Crop residue and tillage effects on carbon
sequestration in a Luvisol in central Ohio
S.W. Duiker, R. Lal
*School of Natural Resources, Ohio State University, 2021 Coffey Road,Columbus, OH 43210-1085,USA
Received 3 July 1998; received in revised form 2 March 1999; accepted 1 June 1999
Abstract
Soils play a key role in the global carbon cycle. They can be a source or a sink of carbon and in¯uence CO2concentrations in
the atmosphere. In order to calculate the carbon budget of a region, the effect of soil management practices on carbon sequestration in soils needs to be quanti®ed. Objectives of this experiment were to determine: (i) the effects of ridge till, plow till and no-till on the soil organic carbon (SOC) pool; (ii) the SOC loss or sequestration for mulch rates of 0± 16 Mg haÿ1
yearÿ1
wheat (Triticum aestivumL.) straw applied in combination with each tillage method, and (iii) impacts of tillage and crop residue treatments on soil physical quality, including aggregation and porosity. The experiment was initiated in 1989 on a Crosby silt loam (Stagnic Luvisol) in Central Ohio. Seven years after initiation of the experiment, there was a positive, linear effect of residue application rate on SOC contents in all tillage treatments. In the eighth year of the experiment these trends were con®rmed for plow and no-till, but not for ridge till. Linear-regression equations, relating SOC content for the 0±10 cm soil depth to mulch rate, were: for no-till: SOC (Mg haÿ1
)15.210.32 [Residue (Mg haÿ1
yearÿ1
)] (r0.68) and for plow till: SOC11.950.27 [Residue] (p0.72). The carbon conversion ef®ciencies were 8% per year for plow till
and 10% per year for no-till. Detailed sampling at different depths revealed that increases in SOC content were only signi®cant for the 0±5 cm depths of plow and no-till treatments. Effects of crop residue application on water stable aggregation in the 0± 10 cm layer were most pronounced with plow till and ridge till but not with no-till. Water retention characteristics, a measure of pore size distribution, was not in¯uenced by tillage system, but crop residue application had a signi®cant effect on water retention in the 0±10 cm layer at matric suctions of 30±300 kPa. This means that residue application increased macropores of diameters 1±10mm. It is concluded that, depending on the amount of crop residue returned to the soil, the large numbers
of farmers converting from plow to no-till cultivation in the Corn Belt may create an important sink for atmospheric CO2. #1999 Elsevier Science B.V. All rights reserved.
Keywords:Soil organic matter; Aggregation; Carbon sequestration; Mulching; Conservation tillage; Greenhouse effect; Soil quality
1. Introduction
Concern with global warming has led to a surge of interest in evaluating the effect of management prac-tices on carbon sequestration in soils. This interest is justi®ed because soils play a key role in the global
*Corresponding author. Tel.: 292-9069; fax: +1-614-292-7432
E-mail address:[email protected] (R. Lal)
carbon budget, containing 3.5% of the carbon reserves of the earth, compared with 1.7% in the atmosphere, 8.9% in fossil fuels, 1.0% in biota and 84.9% in the oceans (Lal et al., 1995). Land use change is still a major source of carbon emission due to burning and decomposition of vegetation and declining soil organic carbon (SOC) contents in soils. Depending on management, soils can be an important sink for carbon. However, realization of the sink capacity of soils requires adoption of appropriate soil manage-ment practices that increase SOC content.
In the U.S., many farmers have shifted from plow till to conservation tillage systems. The area under conservation tillage (leaving >30% of crop residue on the soil surface) has increased from 29 million ha (26% of planted area) in 1989 to 40 million ha (35%) in 1994 (Bull and Sandretto, 1996), and is expected to increase to 75% by the year 2020 (Lal, 1997). The Corn Belt and Northern Plains are the regions where conservation tillage (especially no-till) is most popular. In 1994, about 14 million ha of cropland in the Corn Belt had >30% residue coverage, while 17 million ha had <30% residue coverage (Bull and Sandretto, 1996). The implications for carbon sequestration of the change to conservation tillage are far reaching and need to be clari®ed for different ecosystems.
Conservation tillage impacts the soil environment in different ways. Crop residue returned to the land can increase or maintain SOC content (Larson et al., 1972; Havlin et al., 1990; Paustian et al., 1997). Many studies have shown that SOC content increases soil aggregation (Christensen, 1986; Skidmore et al., 1986; Unger, 1997a). Further, SOC in microaggre-gates is usually more resistant to decomposition and has a longer turnover time compared with SOC in macroaggregates or labile fractions (Beare et al., 1994; Carter, 1996). Results of many studies con®rm that plow till reduces SOC contents relative to no-till in the topsoil (e.g. Angers et al., 1997; Paustian et al., 1997; Unger, 1997b), although the reverse may be true deeper in the pro®le (Dick, 1983).
