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DIVISION S-3—SOIL BIOLOGY & BIOCHEMISTRY

Soil Aggregation and Carbon and Nitrogen Storage under Soybean Cropping Sequences

Dep. of Soil and Crop Sciences, Texas A&M Univ., 2474 TAMU, College Station, TX 77843-2474

^* Corresponding author (awright{at}ag.tamu.edu).

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Management practices, such as no-tillage (NT) and high-intensity cropping sequences, have the potential to enhance C and N sequestration in agricultural soils. The objectives of this study were to investigate the impacts of conventional-tillage (CT), NT, and multiple cropping sequences on soil organic carbon (SOC) and nitrogen (SON) sequestration and on distribution within aggregate-size fractions in a southcentral Texas soil after 20 yr of treatment imposition. No-tillage management increased soil aggregation compared with CT, with the bulk of SOC and SON storage present in larger aggregate-size fractions (>2 mm, 250 µm to 2 mm) at both soil depths. Multiple cropping systems, such as a grain sorghum [Sorghum bicolor (L.) Moench]/wheat (Triticum aestivum L.)/soybean [Glycine max (L.) Merr] (SWS) rotation and a wheat/soybean (WS) doublecrop had the highest SOC and SON storage, while the continuous monoculture soybean treatment had the lowest storage. Soil organic C and SON storage were significantly greater under NT than CT for all cropping sequences at 0 to 5 cm and for SWS and WS at 5 to 15 cm. At the 0- to 5-cm depth, NT increased SOC storage by 64% and SON storage by 76% compared with CT. However, at 5 to 15 cm, NT only increased SOC storage by 28% and SON storage by 40%. The use of NT showed a greater impact for increasing SON storage than for SOC storage, suggesting that N cycling is an important factor related to soil C sequestration potential.

Abbreviations: CT, conventional-tillage management • NT, no-tillage management • SOC, soil organic carbon • SON, soil organic nitrogen • SOM, soil organic matter • WS, wheat/soybean doublecrop • SWS, sorghum/wheat/soybean rotation

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
INCREASING IMPORTANCE has been placed on the use of agricultural soils for the mitigation of atmospheric CO₂ through sequestration of soil C. This may be achieved by adoption of best management practices, such as NT and improved residue management. Changes in agricultural practices often influence both the quantity and quality of soil organic matter (SOM) and turnover rates. Impacts of tillage on SOM have been well documented, but results vary because of many factors such as soil type, cropping systems, residue management, and climate (Reicosky et al., 1995). Cultivation reduces SOM and alters the distribution and stability of aggregates (Six et al., 1998). No-tillage management can increase soil aggregation and C and N storage, and improve soil physical, chemical, and biological properties (Paustian et al., 1997; Hendrix et al., 1998).

Most impacts of NT on C sequestration have been observed in surface soils near the rooting zone and crop residues (Dick, 1983; Potter et al., 1998; Bossuyt et al., 2002). The greatest increases in SOC are observed in highly-intensive cropping systems, where multiple crops are grown yearly (Wood et al., 1991; Franzluebbers et al., 1995; Ortega et al., 2002). Types of crop residues play important roles in C sequestration and soil aggregation because of the C/N ratios or quality of the residues (Lynch and Bragg, 1985; Franzluebbers et al., 1995; Potter et al., 1998). The degradation of fresh crop residues is often governed by C/N ratios (Oades, 1988; Chesire and Chapman, 1996), but as residues undergo decomposition, they become more recalcitrant and degradation then becomes controlled by lignin contents or lignin/N ratios (Melillo et al., 1982; Tian et al., 1992). Thus, the ability of soils to sequester C is closely related to N.

