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Published online 20 September 2006
Published in Soil Sci Soc Am J 70:1896-1905 (2006)
DOI: 10.2136/sssaj2005.0229
© 2006 Soil Science Society of America
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Soil Biology & Biochemistry

Tillage and Nitrogen Effects on Soil Organic Matter Fractions in Wheat-based Systems

Fugen Dou* and Frank M. Hons

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

* Corresponding author (1896figdou{at}iarc.uaf.edu)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Management practices that alter plant residue production and distribution influence SOC (soil organic carbon) dynamics. The objectives of this study were to investigate the impacts of tillage, cropping system, and N fertilization on SOC and soil N pools through physical fractionation of a central Texas soil after 20 yr. Nitrogen fertilization and no-tillage (NT) significantly increased wheat (Triticum aestivum L.) straw yield. Compared with conventional tillage (CT), SOC under NT in surface (0–5-cm) samples was 38, 69, and 68% greater for continuous wheat (CW), wheat–soybean [Glycine max (L.) Merr.]–sorghum [Sorghum bicolor (L.) Moench] rotation (SWS), and double-cropped wheat–soybean (WS), respectively. The greatest SOC was observed in WS under NT with N fertilization, and the lowest occurred in CW under CT without N. Increased cropping intensity increased SOC compared with monoculture. Nitrogen fertilization only significantly increased SOC sequestration under NT. No-tillage increased SOC concentration in all physical size fractions compared with CT. Increased cropping intensity and N fertilization significantly increased SOC sequestration in most size fractions only under NT. Intraparticulate organic matter C (IPOM-C) was proportionally more affected by tillage than total SOC, indicating that this fraction was more sensitive to management. Carbon concentrations in all size fractions were significantly correlated with each other as well as SOC. Our results indicated that NT associated with enhanced cropping intensity and N fertilization sequestered greater SOC and soil total N.

Abbreviations: CT, conventional tillage • CW, continuous wheat • IPOM-C, intraparticulate organic matter carbon • NT, no-tillage • POM, particulate organic matter • ROC, resistant organic carbon • SOC, soil organic carbon • SOM, soil organic matter • STN, soil total nitrogen • SWS, wheat–soybean–sorghum rotation • WS, double-cropped wheat–sorghum


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
SOIL TILLAGE can affect the amount and turnover of SOM (Angers and Carter, 1996; Lal et al., 2004). Compared with CT, NT can alter plant residue input and soil aggregate turnover (Franzluebbers et al., 1995b). Cultivation incorporates aboveground plant residues into the soil matrix and disturbs soil structure by destroying soil macroaggregates and exposing protected organic matter to decomposers (Cambardella and Elliott, 1993), accelerating SOM decomposition. In contrast, NT results in more plant residue on the soil surface, and reduces water and energy exchange between the soil surface and the atmosphere. These reductions decrease soil temperature and increase soil water content, thereby favoring C accumulation (Franzluebbers et al., 1995c; Grant et al., 1997). In addition, greater fungal growth, which contributes to the formation and stabilization of macroaggregates, has been reported under NT (Tisdall and Oades, 1982; Holland and Coleman, 1987). Six et al. (2000a) observed that the rate of macroaggregate formation and degradation (i.e., aggregate turnover) was reduced under NT compared with CT, and led to the formation of stable microaggregates that favored long-term C sequestration. Some studies, however, have reported that reduced tillage or NT did not increase C sequestration compared with CT (Horne et al., 1992; Hussain et al., 1998; Karlen et al., 1989). A slight decrease in SOC with continuous soybean under NT compared with CT was reported by Havlin et al. (1990) in a Grundy silty clay loam. Therefore, to better understand SOC turnover, comparisons of SOC between NT and CT within different cropping systems is necessary.

Crop rotation diversity and N fertilization can also affect SOC dynamics by altering plant residue production and distribution (Franzluebbers et al., 1994; Haynes and Beare, 1996). West and Post (2002) examined a total of 67 global, long-term experiments consisting of 276 paired treatments. In this analysis, enhancement of rotation complexity referred to a change from monoculture to continuous cropping, from crop–fallow systems to continuous monoculture or rotation cropping, and an increase in the number of crops used in a rotational cropping system. They indicated that enhancing rotational complexity could sequester an average extra 20 ± 12 g C m–2 yr–1. In addition, Dick (1992) suggested that crop rotation promotes crop productivity by suppressing deleterious microorganisms that flourish under monocultures.

