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a J.C.M. Sá, Universidade Estadual de Ponta Grossa, Cx. Postal 992/3, 84010-330, Ponta Grossa-PR, Brasil
b Universidade de São Paulo-Centro de Energia Nuclear na Agricultura, Av. Centenário 303, 13416-970, Piracicaba-SP, Brasil
c The Ohio State University, School of Natural Resources, 1680 Madison Avenue, Wooster, OH 44691
d The Ohio State University, School of Natural Resources, 2021 Coffey Rd., Columbus, OH 43210
* Corresponding author (dick.5{at}osu.edu)
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSoil Organic Carbon and...Effect of Crop Residues...CONCLUSIONSREFERENCES |
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0.05). There were increased SOC concentrations in the finer particle-size fractions (<20 µm) of no-tillage surface soil compared with the NF or CT-22 soils. However, the percentage of SOC derived from crop residues in no-tillage treatments, as assessed by 13C natural abundance (
), was generally greater in the coarse (>20 µm) than in the finer (<20 µm) particle-size fractions. The C sequestration rate for no-tillage was 80.6 g C m-2 yr-1 for the 0- to 20-cm depth and 99.4 g C m-2 yr-1 for the 0- to 40-cm depth. The no-tillage C sequestration potential for South Brazil was estimated as 9.37 Tg C yr-1.
Abbreviations: CT-22, conventional tillage for 22 yr • NF, native field • NT-10, no-tillage for 10 yr • NT-20, no-tillage for 20 yr • NT-22, no-tillage for 22 yr • PNF-1, 1-yr conversion of native field to cropland by plow tillage • SOC, Soil organic C • TN, total nitrogen • , natural abundance • *,**,***, Significant at the 0.05, 0.01, and 0.001 probability levels, respectively
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSoil Organic Carbon and...Effect of Crop Residues...CONCLUSIONSREFERENCES |
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Agricultural practices can render a soil either a sink or a source of the atmospheric CO2, with direct influence on the greenhouse effect (Lugo and Brown, 1993; Lal et al., 1995). The CO2 contribution to radiative forcing is about 50%, and 22.9% of total CO2 emissions to the atmosphere is attributed to agriculture, deforestation, and land use (Intergovernmental Panel on Climate Change, 1996).
In temperate zones, grassland soils tend to lose 30 to 50% of their original SOC content in the first 40 to 50 yr of cultivation (Campbell and Souster, 1982; Mann, 1985). In contrast, the SOC loss in tropical regions may be several times higher (Lal and Logan, 1995). In Northeast Brazil, Resck (1998) reported a SOC loss of 69% within 5 yr of cultivation by a heavy disk harrow in quartz sand (<15% clay content) and 49% in a Typic Hapludox—Dark Red Latosol (>30% clay content). Plowing decreases aggregate stability, disrupts macroaggregates and exposes SOC to microbial processes (Tisdall and Oades, 1982). As a consequence, the mineralization rates increase due to high aeration, resulting in high CO2 flux to the atmosphere (Elliot, 1986; Reicosky et al., 1995).
Several reports have shown that crop residue mulch associated with no-tillage management improves soil aggregation and increases SOC content (Havlin et al., 1990; Carter, 1992; Cambardella and Elliot, 1992, 1993). However, this increase is generally restricted to the surface soil. Kern and Johnson (1993) reviewed data from 17 field studies comparing no-tillage with conventional-tillage plots in the USA, and observed that SOC gains were 27% for the 0- to 8-cm layer, 16% for the 8- to 15-cm layer, and no gains for depths >15 cm. In tropical zones, a significant impact on SOC concentrations has been observed for the 0- to 10-cm layer (Lal, 1976; Sá, 1993, p. 96; Resck, 1998; Bayer et al., 2000b).