Macroaggregates often form around particles of undecomposed SOM, thus providing protection from decomposition (Gupta and Germida, 1988; Gregorich et al., 1989; Six et al., 2000). Macroaggregates also form from microaggregates because of the effects of binding agents such as polysaccharides and fungal hyphae (Tisdall and Oades, 1982; Beare et al., 1997). No-tillage management may promote soil aggregation through enhanced binding of soil particles because of increased SOM content (Paustian et al., 2000; Six et al., 2000). Tillage disrupts macroaggregates, releasing bound microaggregates and organic matter contained within macroaggregates, making it more susceptible to decomposition (Six et al., 1998; Gale et al., 2000; Six et al., 2000). Microaggregates are more stable than macroaggregates, and tillage subsequently disrupts large aggregates more than smaller aggregates (Elliott, 1986; Cambardella and Elliott, 1993). Since tillage often increases the proportion of microaggregates to macroaggregates (Six et al., 2000), there may be less crop-derived SOM in CT than in NT soils (Six et al., 1999). Thus, soil C sequestration may be enhanced by maintaining large aggregate-size fractions through the utilization of NT. Data regarding the long-term impacts of tillage and cropping sequences on SOC and SON sequestration and on distribution within aggregate-size fractions is lacking for the southern USA. Specific objectives of this study were to determine the long-term impacts of tillage and multiple cropping sequences on SOC and SON storage and distribution within aggregate-size fractions 20 yr after imposition of NT.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Site Description
A long-term field experiment was initiated in 1982 along the Brazos River floodplain in south-central Texas (30°32' N, 96°26' W). Long-term average annual rainfall in this area is approximately 980 mm and average annual temperature is 20°C. The soil used was a Weswood silty clay loam (fine-silty, mixed, superactive, thermic Udifluventic Haplustepts) with pH 8. Three cropping sequences were established under both CT and NT, and included a grain sorghum/wheat/soybean rotation, a wheat/soybean doublecrop, and a continuous monoculture soybean treatment. Conventional tillage in sorghum and soybean consisted of disking to a depth of 10 to 15 cm after harvest, followed by chiseling to 25 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. For wheat under CT, soil was disked three to four times after harvest. Under NT, no soil disturbance occurred except for banded fertilizer application and planting.

Continuous soybean produced one crop per year. Soybean was planted in 1-m wide rows in June and harvested in October. For the WS doublecrop, wheat was planted in 0.2-m wide rows in November and harvested in May, followed by the soybean crop. For the SWS rotation, sorghum was planted in 1-m wide rows in March and harvested in August, followed by wheat in winter, then soybean the following summer. The rotation resulted in three crops every two years. Soybean received 34 kg P₂O₅ ha^–1 banded preplant with no added N. Nitrogen, as NH₄NO₃, was subsurface applied preplant at 90 kg N ha^–1 for sorghum. Wheat received 68 kg N ha^–1, with half surface broadcast applied shortly after emergence, and half in late February. Field plots measured 4-m wide by 12.2-m long and treatments were replicated four times.

Soil Sampling
Soil samples were taken in November 2002 after soybean harvest from continuous soybean, WS, and SWS treatments. Soil cores were taken to a depth of 20 cm with a 2.5-cm-diam. probe, with 25 cores being taken per plot. The top 15 cm were sectioned into 0- to 5- and 5- to 15-cm depth intervals. Samples from respective depths in each plot were then 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 for measurement of bulk density (Blake and Hartge, 1986).

Soil Analyses
Fractionation of soil aggregates was performed using a wet-sieving procedure (Elliott and Cambardella, 1991; Cambardella and Elliott, 1994). Approximately 90-g soil samples were capillary-wetted to field capacity to minimize slaking following immersion. Wetted soil was immersed in water on a nest of sieves (2 mm, 250 µm, and 53 µm) and shaken vertically 3 cm for 50 times during a 2-min period. Soil aggregates retained on sieves were then backwashed into preweighed containers, oven dried at 50°C for 2 to 3 d, and weighed. Material that passed through the 53-µm sieve was not collected, but the mass of this fraction was determined by calculating 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 from each aggregate-size fraction were then ground to pass a 0.5-mm sieve and analyzed for SOC and SON. Whole soil samples that were not used for aggregate-size fractionation were also analyzed.