Physical fractionation has been widely used to investigate C and N dynamics of different SOM pools because of its better relationships with SOM structure and function compared with chemical fractionation (Christensen, 1992, 2001). During physical fractionation, SOM may be separated into various pools with different turnover rates. Particulate organic matter (POM) has been reported to be more labile and therefore more responsive to management practice (Cambardella and Elliott, 1992). Compared with POM, finer organic matter more tightly associated with soil minerals is considered more resistant, with turnover times of hundreds or thousands of years (Christensen, 1992; Golchin et al., 1994). Additional research is needed to determine if these effects also occur in warmer climates and if magnitudes of response are altered. Our study had three hypotheses: (i) NT sequesters more SOC than CT, (ii) an enhanced cropping sequence will increase SOC more than a monoculture, and (iii) labile SOC pools will respond more sensitively to management than slow or resistant pools.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Crop Management and Site Description
A long-term field experiment was initiated in 1982 in the Brazos River floodplain in central Texas (30°32' N, 94°26' W). The soil is classified as a Weswood silty clay loam (fine-silty, mixed, superactive, thermic Udifluventic Haplustept) and contains an average of 115, 452, and 433 g kg–1 of sand, silt, and clay, respectively. The soil has a pH of 8.2 (1:2 soil/water) and an organic C concentration of ~8 g C kg–1 soil. Annual temperature is 20°C and rainfall is 978 mm. A split-split plot within a randomized complete block design was established with tillage as the main plot, cropping sequence as the split plot, and N fertilizer rate as the split-split plot. Split-split plots measured 4 by 12.2 m and treatments were replicated four times.

Three crop species were grown annually under CT and NT. Cropping systems were CW, WS, and SWS. 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 stubble 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 (Ribera et al., 2004). Nitrogen, as NH4NO3, was preplant banded at 90 kg N ha–1 for sorghum. Wheat received 68 kg N ha–1, with half surface broadcast by hand shortly after emergence, and half in late February. Soybean received 15 kg P ha–1 as triple superphosphate, which was preplant banded with no added N. Soil tests indicated that all nutrients except N were adequately available.

Soil Sampling and Analysis
Soil samples were collected shortly after wheat harvest in May 2002. Individual samples consisted of 25 composited cores (25-mm diameter) per split-split plot that were divided into depth increments of 0 to 5, 5 to 15, and 15 to 30 cm. Only results for the 0- to 5-cm depth are reported here since most differences occurred in this layer. Soil was sieved to pass a 2.0-mm screen (visible pieces of crop residues and roots removed) and oven dried for 24 h at 40°C. A portion of the sieved, moist soil was also dried at 60°C for 48 h for chemical and physical analyses. Soil bulk density was determined by the soil core (i.d. 5 cm) method when oven dried at 105°C (Blake and Hartge, 1986). Two cores were taken per split-split plot and at the respective depths and oven dried at 105°C. Means of the two cores per depth represented the bulk density of each split-split plot.

Size and density fractionation procedures were conducted on soil samples to isolate SOC fractions described in the conceptual model of Six et al. (2000b). In brief, soil samples (20 g) on a 250-µm sieve were immersed in deionized water and shaken with glass beads. A continuous and steady water flow through the screen ensured that microaggregates were immediately flushed onto a 53-µm sieve and not exposed to further disruption. After all macroaggregates were broken, material on the 53-µm sieve was washed to ensure that isolated microaggregates were water stable. The intermicroaggregate POM retained together with the microaggregates on the sieve was isolated by density flotation in 1.85 g cm–3 Na polytungstate (Six et al., 2000a). This procedure resulted in the following fractions: (i) coarse, POM (>250 µm), (ii) fine, inter-(intermicroaggregate) POM (53–250 µm), (iii) fine, intra- (intramicroaggregate) POM (53–250 µm), (iv) protected (intramicroaggregate) <53-µm fraction, (v) unprotected <53-µm fraction, (vi) resistant organic C (ROC) in the unprotected <53-µm fraction, and (vii) ROC in the protected <53-µm fraction.

Soil organic C was determined in each fraction using the modified Mebius method (Nelson and Sommers, 1982) and STN (soil total nitrogen) was determined following the procedure of Gallaher et al. (1976), with analysis by an automated salicylic acid modification of the indophenol blue method (Technicon, 1977).