The combination of determining size distribution of water stable aggregates, particle-size fractionation, and 13C techniques is a useful tool to investigate the relationship between crop-residue management and no-tillage on SOC dynamics (Balesdent et al., 2000; Havlin et al., 1990; Carter, 1992; Christensen, 1992; Cambardella and Elliot, 1994; Beare et al., 1994; Jastrow et al., 1996; Rasmussen et al., 1998; Bayer et al., 2000a). In soils of the tropics, particle-size fractionation techniques have been used to characterize relationships between SOC and aggregation at the macro and microaggregate scale (Feller et al., 1996). The concept is that soil organic fractions associated with different sized particles differ in structure and function, and therefore play different roles in SOC turnover (Christensen, 1992). Developing such relationships is crucial to understanding the SOC dynamics, the effect of crop residues on the SOC pool and composition, and C-turnover time in soil. Bayer et al. (2000a)(2000b), using a particle-size fractionation technique combined with electron spin resonance and 13C nuclear magnetic resonance showed that, in southern Brazil, crop residues input from no-tillage and cropping systems resulted in a SOC sequestration rate of 1.33 Mg C ha-1 yr-1. They also reported that SOC associated with sand and silt fractions was less humified than that associated with the finer-size fractions. Nevertheless, there are also studies (Balesdent et al., 1987; Anderson and Paul, 1984; Oades et al., 1987; Six et al., 2000) reporting that the most humified or oldest faction is associated with silt particles.
There are few comparison between plow and no-tillage systems for SOC dynamics in oxisols that consider soil under natural vegetation as a base line representing the steady-state level. Obtaining data on base-line SOC pool is essential for understanding the magnitude of the SOC gain or losses because of the confounding effects of microbial respiration and soil erosion on SOC pool and fluxes. Information about SOC sequestration potential in a no-tillage chronosequence is rarely available, yet it is important for developing strategies for sustainable management of soils. In this study, a chronosequence is understood to mean a series of similar soils that differ from each other in certain properties primarily as a result of time.
This study evaluated the effects of a long-term no-tillage chronosequence in a Brazilian Oxisol on (i) SOC and total N (TN) contents in whole soil and in particle-size fractions, (ii) the amount of soil C derived from crop residues using the 13C technique, and (iii) SOC sequestration rates.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSoil Organic Carbon and...Effect of Crop Residues...CONCLUSIONSREFERENCES |
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In 1989 a border area, which had been converted to cropland at the same time as the NT-20 treatment, but after 2 yr was reverted back to fallow, was converted to no-tillage. This area represents the NT-10 treatment with prior history being 2 yr of cultivation and 8 yr of fallow prior to adoption of no-tillage. The fallow period represented a time when nothing was done to the soil. For the NT-22 treatment, lime was broadcast four times (two times for the NT-10 treatment) on the soil surface at a rate of 1.5 Mg ha-1. Cropping during the summer season in this area consisted of seven crops of soybean and three of corn. Cropping during the winter season comprised of four crops of wheat and six of black oat.
Another site was converted to cropland in June 1996 and represents the plow tillage conversion of the native field (PNF-1). The conversion involved application of lime (3.5 Mg ha-1) and triple superphosphate (140 or 72 kg ha-1 of P) incorporated to 20-cm depth by three separate disking operations. The conversion began 18 mo prior to sampling. The crops sown were soybean (October 1996), black oat (May 1997), and corn (September 1997).
The previous land use at the Ponta Grossa site (Frankanna Farm) was also natural vegetation and the conversion to agriculture was initiated in 1961 (Fig. 1). Soil management practices used during 1961 to 1976 were similar to those of the Tibagi site with the exception of two more years of rice crop before changing to a soybean–wheat rotation. In 1976, a tillage variable was imposed thus permitting comparison of NT-22 with CT-22. The latter involved plow tillage after summer harvest and again after winter harvest followed by two diskings to break the clods. Cropping during the summer season between 1976 until 1998 comprised of 15 crops of soybean, six of corn, and two of black bean (Phaseolus vulgaris L.). Black bean was not considered part of the regular rotation. Cropping during the winter season was comprised of 10 crops of wheat, four of black oat and one of lupine. Winter ryegrass (Lolium multiflorum Lam.) was sown during the last four seasons and removed as forage. Also, this area received liquid cattle manure at the rate of 15 to 20 m3 ha-1 in 1996 and 1997.