Soil organic C was determined using a modified Mebius method (Nelson and Sommers, 1982). Briefly, 0.5 g of soil was digested with 5 mL of 1.0 N K₂Cr₂O₇ and 10 mL of H₂SO₄ at 150°C for 30 min, followed by titration of digests with standardized FeSO₄. Soil organic N was quantified using a Kjeldahl digestion procedure (Gallaher et al., 1976), and NH₄–N was analyzed colorimetrically (Technicon Industrial Systems, 1977).

Since sand-associated C is deemed to be minimal (Elliott et al., 1991), adjustments were made for sand contents in aggregate-size fractions. Approximately 5-g of soil was dispersed in 20 mL of 0.01 M Na₄P₂O₇, shaken for 16 h, and filtered through a 53-µm sieve. Sand retained on sieves was backwashed into pre-weighed containers, dried at 50°C for 2 to 3 d, and weighed. Sand contents of aggregate-size fractions were determined using the proportion of the mass of material remaining on the sieve to the initial 5-g sample.

Statistical Analyses
The experimental design was a split-split plot within a randomized complete block. Tillage treatment served as the main plot, cropping sequence was the split plot, and N rate was the split-split plot, although only one N rate was tested in this study. Data were analyzed with JMP Software (SAS Institute, 1995). Analysis of variance was performed for individual treatment comparisons at P < 0.05 with separation of means by the LSD. The determination of differences between aggregate-size fractions was performed using a three-way ANOVA with factors being cropping sequence, tillage regime, and aggregate-size fractions, with separation of means using the LSD at P < 0.05. Soil organic C and SON storage were calculated as a function of SOC and SON concentrations, percentage aggregate-size fractions of whole soil, and soil bulk density.

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Soil bulk density was not influenced by cropping sequence. No-tillage management increased bulk density compared with CT at 5 to 15 cm but not at 0 to 5 cm. Mean bulk density was 1.19 g cm^–3 at 0 to 5 cm. At 5 to 15 cm, bulk density averaged 1.35 g cm^–3 for CT and 1.63 g cm^–3 for NT.

On a whole soil basis, both SOC and SON concentrations at 0 to 5 cm were significantly higher under NT than CT for all cropping sequences except for SOC under WS (Fig. 1) . No impacts of tillage were observed at 5 to 15 cm, except that SWS has higher SON under NT than CT. Continuous soybean generally had the lowest SOC and SON concentrations at 0 to 5 cm. This result was likely due to continuous soybean contributing less C to soil compared with residues from more-intensive cropping sequences (Franzluebbers et al., 1995). Soil organic C and SON concentrations were significantly higher at 0- to 5-cm than at 5- to 15-cm soil under both CT and NT.

View larger version (43K): [in this window] [in a new window]   Fig. 1. Soil organic carbon (SOC) and organic nitrogen (SON) concentrations at the 0- to 5-cm and 5- to 15-cm soil depths. CT and NT refer to conventional and no-tillage management. SWS, WS, and soybean denote sorghum/wheat/soybean, wheat/soybean, and continuous soybean sequences, respectively. Error bars represent the standard error of the mean.

View larger version (61K): [in this window] [in a new window]   Fig. 2. The percentages of aggregate-size fractions of whole soil as influenced by tillage and cropping sequences at the 0- to 5-cm and 5- to 15-cm soil depths. CT and NT refer to conventional and no-tillage management. SWS, WS, and soybean denote sorghum/wheat/soybean, wheat/soybean, and continuous soybean sequences, respectively. Error bars represent the standard error of the mean.

View larger version (62K): [in this window] [in a new window]   Fig. 3. Sand-free soil organic carbon (SOC) concentrations of four aggregate-size fractions at the 0- to 5-cm and 5- to 15-cm soil depths. CT and NT refer to conventional and no-tillage management. SWS, WS, and soybean denote sorghum/wheat/soybean, wheat/soybean, and continuous soybean sequences, respectively. Error bars represent the standard error of the mean.