Resistant organic C and N in free form in the <53-µm fraction and within microaggregates were determined using the method suggested by Rovira and Vallejo (2002) with the following modifications. One gram of oven-dry <53-µm sample was hydrolyzed with 25 mL of 6 M HCl at 110°C for 18 h with occasional shaking. After cooling, the unhydrolyzed residue was recovered by centrifuging at 2851g. The process of centrifugation (at 20°C) and decantation was repeated several times with deionized water until neutral pH was reached. Residues were then transferred to preweighed vials, dried at 60°C to constant weight, with C and N being measured using an elemental analyzer (Carlo Erba EA-1108, Lakewood, NJ).

The mass of SOC in different fractions was determined by accounting for organic C concentration, soil bulk density, and the proportion of fractions within the respective depth of soil.

Statistical Analysis
Data were analyzed using SPSS (SPSS, 2002). A three-way ANOVA model was used for individual treatment comparisons at P < 0.05, with separation of means by LSD. If equal variance was not met (Levene's test), then data were transformed according to the minimal lambda value. In the case of significant interaction, comparisons were done within cropping sequence. Significant correlation coefficients were based on P < 0.05.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Soil Organic Carbon and Nitrogen
Soil organic C was significantly affected by tillage, cropping sequence, and N fertilization at both 0- to 5- and 0- to 30-cm depths (Fig. 1a and 1b). The greatest SOC was observed in WS under NT with N fertilization and the lowest with CW under CT without N. Compared with CT, SOC at 0 to 5 cm under NT was 38, 69, and 68% greater in CW, SWS, and WS, respectively (Fig. 1a). When considered at a depth of 0 to 30 cm, NT resulted in 6, 32, and 25% more SOC in CW, SWS, and WS. Thus NT did sequester more SOC than CT but results varied somewhat with cropping intensity and depth. Soil organic C without N fertilization under NT was 20% greater in SWS and 27% greater in WS than CW, possibly because of N contributed by soybean. With N fertilization, SOC under NT was only 18% greater in SWS and 15% greater in WS than CW (Fig. 1a). Under CT, however, SOC was not significantly different among cropping sequences and also was not affected by N fertilization. These results were similar to those reported by Franzluebbers et al. (1994).


Figure 1
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  		Fig. 1. Soil organic C at (a) 0 to 5 and (b) 0 to 30 cm, and distribution of 0- to 5-cm soil into size fractions of (c) >250 µm, (d) 53 to 250 µm, and (e) <53 µm as affected by cropping sequence, tillage, and N fertilization. CW, SWS, and WS indicate continuous wheat, sorghum–wheat–soybean rotation, and wheat–soybean doublecrop, while NT and CT denote no-tillage and conventional tillage.

 
Soil organic C concentration and STN were significantly correlated and therefore similar patterns were also observed for STN (data not shown). Greater SOC and STN under NT with N fertilization may result from (i) greater crop residue input with N fertilization and NT and (ii) the lack of residue redistribution and disturbance under NT. Franzluebbers et al. (1994) proposed that, in contrast to mixing of roots and residues with soil under CT, NT probably reduced the extent of decay due to a less favorable environment for microbial activity. Lower turnover of intra-POM under NT than CT may also partly contribute to C sequestration (Six et al., 2000a).

Distribution of Soil into Size Fractions
Because most changes occurred in surface soil and since both 0- to 5- and 0- to 30-cm depths showed similar results, size fractionation was only done for the 0- to 5-cm samples. About 99% of the soil was distributed into the two smaller size fractions (<53 and 53–250 µm; Fig. 1c, 1d, and 1e), and no significant difference was observed between the proportions of the <53- and 53- to 250-µm fractions. When averaged across all treatments, the amount of soil in the <53-µm fraction was 8% greater than in the 53- to 250-µm fraction. The proportion of soil in the <53-µm fraction was greater under CT than NT with N applied, except for CW. In contrast to the finest fraction, soil distribution into the 53- to 250-µm fraction was numerically greater under NT than CT for all cropping sequences but was significant only for WS. No-tillage significantly increased the proportion of soil in the >250-µm fraction compared with CT (Fig. 1c). Nitrogen application also increased the quantity of soil found in this fraction, at least partially because of increased residue production (Franzluebbers et al., 1995a). The portion of soil distributed into this largest aggregate fraction, however, was <0.015 for all treatments.

