggregate-size fractions. Significant correlation coefficients were based on P < 0.05.
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RESULTS
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Aggregate Distribution
Aggregate-size distribution was significantly impacted by tillage and crop species at the 0- to 5-cm depth, but seldom at the 5- to 15-cm depth (Fig. 1)
. Under CT at the 0- to 5-cm depth, wheat produced a greater percentage of macroaggregates (>250 µm) than other crops, while soybean produced the lowest percentage of macroaggregates under NT. Sorghum and soybean generally had greater percentages of <250-µm aggregates at the 0- to 5-cm depth under both CT and NT than wheat. No-tillage increased the size of the >2-mm fraction for wheat and sorghum, and the size of the 250-µm to 2-mm fraction for sorghum. For CT, the 250-µm to 2-mm fraction comprised the greatest proportion of soil, and the >2-mm fraction the lowest. Under NT, the 250-µm to 2-mm fraction also comprised the greatest proportion of soil, followed by the >2-mm fraction. No-tillage increased the size of the >2-mm fraction compared with CT in the 0- to 5-cm depth by 389, 23, and 106% for sorghum, wheat, and soybean, respectively, but only increased the size of the 250-µm to 2-mm fraction by 19%.
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Fig. 1. Distribution of soil aggregates in sorghum, wheat, and soybean monocultures under no-tillage (NT) and conventional tillage (CT) at 0- to 5- and 5- to 15-cm depths. Error bars represent the standard error of the mean.
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At the 5- to 15-cm depth, wheat had the highest percentage of >2-mm aggregates under CT, but the lowest 250-µm to 2-mm fraction under CT (Fig. 1). No differences between crop species or effects of tillage were observed for the two smallest aggregate-size fractions. No-tillage generally failed to increase the size of macroaggregate fractions compared with CT at the 5- to 15-cm depth, except for wheat in the 250-µm to 2-mm fraction. At the 5- to 15-cm depth under both tillage treatments, macroaggregate fractions comprised the greatest proportion of soil.
Soil Organic Carbon and Soil Organic Nitrogen Concentrations
The SOC concentrations were significantly impacted by tillage and crop species (Fig. 2)
. At the 0- to 5-cm depth, SOC under CT was highest for wheat, and under NT was lowest for soybean. At the 5- to 15-cm depth, SOC under CT was again highest for wheat, but under NT was highest for soybean. No-tillage increased SOC for all crop species at the 0- to 5-cm depth, but tillage had no effect at the 5- to 15-cm depth.
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Fig. 2. Soil organic C and N concentrations (g kg–1) in sorghum, wheat, and soybean monocultures under no-tillage (NT) and conventional tillage (CT) at 0- to 5- and 5- to 15-cm depths. Error bars represent the standard error of the mean.
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Similar to SOC, wheat under CT had the highest SON concentrations, and under NT, soybean had the lowest concentrations at the 0- to 5-cm depth. At the 5- to 15-cm depth, SON was lowest for soybean under both CT and NT. No-tillage increased SON for all crops at the 0- to 5-cm depth, but had no effect at the 5- to 15-cm depth. The SOC and SON concentrations were significantly correlated (r = 0.87) at the 0- to 5-cm depth. The assumed SOC/SON ratios were not impacted by tillage or depth, but were significantly higher for soybean (C/N = 13) than other crops (C/N = 10), and lower for the >2-mm fraction (C/N = 10) than the average of the three smaller aggregate-size fractions (C/N = 12).
Soil Organic Carbon and Soil Organic Nitrogen Storage
Soil bulk density was used for calculation of total SOC and SON on a per-hectare-by-depth basis (Table 1). Bulk density increased with depth and was often higher for wheat than sorghum or soybean. The SOC and SON were significantly impacted by tillage and crop species (Fig. 3)
. At both soil depths, SOC under CT was greater for wheat than sorghum or soybean, and under NT, greater for wheat than sorghum. No-tillage increased SOC over CT for all crops at the 0- to 5-cm depth. At the 5- to 15-cm depth, however, SOC was greater under NT than CT only for soybean. No-tillage increased SOC over CT for sorghum, wheat, and soybean by 97, 47, and 72%, respectively, at the 0- to 5-cm depth, and for soybean at the 5- to 15-cm depth by 50%.