View larger version (59K): [in this window] [in a new window]   Fig. 4. Sand-free soil organic nitrogen (SON) concentrations of four aggregate-size fractions at the 0- to 5-cm and 5- to 15-cm soil depths. CT and NT refer to conventional and no-tillage management. SWS, WS, and soybean denote sorghum/wheat/soybean, wheat/soybean, and continuous soybean sequences, respectively. Error bars represent the standard error of the mean.

Soil aggregation and the stability of macroaggregates were affected by the C/N ratio or the quality of crop residues returned to soil. Residue quality plays an important role in regulating long-term SOM storage (Lynch and Bragg, 1985). Residues increase macroaggregate formation due to enhancement of microbial activity and production of binding agents (Golchin et al., 1994; Jastrow, 1996). Sorghum and soybean residues generally have higher quality residues (lower C/N) than wheat straw (higher C/N), and thus are more susceptible to rapid microbial degradation (Ghidey and Alberts, 1993; Franzluebbers et al., 1998; Potter et al., 1998). Hence, residues from cropping sequences with sorghum or soybean are more readily decomposed than wheat residues. Organic matter that quickly decomposes exerts a rapid aggregate stabilization effect, but this effect may be transient (Griffiths and Burns, 1972). However, more slowly decomposed residues take longer for the maximum impact of soil aggregation, and the impacts are often more effective in the long term (Martin and Waksman, 1941).

At 0 to 5 cm, SOC storage was significantly greater under NT than CT for all cropping sequences (Fig. 5) . At 5 to 15 cm, SWS and continuous soybean showed higher SOC storage under NT than CT, but this effect was not observed for WS. No differences between cropping sequences on SOC storage were observed at the 0- to 5-cm depth. At the 5- to 15-cm depth, continuous soybean had the lowest SOC storage under CT and NT, which was related to lower residue production than for higher-intensity cropping sequences.

View larger version (52K): [in this window] [in a new window]   Fig. 5. Soil organic carbon (SOC) and organic nitrogen (SON) storage at the 0- to 5-cm and 5- to 15-cm soil depths. CT and NT refer to conventional and no-tillage management. SWS, WS, and soybean denote sorghum/wheat/soybean, wheat/soybean, and continuous soybean sequences, respectively. Error bars represent the standard error of the mean.

No-tillage management increased SOC storage compared with CT by 70, 51, and 72% at 0 to 5 cm and 24, 10, and 51% at 5 to 15 cm for SWS, WS, and continuous soybean, respectively. No-tillage management increased SON storage compared with CT by 94, 89, and 44% at 0 to 5 cm and 58, 4, and 58% at 5 to 15 cm for SWS, WS, and continuous soybean, respectively. Tillage regime did not have as significant an influence on SOC and SON storage for WS as for SWS and continuous soybean at the 5- to 15-cm depth, even though SOC and SON storage for WS was comparable with SWS. This result occurred because SOC and SON storage under CT was similar for WS and SWS, but under NT, storage was greater for SWS than WS. The percentage increase in SOC and SON storage between CT and NT was comparable between SWS and continuous soybean, even though total SOC and SON storage were significantly greater for SWS than continuous soybean. This was a result of continuous soybean having the lowest SOC and SON storage under CT at both depths, but similar storage to SWS and WS under NT. This was because of enhanced turnover of soybean residues when mixed with soil during tillage, as soybean residues typically have higher N contents than either sorghum or wheat (Franzluebbers et al., 1995).