Soil Organic Carbon and Nitrogen in Unprotected and Protected <53-µm Fractions
No-tillage significantly increased the concentration of organic C in the <53-µm fraction compared with CT for all cropping sequences (Fig. 2a ). Unprotected organic C in this fraction was 25, 38, and 36% greater for CW, SWS, and WS, respectively, under NT than CT (Fig. 2b). Protected organic C in this fraction under NT was 31, 52, and 50% greater in CW, SWS, and WS, respectively, than under CT (Fig. 2b). Unlike NT, no differences in organic C under CT were observed among cropping sequences (Fig. 2a), indicating that the organic C concentration of this size fraction increased with enhanced cropping intensity only under NT.


Figure 2
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  		Fig. 2. Concentration of unprotected and protected organic C at 0 to 5 cm (a) in the <53-µm fraction, (b) calculated on a whole-soil mass basis, and (c) as a proportion of SOC (soil organic C) as affected by cropping sequence, tillage, and N fertilization. CW, SWS, and WS indicate continuous wheat, sorghum–wheat–soybean rotation, and wheat–soybean doublecrop, while NT and CT denote no-tillage and conventional tillage.

 
Nitrogen application increased organic C concentration in this size fraction with NT, but results were only significant for CW (Fig. 2a). Nitrogen addition did not influence organic C under CT. In contrast, Cambardella and Elliott (1992) did not observe any difference in organic C between NT and CT in the <53-µm fraction. One explanation for this observation may be the difference in cropping intensity. The cropping sequences for our study were CW, SWS, and WS instead of a wheat–fallow system. A trend similar to that of Cambardella and Elliott (1992) was observed for CW, especially with no N fertilization. Another factor might be the difference in sampling depth. We used surface (0–5-cm) soil, while Cambardella and Elliott (1992) used 0 to 20 cm. We found no significant difference for SOC in the <53-µm fraction between NT and CT at the 5- to 15-cm depth (data not shown). Similar results of SOC stratification under NT have been reported (Dick et al., 1991; Needelman et al., 1999; Six et al., 2000a). The concentration of SOC reported by Cambardella and Elliott (1992) was three times larger than our results. This result might be explained by differences in climate and soil texture (Christensen, 1996). The proportion of whole SOC in the <53-µm fraction was greater under CT than NT (Fig. 2c), with proportions ranging from 23 to 39%. Cropping sequence had little impact on the proportions, while N addition tended to decrease it.

Physical protection by microaggregates affected organic C concentration of the <53-µm fraction and the proportion of SOC in this fraction. Compared with the unprotected <53-µm fraction, greater C concentrations were observed in the protected <53-µm fraction for both CT and NT (Fig. 2a). In addition, differences in C concentrations between NT and CT for protected <53-µm fractions were greater than for unprotected <53-µm fractions. In contrast to C concentration, differences in the proportions of SOC in the unprotected <53-µm fraction between NT and CT were greater than the protected <53-µm fraction (Fig. 2c). Total N in the <53-µm fraction was highly related to organic C (Table 1).


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  		Table 1. Correlation matrix of soil C and N pools in physical fractions of 0- to 5-cm soil samples following wheat (n = 48). All pools were significantly correlated at P = 0.01.

 
The <53-µm fraction consists of primary particles, such as clay and silt, and small microaggregates, as well as smaller sized POM (Shang and Tiessen, 2000). Using a size and density fractionation method, Hassink et al. (1997) observed a positive relationship between the proportion of <20-µm particles in a soil and the amount of C that was associated with this fraction in both surface grassland and arable soils. In our case, if we assume that the soil under both NT and CT has the same proportion of particles <53 µm, then the difference in SOC concentration between NT and CT might be explained by several factors. First, organic C storage potential in this fraction under CT did not reach a maximum compared with NT. Second, it may have been influenced by the difference in sieve size. We included larger particles in this fraction. Hassink et al. (1997) also observed increased C for larger size fractions. If we consider the effect of fraction source, however, it appears that the free fraction still did not reach a maximum compared with the SOC concentration under NT from microaggregates.

A model for formation of organo-mineral complexes proposed that SOM attaches to basic cations adsorbed onto soil particles (Edwards and Bremner, 1967). Moreover, Turchenck and Oades (1978) suggested that C associated with this fraction is very stable and not easily affected by agricultural practices. Our results, however, contrast with this observation, indicating that microaggregates may have provided extra physical protection from microbial decomposers compared with free (unprotected) fractions.