At both sites, the 3-yr crop rotation generally was as follows: wheat–soybean—Year 1; black oat–soybean—Year 2; and black oat–corn—Year 3. Two crops per each year corresponded to winter and summer seasons, respectively. Specific details of no-tillage treatments, fertilizers used, total dry biomass (aboveground + root dry biomass) production, and the percentage of dry biomass contributed by each crop in the crop rotation are summarized in Table 3. The aboveground dry biomass was estimated from an index based on the grain yield/shoot ratio. The index was 0.9 for soybean, 1.0 for corn, and 1.0 for wheat. The aboveground biomass for each crop was estimated by multiplying the grain yield by the respective index. The same technique was used to estimate the root dry biomass. The index to obtain root dry biomass for each crop was 0.2 for soybean, 0.25 for corn, 0.2 for wheat, and 0.3 for oat. The data obtained from the grain yield was multiplied by the root index to estimate total root biomass. Total biomass was calculated as the sum of shoot and root biomass.
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Soil Chemical and Mineralogical Analyses for Characterization of Soil Profiles
Soil pH was measured using a 1:2.5 ratio of soil/0.01 M CaCl2 solution (EMBRAPA, 1979). The potential acidity was determined using a 0.01 M calcium acetate solution buffered at pH 7.0 (EMBRAPA, 1979). The exchangeable Al3+, Ca2+, Mg2+, K+, and available P were extracted using a cation- and anion-exchange resin (Raij and Quaggio, 1983, p. 31). The cation-exchange capacity (CEC) was obtained by summing the value of potential acidity and the exchangeable cations. The Bo1 horizon was sampled for each site to identify the clay material by x-ray diffraction (Jackson, 1966, p. 849). Relative quantities of kaolinite and gibbsite were determined using the thermal differential analysis (Jackson, 1966, p. 849) in Ap and Bo1 horizons. The soil texture of all horizons was measured by the pipette method (Gee and Bauder, 1986).
Carbon and Nitrogen Analyses in a Whole Soil Layer
Soil samples from each depth were air dried and ground to pass through a 2-mm sieve. A portion of each sample was ground to pass through 150-µm sieve to determine the SOC and TN contents. The latter was determined by micro Kjeldahl, and SOC by the loss-on-ignition method (Nelson and Sommers, 1982) using a C analyzer. Soil bulk density for each layer was measured by the core method (Blake and Hartge, 1986) using cores of 5.0-cm diam. and 5.0 cm deep for the 5- to 10-, 10- to 20-, and 20- to 40-cm depths. Cores of 5.0-cm diam. by 2.5-cm deep were used for the 0- to 2.5-, and 2.5- to 5-cm depths. The core was taken in the middle of the layer for the 10- to 20- and 20- to 40-cm depths. The SOC and TN pools, expressed as megagrams per hectare for a specific depth, were computed by multiplying the SOC and TN content (g kg-1) with bulk density (g cm-3) and depth (cm).