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Fig. 3. Soil organic C and N (Mg ha–1) in sorghum, wheat, and soybean monocultures under no-tillage (NT) and conventional tillage (CT) at 0- to 5- and 5- to 15-cm depths. Error bars represent the standard error of the mean.
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Under CT, wheat (22.3 Mg C ha–1) had 46% greater SOC in the top 15 cm of soil than sorghum (15.3 Mg C ha–1), and 58% greater SOC than soybean (14.1 Mg C ha–1). Under NT, wheat (23.9 Mg C ha–1) had 22% greater SOC in the top 15 cm than sorghum (19.6 Mg C ha–1) and 7% greater SOC than soybean (22.3 Mg C ha–1).
Soil organic N exhibited similar responses to tillage and crop species as SOC (Fig. 3). At the 0- to 5-cm depth, wheat had the highest SON under both CT and NT, and sorghum had higher SON than soybean under NT. No-tillage also increased SON compared with CT for all crops at the 0- to 5-cm depth. At the 5- to 15-cm depth, wheat had the highest SON under both CT and NT, while sorghum had higher SON than soybean under both tillage regimes. At the 5- to 15-cm depth, NT increased SON compared with CT only for soybean. No-tillage increased SON over CT for sorghum, wheat, and soybean by 117, 56, and 44%, respectively, at the 0- to 5-cm depth. For soybean, SON increased 58% at the 5- to 15-cm depth. Soil organic C and SON were significantly correlated at both the 0- to 5-cm (r = 0.93) and 5- to 15-cm depths (r = 0.82).
Tillage had less significant impacts on SON sequestration for wheat than other crop species. Total SON in the top 15 cm of soil was 36, 15, and 53% greater under NT than CT for sorghum, wheat, and soybean, respectively. Wheat (2.1 Mg N ha–1) under CT had 40% greater SON at 0 to 15 cm than sorghum (1.5 Mg N ha–1) and 90% greater SON than soybean (1.1 Mg N ha–1). Under NT, wheat (2.5 Mg N ha–1) had 19% greater SON at 0 to 15 cm than sorghum (2.1 Mg N ha–1) and 44% greater SON than soybean (1.7 Mg N ha–1).
Distribution of Soil Organic Carbon and Soil Organic Nitrogen among Aggregate-Size Fractions
The SOC for sorghum was lowest under CT, while wheat had the highest levels under NT in the >2-mm fraction at the 0- to 5-cm depth (Fig. 4)
. In the 250-µm to 2-mm fraction, wheat had the highest SOC under both CT and NT. No-tillage increased SOC in the >2-mm fraction for sorghum and wheat, and increased SOC for all crops in the 250-µm to 2-mm fraction. Under both CT and NT, the greatest quantity of SOC was present in the 250-µm to 2-mm fraction. No-tillage increased SOC over CT at the 0- to 5-cm depth by 217 and 98% for the >2-mm and 250-µm to 2-mm fractions, but only 40% for the two smallest aggregate-size fractions.
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Fig. 4. Distribution of soil organic C (Mg C ha–1) among aggregate-size fractions in sorghum, wheat, and soybean monocultures under no-tillage (NT) and conventional tillage (CT) at 0- to 5- and 5- to 15-cm depths. Error bars represent the standard error of the mean.
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In the >2-mm fraction at the 5- to 15-cm depth, wheat had the highest SOC and soybean the lowest under CT (Fig. 4). In the 250-µm to 2-mm fraction, wheat and sorghum had higher SOC under CT than soybean, and wheat had the highest levels under NT. No-tillage increased SOC compared with CT at the 5- to 15-cm depth for sorghum in the >2-mm fraction, wheat and soybean in the 250-µm to 2-mm fraction, and for soybean in the 53- to 250- and <53-µm fractions. Similar to the 0- to 5-cm depth, the 250-µm to 2-mm fraction at the 5- to 15-cm depth had the highest SOC.