Short-term increases in SOC under NT are seldom observed in subsurface soils. After 4 yr of NT, SOC increased in the surface 0 to 5 cm, but not in subsurface soil (Wood et al., 1991). Even after 8 yr of NT, increases in SOC storage were observed only at 0 to 5 cm (Ortega et al., 2002). Thus, many years of NT may be required for increases in SOC and SON storage at lower soil depths to occur. In our study, after 20 yr of NT, significant increases in both SOC and SON sequestration were observed for some treatments at 5 to 15 cm, although increases were generally less than those observed at the 0- to 5-cm depth. The use of NT increased SOC and SON storage at 0 to 5 cm by an average of 64 and 76%, respectively. At the 5- to 15-cm depth, NT increased SOC and SON storage by 28 and 40%, respectively. In a related study, after 9 yr of NT, SOC in the top 20 cm of soil was 25% higher under NT than CT (Franzluebbers et al., 1998). Thus, long-term NT and high-intensity cropping systems have potential for significant increases in SOC and SON storage. The use of NT showed a greater impact for increasing SON storage than for SOC storage, suggesting that N cycling is an important factor related to soil C sequestration potential.

Soil organic C and SON storage in aggregate-size fractions were calculated as the total C or N stored ha^–1 within either the 0- to 5-cm or 5- to 15-cm soil depths. Averaged across cropping sequences, SOC storage was significantly greater under NT than CT for all aggregate-size fractions at 0 to 5 cm (Fig. 6) . Continuous soybean exhibited the highest SOC storage in the >2-mm fraction, but exhibited the least storage in the <53-µm fraction. The SWS sequence had the highest SOC storage in the <53-µm fraction under both CT and NT. The largest SOC storage at this depth occurred in the 250-µm to 2-mm fraction. At 5 to 15 cm, SOC storage was greater under NT than CT only in the 250-µm to 2-mm and <53-µm fractions. Among aggregate-size fractions, continuous soybean had lower SOC storage than SWS or WS, and few differences between SWS and WS were observed. The largest SOC storage at the 5- to 15-cm depth was also observed in the 250-µm to 2-mm fraction, followed by the <53-µm fraction, while the least SOC storage was observed in the >2-mm and 53- to 250-µm fractions. Similar observations of C storage in aggregate-size fractions under NT and CT were observed for agricultural soils in Georgia (Bossuyt et al., 2002).

View larger version (51K): [in this window] [in a new window]   Fig. 6. Partitioning of soil organic carbon (SOC) storage among aggregate-size fractions at the 0- to 5-cm and 5- to 15-cm soil depths. CT and NT refer to conventional and no-tillage management. SWS, WS, and soybean denote sorghum/wheat/soybean, wheat/soybean, and continuous soybean sequences, respectively. Error bars represent the standard error of the mean.

View larger version (41K): [in this window] [in a new window]   Fig. 7. Partitioning of soil organic nitrogen (SON) storage among aggregate-size fractions at the 0- to 5-cm and 5- to 15-cm soil depths. CT and NT refer to conventional and no-tillage management. SWS, WS, and soybean denote sorghum/wheat/soybean, wheat/soybean, and continuous soybean sequences, respectively. Error bars represent the standard error of the mean.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Management strategies, such as NT and high-intensity cropping sequences, played significant roles in SOC and SON sequestration. Consequences of NT included improved surface soil aggregation and an increase in the proportion of macroaggregates. Higher SOC and SON storage was observed in macroaggregate than in microaggregate fractions. No-tillage management significantly increased SOC and SON storage for all cropping sequences compared with CT in surface soils, and for SWS and continuous soybean at the lower soil depth. No-tillage management had a more significant impact on SON than SOC sequestration, as NT increased SON to a greater extent than SOC storage. Rotations with multiple crops or high-intensity cropping produced the greatest SOC and SON storage, while continuous soybean generally had the lowest SOC and SON storage, especially under CT. The greatest enhancement of SOC and SON sequestration was achieved through high-intensity cropping sequences coupled with NT. Even under CT, SWS and WS stored more SOC and SON than continuous soybean. Thus, for maximum potential SOC and SON sequestration, multiple cropping sequences coupled with NT was the most desirable management strategy. Further studies detailing the stability of SOC and SON under NT are necessary to provide a better understanding of the long-term potential of agricultural soils to sequester C and N.

	ACKNOWLEDGMENTS

 
This research was funded by the Consortium for Agricultural Soils Mitigation of Greenhouse Gases (CASMGS).

Received for publication June 24, 2003.

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