Particle-Size Fractionation
The particle-size fractionation was done according to Feller (1994). A 40-g oven dry subsample sieved through a 2-mm screen, from each treatment and each depth, was prewetted overnight at 4°C in 200 mL of deionized H2O. Aggregate disruption was accomplished by rotary shaking at a frequency of 50 rpm with three agate balls (10-mm diam.) for 2 h. The amount of soil that did not pass through a 200-mm sieve was used to estimated the 200- to 2000-µm fraction. The soil that passed through the 200-µm sieve was ultrasonicated using a probe-type ultrasonic unit at 240 W for 10 min. This energy level was determined to be the minimum required for the breakdown of macroaggregates into sand- and silt-sized microaggregates, associated organic matter, and primary particles. A suspension sample was taken after each sonication to check the degree of disruption under a microscope. The disrupted soil suspension was passed through two sieves (53- and 20-µm) to obtain the 53- to 200-µm and 20- to 53-µm size fractions. The material remaining on the each sieve was washed and added to the corresponding suspension. The silt (2–20 µm) and clay (<2 µm) fractions were obtained by six to seven centrifugations of the soil suspension that passed through 20-mm sieve. The centrifuge was calibrated to 90 x g (700 rpm) and each centrifugation duration was 3 min. The supernatant liquid from each centrifugation was siphoned and stored in a 1-L glass cylinder and 10 mL of deionized H2O was added in each tube. The procedure was repeated until the supernatant in the tube was clear. The soil pellet in each tube was recovered, and it represented the 2- to 20-µm size fraction. The clay suspension in the 1-L glass cylinder was flocculated with 0.77 g CaCl2, and it represented the <2-µm size fraction.
The Natural Abundance of 13C, Carbon, and Nitrogen Analyses in the Particle-Size Fractions
Natural abundance stable isotope ratios were measured in different particle-size fractions for each depth, based on the method of Cerri et al. (1985), Balesdent et al. (1987), and Angers and Giroux (1996). The 13C/12C ratio and SOC and TN contents were determined by a Mass Spectrometer (Delta Plus, Finnigan Mat; Finnigan Corp., Cincinati, OH) equipped with a gas chromatograph model EA 1110 CHN. The 13C value was calculated from the measured C isotope ratio (R) of the sample and standard gas was calibrated versus the Pee Dee Beleminite (PDB) standard (Eq. [1]) available from the National Bureau of Standards.
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| (1) |
The proportion of C derived from crop residues (X) was calculated according to the method of Angers and Giroux (1996) (Eq. [2]):
 | (2) |
cs equals the 13C value of sample fraction of cropped soil (measured in each particle-size fraction for each depth in PNF-1, NT-10, NT-20, NT-22, and CT 22 treatments); nf equals the 13C value of the NF treatment for each particle-size fraction and for each depth (which represented the natural vegetation dominated by C4 species); and cr equals the measured 13C value of the crop residues. The 13C value of crop residues (-23.8 ± 0.26
) was based on the average of 10 subsamples of all the aboveground biomass collected before the harvest of corn in the NT-20 treatment. These residues represented a mixture from all of the previous crops grown in rotation although it was dominated by black oat, the most recent rotational crop.
Statistical Analyses
The data were statistically analyzed for ANOVA, and means were compared using the Tukey test (LSD0.05). The regression equations were developed by the stepwise procedures (SAS Institute, 1990). Pearson correlation coefficients were used to assess the degree of relationships among variables. Regression equations were used to assess the temporal changes in SOC and TN pools for each soil depth considering the native field as the baseline or the reference point. The rates of SOC sequestration were calculated by determining the slope of the regression line (dy/dx) for each depth for the NT-10, NT-20, and NT-22 treatments. Statistical significance were computed at P 0.05 and P 0.01, and P 0.001 represented by *, **, and ***, respectively.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSoil Organic Carbon and...Effect of Crop Residues...CONCLUSIONSREFERENCES |
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0.05) in soils under long-term no-tillage than those for the NF and CT-22 treatments in the top 0- to 5-cm layer (Table 4). In contrast, depletion of SOC and TN contents in the long-term conventional tillage soil (i.e., the CT-22 treatment) as compared with the NF treatment occurred in the top 0- to 5-cm depth. These data are similar to those reported by Bayer et al. (2000b) for a soil in southern Brazil and by Kern and Johnson (1993) and Dick et al. (1998) in different ecoregions in the USA.