Resistant Organic Carbon and Nitrogen
Resistant organic C was highly related to organic C of the <53-µm fraction whether unproptected or protected by microaggregates (Table 1). Concentrations of ROC on both size-fraction and whole-soil bases were significantly greater under NT than CT, except for NT in CW without N application (Fig. 3 ). Based on residue weights after acid hydrolysis, ROC in the unprotected <53-µm fraction under NT was 12, 29, and 46% greater for CW, SWS, and WS, respectively, than under CT (Fig. 3a). Resistant organic C in the protected <53-µm fraction under NT was 18, 58, and 52% greater for CW, SWS, and WS, respectively, than under CT (Fig. 3a). Nitrogen application did not consistently influence ROC under either NT or CT. Resistant organic C increased in the order of SWS > WS > CW, but differences were insignificant (Fig. 3a). Similar tendencies were observed for ROC after conversion to a whole-soil mass basis (Fig. 3b). The proportions of whole-soil SOC as ROC in the protected and unprotected <53-µm fractions were not significantly different (Fig. 3c).


Figure 3
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  		Fig. 3. Concentration of unprotected and protected ROC (resistant organic C) at 0 to 5 cm (a) in the <53-µm fraction, (b) calculated on a whole-soil mass basis, and (c) as a proportion of SOC (soil organic C) as affected by cropping sequence, tillage, and N fertilization. CW, SWS, and WS indicate continuous wheat, sorghum–wheat–soybean rotation, and wheat–soybean doublecrop, while NT and CT denote no-tillage and conventional tillage.

 
Resistant organic N was highly related with ROC in both the unprotected <53-µm fraction or when protected by microaggregates (Table 1). The C/N ratio of resistant organic matter for the protected <53-µm fraction under NT was significantly larger than under CT except for CW (Fig. 4d ). Average C/N ratios of resistant organic matter were 8:1 and 10:1 for CT and NT, respectively. The lower C/N under CT probably indicates that organic matter was more decomposed than under NT. Six et al. (2002) reported a consistently lower C/N of 6 to 8 for the unhydrolyzable fraction, and suggested that it is a microbially-derived SOC pool. Senesi and Loffredo (1999) suggested that resistant organic matter consists of materials from plants such as lignin and suberin, and those from microorganisms such as melanins and paraffinic macromolecules. No-tillage probably exerted more protection for SOC than CT, and thus affected the C/N ratio of resistant organic matter.


Figure 4
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  		Fig. 4. Soil organic C (SOC) at 0 to 5 cm (a) in microaggregates (53–250 um), (b) calculated on a whole-soil mass basis, and (c) as a proportion of SOC, and the C/N ratios of (d) ROM (resistant organic matter) and (e) inter- and (f) intra-POM (particulate organic matter) as affected by cropping sequence, tillage, and N fertilization. CW, SWS, and WS indicate continuous wheat, sorghum–wheat–soybean rotation, and wheat–soybean doublecrop, while NT and CT denote no-tillage and conventional tillage.

 
Microaggregate-Associated Organic Carbon and Nitrogen
A significant interaction (P = 0.02) between tillage and N application was observed for organic C concentrations of microaggregates (53–250 µm) calculated on both fractional and whole-soil bases (Fig. 4a and 4b). Organic C of microaggregates was significantly greater with than without N addition only under NT. Compared with controls, N fertilization under NT increased organic C of microaggregates by 23, 19, and 7% in CW, SWS, and WS, respectively (Fig. 4b). Due to the greater amount of microaggregates with than without N under NT, those differences were enhanced when determined on a whole-soil basis. Under NT, enhanced cropping intensity (SWS and WS) significantly increased SOC associated with microaggregates compared with monoculture (CW). Differences for this fraction, however, were minimal among different cropping sequences under CT.

The proportion of whole-soil SOC in microaggregates under NT was 12, 25, and 35% greater for CW, SWS, and WS, respectively, than under CT (Fig. 4c), and indicated that more organic C was physically protected under NT. Total N in microaggregates was highly related to organic C (Table 1). Greater variation was observed in N than in C. The C/N ratio (8.8) of this fraction was less than that (10.7) of whole soil (data not shown). Tisdall and Oades (1982) suggested that microaggregates consist mainly of <20-µm particles cemented together by plant and fungal debris encrusted with inorganic materials, crystalline oxides, and highly disordered aluminosilicates.