The SON in the >2-mm fraction at the 0- to 5-cm depth was highest for wheat under CT, and was highest for wheat and lowest for soybean under NT (Fig. 5)
. In the 250-µm to 2-mm fraction, wheat had the highest SON under CT. Few differences between crop species were observed in the two smallest aggregate-size fractions. No-tillage increased SON compared with CT at the 0- to 5-cm depth in the >2-mm fraction for all crop species, and for sorghum and soybean in the 250-µm to 2-mm fraction. No-tillage increased SON over CT at the 0- to 5-cm depth by 300, 94, 41, and 39% for the >2-mm, 250-µm to 2-mm, 53- to 250-µm, and <53-µm fractions, respectively. Under NT, the >2-mm and 250-µm to 2-mm fractions had the highest SON, while under CT, the 250-µm to 2-mm fraction had the highest SON.
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Fig. 5. Distribution of soil organic N (Mg N ha–1) among aggregate-size fractions in sorghum, wheat, and soybean monocultures under no-tillage (NT) and conventional tillage (CT) at 0- to 5- and 5- to 15-cm depths. Error bars represent the standard error of the mean.
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Impacts of tillage on SON distribution within aggregate-size fractions were more evident in macroaggregate fractions and in surface soil than at the 5- to 15-cm depth (Fig. 5). At the 5- to 15-cm depth, wheat had the greatest and soybean the lowest SON under CT in the >2-mm fraction. In the 250-µm to 2-mm fraction, wheat also had the highest levels at the 5- to 15-cm depth. Crop species had minimal impact on SON in the 53- to 250-µm fraction. No-tillage increased SON compared with CT only for soybean in the >2-mm and 250-µm to 2-mm fractions. In the two smallest aggregate-size fractions, no impacts of tillage on SON were observed. The 250-µm to 2-mm fraction contained the greatest quantity of SON under CT. This fraction, along with the <53-µm fraction, had the greatest SON under NT.
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DISCUSSION
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Aggregate-size distribution was significantly impacted by tillage and crop species at the 0- to 5-cm depth, but seldom at the 5- to 15-cm depth. Residues incorporated into subsurface soils by tillage may have increased aggregation, countering the destructive impacts of tillage on aggregates. Aggregation was generally greater for wheat than other crop species. The 250-µm to 2-mm fraction had the highest percentage of soil under both tillage regimes. The >2-mm fraction had the second highest percentage distribution of soil under NT. Under CT, however, tillage significantly reduced the proportion of soil in this fraction for sorghum and soybean but not for wheat. Differences in aggregation between the crop species may have been due to differences in total crop residue production or the quality of residues. Residues having low N concentrations, such as wheat, generally decompose at slower rates than residues with higher N, such as sorghum and soybean (Ghidey and Alberts, 1993; Franzluebbers et al., 1995b). Since wheat residues often persist longer and increase SOM more than sorghum or soybean (Ghidey and Alberts, 1993), this may account for the greater proportion of macroaggregates observed for the wheat monoculture. Improved soil aggregation for wheat, and the influence of aggregation on the binding and protection of SOM (Six et al., 2002), may explain greater SOC and SON for wheat than sorghum and soybean (Fig. 3).
No-tillage under wheat and sorghum, and to a lesser extent for soybean, increased the proportion of macroaggregates compared with CT. Conventional tillage reduced the proportion of aggregates in >250-µm fractions, which may explain lower SOM under CT. These results show that tillage disrupts aggregates and exposes protected intraaggregate organic matter to microbial decomposition, which results in decreased SOM. Further research is needed to determine the stability of SOM when CT is introduced to soils that have been under long-term NT.