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0.05) for the surface layers of the no-tillage soils compared with the NF soil and the ratios increased with depth for all treatments except for the PNF-1 treatment. This suggests that the availability of N in the surface soil layers was greater and that the N supply is a key component to reduce C losses and increase the SOC content (Ismail et al., 1994). The significant increase of SOC and TN contents upon initial conversion to cropland (i.e., comparing the PNF-1 and NF treatments) for all depths may be because of rapid mineralization of the biomass in natural vegetation stimulated by soil chemical amelioration by liming and P application (Fox, 1980). In addition, the inputs of C and N in crop residue from soybean, black oat, and corn cultivated prior to soil sampling may have also enhanced SOC and TN contents.
The decrease in SOC and TN contents in the NT-10 treatment is thought to be because of the following factors: (i) 2 yr of cultivation and 8 yr of fallow prior to adopting no-tillage may have enhanced microbial activity and released C by respiration; (ii) the residence time of C in the soil profile may have also changed during the fallow period impacting its availability for microbial breakdown (Cihacek and Ulmer, 1997); and (iii) the rate of residue inputs during this period under no-tillage did not compensate high losses due to mineralization.
In comparison with NF treatment, changes in the SOC pools for the 0- to 40-cm soil layer were +21.9 Mg C ha-1 for PNF-1, -4.83 Mg C ha-1 for NT-10, +17.4 Mg C ha-1 for NT-20, +18.9 Mg C ha-1 for NT-22, and -0.13 Mg C ha-1 for CT-22 (Fig. 2) . The gains and losses of the SOC and TN pools for the different treatments varied with depth of soil layer (Fig. 3) , and were determined taking into account bulk density differences which were higher in the no-tillage treatments than in the NF and CT-22 treatment. Most of the increased SOC found in the no-tillage profiles, as compared with that in the NF treatment, was found in the 0- to 5-cm layer for the NT-10 treatment (59%), in the 0- to 10-cm soil layer for the NT-20 treatment (57.9%), and in the 0- to 10-cm soil layer for the NT-22 treatment (81.8%). In contrast, the same comparison of SOC increase for the PNF-1 treatment in the 0- to 10-cm soil layer was only 31%.
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0.05) among these parameters:
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In comparison with the no-tillage treatments, a highly significant (P 0.05) loss of SOC in the CT-22 occurred in the top 10-cm layer. The percentage of the total SOC loss associated which each soil layer, as compared with the NF treatment, was 62.1% for the 0- to 2.5-cm soil layer, 27.6% for the 2.5- to 5-cm layer, and 10.3% for 5- to 10-cm layer. The comparison between NT-22 and CT-22 for 0- to 40-cm layer showed that no-tillage had 19.0 Mg ha-1 more SOC and 1.91 Mg ha-1 more TN (Fig. 4)
. Although soil erosion was not measured, the visual observations of these plots, sited on about a 1% slope, showed minimal erosion. Because the annual input of crop residues was similar for the NT-22 and CT-22 treatments, the difference in SOC and TN content may be attributed to differences in the rates of assimilation and decomposition of residues for the two tillage treatments.
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Soil Organic Carbon and Total Nitrogen Changes in the Particle-Size Fractions |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSoil Organic Carbon and...Effect of Crop Residues...CONCLUSIONSREFERENCES |
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In the 0- to 2.5-cm and the 2.5- to 5-cm soil layers of the no-tillage treatments, the peak SOC concentrations were observed in 20- to 53-µm size fraction. This is an indication that long-term no-tillage with high crop residues input improved the protection and concentration of organic C through formation of silt-sized mineral-organo complexes. This protection may be because of the accumulation of fungal hyphae debris when crop residues are left on the soil surface. Fungi dominate microbial communities in no-tillage systems (Doran, 1980; Hendrix et al., 1986).