Particulate Organic Carbon and Nitrogen
A significant interaction (P = 0.012) between tillage and N fertilization was observed for IPOM-C (Fig. 5 ). Intraparticulate organic matter C was significantly greater with N fertilization than for unfertilized controls under NT, except for WS. Nitrogen fertilization did not affect IPOM-C for CT calculated on a size-fraction basis (Fig. 5a). Similar results were observed after converting to a whole-soil basis, except that IPOM-C in CW decreased compared with the other cropping sequences (Fig. 5b). The concentration of inter-POM C was significantly lower in CW than the other cropping sequences.


Figure 5
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  		Fig. 5. Concentration of inter- and intra-POM (particulate organic matter) C at 0 to 5 cm (a) in the 53- to 250-µm fraction, (b) calculated on a whole-soil mass basis, and (c) as a proportion of SOC (soil organic C) as affected by cropping sequence, tillage, and N fertilization. CW, SWS, and WS indicate continuous wheat, sorghum–wheat–soybean rotation, and wheat–soybean doublecrop, while NT and CT denote no-tillage and conventional tillage.

 
The proportion of SOC as IPOM-C was significantly greater under NT than CT (Fig. 5c). Nitrogen application also tended to increase this proportion, but was significant only for NT with CW. Cropping sequence also had minimal effect on this proportion, especially under CT. Cambardella and Elliott (1992) observed similar results for tillage treatments. The proportion of SOC as IPOM-C tended to be more sensitive to changes in management than the proportion of SOC as C within the protected <53-µm fraction (Fig. 2c), indicating that labile C was more sensitive than the slower C pool to changes in tillage practice.

Intraparticulate organic N generally mirrored the results observed for C (Table 1). The overall C/N ratio of coarse, inter-POM was greater than that of mineral-associated, or IPOM (Fig. 4e and 4f). The C/N ratio of POM under NT was lower than CT across cropping and N treatments. Based on the morphology and chemical structure of organic materials contained in occluded POM forming the core of microaggregates, Golchin et al. (1995) proposed that the types and amounts of occluded organic materials are dependent on the nature of organic matter input to soil and the microenvironment within soil aggregates. Furthermore, Golchin et al. (1997) suggested that occluded organic materials are more recalcitrant and have higher alkyl and aromatic C contents than free organic materials.

Compared with IPOM-C, coarse inter-POM C concentration was not significantly affected by tillage when calculated on a size-fraction basis (Fig. 5a). On a mass basis, however, NT increased coarse, inter-POM C compared with CT (Fig. 5b). Nitrogen addition also tended to increase the concentration of this fraction calculated on a mass basis, but was only significant for SWS. The proportion of whole-soil SOC as coarse inter-POM followed similar trends as the C concentration of this fraction on a mass basis, with the proportion being greater with NT (Fig. 5c).


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Soil organic C and N were significantly affected by tillage, cropping sequence, and N fertilization. Highest SOC was observed in WS under NT with N fertilization, while the lowest SOC was found in CW under CT without added N. The distribution of SOC and N into different pools was affected by these three factors. Tillage exerted a greater effect on SOC and N distribution than cropping intensity and N fertilization. Organic C and N in all pools were greater under NT than CT. Soil microaggregate pools contributed most to increased SOC sequestration. Compared with total SOC and N, IPOM-C was most affected by tillage. No-tillage affected not only the quantity of SOC and N sequestered, but also the quality. Greater quantities of ROC were found under NT than CT in the <53-µm fractions, whether unprotected or protected by microaggregates. The C/N ratio of the 53- to 250-µm pool, which contained the most organic C and N, was lower under NT than CT, indicating N enrichment in this fraction. All C pools were significantly related with each other, as well as with the N in each pool. Our study indicated that NT in conjunction with increased cropping intensity and N application can effectively increase soil C and N sequestration.


   ACKNOWLEDGMENTS
 
This material is based on work supported by the Cooperative State Research, Education, and Extension Service, USDA, under Agreement no. 2001-38700-11092, by the Consortium for Agricultural Soils Mitigation of Greenhouse Gases (CASMGS).

Received for publication July 13, 2005.


   REFERENCES
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 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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F. Dou, A. L. Wright, and F. M. Hons
Sensitivity of Labile Soil Organic Carbon to Tillage in Wheat-Based Cropping Systems
Soil Sci. Soc. Am. J., August 20, 2008; 72(5): 1445 - 1453.
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