Greater C and N sequestration for wheat than sorghum or soybean may be due to differences in aggregation, crop residue production, N contents of residues, or differences in biochemical aspects of residues. Even though crop residue production was similar between wheat, sorghum, and soybean (Table 1), wheat had significantly greater SOC and SON in surface soil under both CT and NT and under CT in subsurface soil (Fig. 3). Thus, crop residue production alone fails to explain greater SOM for wheat than sorghum and soybean. In Texas, SOC was greater for continuous wheat than sorghum, even though sorghum produced up to 200% more aboveground biomass (Potter et al., 1998). This result was attributed to the lower N content of wheat than sorghum residues.
Residue quality often plays an important role in regulating long-term SOM storage (Lynch and Bragg, 1985). Crop rotations under CT that provide residues with low C/N ratios stimulated decomposition of native SOM to a greater extent than rotations providing residues with higher C/N ratios (Ghidey and Alberts, 1993; Sisti et al., 2004). Average N contents of residues from monocultures varied, with wheat residues having lower N concentrations than sorghum and soybean (Table 1). Under NT, crop rotations have been shown to have minimal effect on native SOM decomposition (Sisti et al., 2004). In our study, the greatest differences in SOM between crop species occurred under CT rather than NT, especially in subsurface soil. The higher N content of soybean residues likely resulted in more rapid decomposition, contributing to lower SOM for soybean than other crops. Biochemical differences between crop residues may also explain variable effects of crop species on aggregation, and ultimately SOC and SON sequestration. Decreases in aggregate stability after soybean growth have been attributed to low phenolic acid content of soybean residues (Martens, 2000a), which may explain the low proportion of >2-mm aggregates at the 0- to 5-cm depth under NT, and generally lower SOC and SON for soybean.
Even though total SOM is often greater under NT than CT, SOM in NT soils is often more labile and susceptible to microbial degradation on disruption of aggregates (Gupta and Germida, 1988; Six et al., 2002). Thus, increases in SOC and SON observed in macroaggregate fractions may be dependent on continuation of NT and residue management. In subsurface soils, tillage had a minimal effect on C and N sequestration, with the exception of the soybean treatment. Residues at the soil surface often experience much wider fluctuations in moisture levels compared with buried residues, which often decompose faster than surface residues (Ghidey and Alberts, 1993). Conventional tillage incorporates residues into the 5- to 15-cm depth, which would not occur under NT. Faster decomposition of crop residues under CT would explain lower SOM for CT than NT in surface soils. However, after 20 yr, soils under CT had similar SOM levels to NT in the 5- to 15-cm depth. Thus, higher residue inputs to subsurface soils under CT were likely offset by higher decomposition rates.
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CONCLUSIONS
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Surface soils in the southern USA, after 20 yr of monocropping with sorghum, wheat, and soybean, showed potential for C and N sequestration. Significant increases in soil C and N were evident after 20 yr of NT in surface soils for all crops, and for soybean in subsurface soils. No-tillage increased the proportion of macroaggregates in soil compared with CT, which contributed to higher SOM levels. Wheat improved soil aggregation and increased the proportion of macroaggregates more than sorghum or soybean. Tillage had less of an effect on soil C and N sequestration for wheat than other crop species. Differences in SOM between crop species were evident, with the wheat monoculture exhibiting the highest SOC and SON levels. The N contents of crop residues likely played important roles in C and N sequestration, as crops producing residues having lower N concentrations resulted in higher SOM levels. Thus, crop species producing lower-quality residues may have a greater potential for C and N sequestration. Tillage reduced the percentage of macroaggregates and exposed SOM to microbial decomposition, which decreased SOM levels. No-tillage increased aggregate formation, which would promote protection of SOM and conservation of soil C and N. Further studies are needed to determine factors influencing the composition and stability of SOM within aggregates.
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ACKNOWLEDGMENTS
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This study was partially funded by the Consortium for Agricultural Soils Mitigation of Greenhouse Gases (CASMGS).
Received for publication March 2, 2004.
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