It is well known that microorganisms secrete large amounts of polysaccharides that can serve as a strong binding agent (Greenland et al., 1962). According to Baldock et al. (1992) the organic matter in the silt-sized fraction is less decomposed, with higher C/N ratio and higher O–alkyl C concentrations, than organic matter in the <20-µm fraction. In contrast, the silt- (2- to 20-µm) and clay- (<2-µm) sized fractions had much lower C/N ratios than observed for the larger sized fractions. This is because of microbial alterations of soil organic matter and the stabilization of the finer fractions by microbial products including polysaccharides, fungal hyphae, and bacterial cells or colonies encrusted with clay particles (Oades and Waters, 1991). As much as 40 to 60% of the microbial biomass may be associated with microaggregates 2 to 20 µm, depending on the amount and type of clay (Monrozier et al., 1991). Under tropical forest or savanna conditions, bacterial cell walls or colonies at different stages of decomposition can be observed in fractions <20 µm. The amorphous organic matter of microbial origin in clay fractions is indicated by very low xylose/mannose ratios (Feller et al., 1996). The low C/N ratio observed in the finer-size fractions is an indication that the SOM is more stabilized and more aromatic (Bayer et al., 2000b).
Conversion to cropland caused a significant decrease in concentration of SOC in the 200- to 2000-µm size fraction for all depths in the PNF-1 treatment (Fig. 5). This size fraction is easily mineralizable because it is composed of fresh plant residues and debris, a potential source of energy for the microbial biomass.
The trends for TN, although not shown, were found to be similar in all cases to those of SOC. This is because of the highly significant correlation between SOC and TN (r = 0.97, n = 150, P 0.001) in these surface soils.
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Effect of Crop Residues and Long-Term No-Tillage on Soil Organic Carbon Assessed by 13C Natural Abundance and Sequestration Rates |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSoil Organic Carbon and...Effect of Crop Residues...CONCLUSIONSREFERENCES |
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13C signature for the various treatments in the chronosequence (Fig. 6)
. The degree of 13C enrichment remained greater in the deeper layers and finer fractions. These results confirm that the crop-residue input with predominant C3 species changed the organic matter composition and suggest different degrees of humification in different particle-size fractions in the top layers.
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13C values compared with the NF treatment. The introduction of C3 carbon with its unique 13C signature, to the organic C originally present in the soil of the NF treatment, was especially evident in the coarsest particle-size fractions. The differences in 13C values were -6.83 to -4.77 for the 0- to 2.5-cm layer, -5.54 to -4.08 for the 2.5- to 5-cm layer, and -4.0 to -3.05 for the 5- to 10-cm layer (Fig. 6). There were no significant differences amongst treatments below 0- to 10-cm depth, suggesting that the crop residues maintained on the surface contributed a greater amount to the SOC than did the root system. The 13C values in CT-22 were generally between the no-tillage and NF treatments.
The effect of C3 species in the crop rotation is diluted by the 13C corn signal. However, the ratio of C3/C4 species was 7:1, and we did not attempt to isolate the source of error caused by the 13C corn signal in the rotation. Both the crop in rotation and the mineralization of the crop residues control the 13C signal and the mineralization of organic residues is greatly affected by the C/N ratio of the residues. The C/N ratio of crop residues estimated by Derpsch (1983) was 13 to 16 for soybean, 28 to 32 for black oat, 34 to 42 for wheat, and 64 to 68 for corn. These ratios suggest that the rate of mineralization can increase after soybean harvest and the influence of the soybean residues can extend to black oat residues, i.e., the N supplied by the soybean residues can also stimulate high biological activity in C-rich crops that the follow in the rotation. Corn residues provide high C input for microbial biomass. The crop residues with a high C/N ratio, when followed by a crop with a low C/N ratio, may be used by the microbial biomass thus stabilizing this C in soil (Smith et al., 1992).
In the 0- to 10-cm soil layer under no-tillage treatments, the estimate of SOC derived from crop residues was significantly greater in the coarse (200–2000, 53–200, and 20–53 µm) than the fine particle-size fractions (Fig. 7) . The SOC derived from crop residues in these coarse- sized fractions ranged from: (i) 59.2 to 100% for the 0- to 2.5-cm layer, 76.1 to 78.4% for the 2.5- to 5-cm layer, and 48.2 to 96.8% for the 5- to 10-cm layer of the NT-10 treatment; (ii) 52.8 to 100% for the 0- to 2.5-cm layer, 55.7 to 68.2% for the 2.5- to 5-cm layer, and 40.4 to 49.0% for the 5- to 10-cm layer of the NT-20 treatment; and (iii) 69.7 to 100% for the 0- to 2.5-cm layer, 57.8 to 71.7% for the 2.5- to 5-cm layer, and 32.9 to 45.5% for the 5- to 10-cm layer of the NT-22 treatment. In the PNF-1 treatment a significant increase in SOC from crop residues occurred in the coarser fraction and in 0- to 10-cm layer.
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The decline in SOC concentrations by conversion from natural to agricultural ecosystems attains a new equilibrium level in 30 to 50 yr (Wagner, 1981). The SOC loss in the present study during 20 yr (representing the first phase of cropping comprising 10 yr with plow tillage and 10 yr of no-tillage) was 1.09 Mg ha-1 yr-1. The new equilibrium or steady-state level was predicted to occur 
40 yr after the adoption of no-tillage with high inputs of crop residues (Fig. 8)
. The new equilibrium SOC content estimate for the 0- to 20-cm layer ranged from 88.0 to 90.0 Mg ha-1 and represented an increase of 47.1 to 50.5% from the original SOC content in the native field.
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13C technique developed by Cerri et al. (1985) and Balesdent et al. (1987) can be used to assess C input and turnover as affected by crop rotation and tillage (Huggins et al., 1998). The 13C variation between native prairie and no-tillage crop rotation in this study were similar to that reported by Balesdent et al. (1988). The SOC sequestration rate associated with no-tillage in this major ecological region of south Brazil was calculated using the NF treatment as a benchmark. The sequestration rate was 80.6 g C m-2 yr-1 for the 0- to 20-cm layer and 99.4 g C m-2 yr-1 for the 0- to 40-cm layer. The largest contribution to the total sequestration rate was associated with the 0- to 5-cm layer. The contribution of different depths was 31.9 g C m-2 yr-1 for the 0- to 2.5-cm layer, 21.2 g C m-2 yr-1 for the 2.5- to 5-cm layer, 12.5 g C m-2 yr-1 for the 5- to 10-cm layer, 15.1 g C m-2 yr-1 for the 10- to 20-cm layer, and 18.7 g C m-2 yr-1 for the 20- to 40-cm layer. These results of SOC sequestration are higher than the 30 to 70 g C m-2 yr-1 reported by Lal et al. (1998).
In Brazil, 27% of cropland (13.4 million hectares) are cultivated using a no-tillage system (Febrapdp, 2000) of which 70.5% (i.e., 9.43 million hectares) is located in south region (Paraná, Santa Catarina, and Rio Grande do Sul State). Therefore, the SOC sequestration potential of this region is 9.37 Tg C yr-1 (data of this study) to 12.54 Tg C yr-1 (data from Bayer et al., 2000b). This potential is equivalent to assimilation (1 unit of C convert to 3.67 units of CO2) of 34.3 Tg CO2 yr-1 to 46.0 Tg CO2 yr-1.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSoil Organic Carbon and...Effect of Crop Residues...CONCLUSIONSREFERENCES |
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13C, was evident in the 200- to 2000-, 53- to 200-, and 20- to 53-µm particle-size fractions. These trends imply that long-term no-tillage systems protect soil organic matter through formation of stable sand- and silt-sized particles. The C sequestration rate for the top 40-cm layer was 99.4 g m-2 yr-1, and the C sequestration potential for south Brazil is estimated at 9.37 Tg C yr-1. Received for publication June 30, 2000.
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSoil Organic Carbon and...Effect of Crop Residues...CONCLUSIONSREFERENCES |
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