Although the positive effect of crop residue on SOC content is well established, only a limited number of studies has evaluated the effect of different residue application rates on SOC content under a range of tillage systems (e.g. Larson et al., 1972). The objec-tives of this experiment were to determine:
1. the effects of ridge till, plow till and no-till on the SOC pool of a Crosby silt loam of central Ohio (the eastern part of the Corn Belt);
2. the SOC sequestered or lost for mulch rates of 0± 16 Mg haÿ1
yearÿ1
wheat straw with each tillage method, and
3. impacts of tillage and crop residue treatments on soil physical quality including aggregation and porosity.
The hypotheses tested were:
1. plow and ridge till result in more decomposition and less carbon sequestration than no-till; 2. residue application increases SOC content; 3. aggregation and porosity increase more due to crop
residue application under no-till compared with ridge and plow till treatments due to decreased soil disturbance.
2. Materials and methods
2.1. Experimental site and statistical design
The experiment was located on Waterman Farm of the Ohio State University (408000 N latitude and
838010 W longitude). Average annual temperature
is 118C and precipitation 932 mm. The soil is a Crosby silt loam (Stagnic Luvisol in the FAO classi-®cation and a ®ne, mixed, mesic Aeric Ochraqualf in the USDA classi®cation). The experiment was initiated in the summer of 1989 as a split plot design with three replicates. Tillage was the main plot and residue rate was the sub-plot (22 m). Tillage treatments were: ridge till, plow till, and no-till. Residue treatments (based on air-dry weight) were 0, 2, 4, 8 and 16 Mg haÿ1
yearÿ1
the soil was not tilled any more during the year. No crop was planted and no fertilizer applied. Herbicides (usually glyphosate) were used to control weeds when necessary.
2.2. Measurements and analyses
Soil physical properties and SOC content were measured from September to November 1996 and from May to July 1997. In fall 1996, one core (7.62 cm high, 7.62 cm diameter) and one bulk soil sample were taken in the center of each plot at 0± 10 cm, 10±20 cm and 20±30 cm depths. Total carbon content of samples passed through a 100-mesh (149mm) sieve was determined by the dry combustion
method (Nelson and Sommers, 1986), neglecting car-bonate content (pH of the Crosby silt loam is ca. 7). In summer of 1997, SOC content at 0±1, 1±3, 3±5 and 5±10 cm depths was measured. A total of three or four samples were taken per plot at each depth with a 5-cm diameter auger, removing soil layer by layer. Samples were taken halfway up the ridge in the case of ridge till treatments to obtain a representative sample for the ridge. The pipet method was used to determine USDA particle size distribution (Gee and Bauder, 1986). Bulk density was determined using the Troxler density probe in summer 1997 (Blake and Hartge, 1986). Volumetric SOC content (kg mÿ3
) was calculated by multiplying the SOC content (g gÿ1
) by bulk density (kg mÿ3
). Water retention characteristics were determined using core samples on a tension table (0±6 kPa) and pressure plates (30± 300 kPa) (Klute, 1986). Water retention was included because it is a measure of pore size distribution (Marshall and Holmes, 1979). Percentage water stable aggregation (%WSA) was determined on50 g air-dry aggregates of diameters 5±8 mm obtained from the bulk samples taken in fall 1996. A sample was placed on the top sieve of a set with diameters of 5, 2, 1, 0.5, and 0.25 mm, wetted under tension and then oscillated submersed in tap water for 30 min. The water stable aggregates in each size fraction were dried at 1058C and corrected for the coarse fraction (Yoder, 1936). The %WSA >0.25 mm and mean weight diameter (MWD) were calculated according to the method described by Kemper and Rosenau (1986).
2.3. Calculations and statistical analysis
Calculation of SOC contents on a per-hectare basis used weighted averages for each sampling layer of the volumetric SOC content of the 0±10 cm layer deter-mined in 1997. Calculation of carbon application rate involved the assumption that the water content of wheat straw was 10% and that its carbon content was 44%, i.e. Carbon(Residue/1.10)0.44 (Lal, 1995). Stepwise linear regression analysis related total %WSA to clay, silt and SOC content. Analysis of variance (F-test) determined signi®cant residue or tillage effects or interactions between tillage and residue rates. Polynomial regression analysis was employed to detect linear, quadratic or cubic relation-ships between residue rates and the dependent vari-ables. Means of tillage treatments were compared using Tukey's test.
3. Results
3.1. Soil organic carbon content
Measurements in 1996 indicated that seven years of residue application had a positive effect on SOC content in the 0±10 cm layer of the soil, but not in the 10±30 cm depth (Fig. 1). Similar results were reported by Dick et al. (1986b) and Havlin et al. (1990). Some authors (Blevins et al., 1983; Dick et al., 1986a; Angers et al., 1997) have reported higher SOC levels at deeper depths under conventional tillage
at the 10±20 cm depth compared with no-till. Climatic factors, drainage, texture, type and depth of soil tillage may be among important factors responsible for dif-ferences among locations and experiments.
The most signi®cant relationship of residue appli-cation rate and SOC content at 0±10 cm depth was linear for all three tillage treatments (Table 1). The data indicated that, on a weight basis, the rate of SOC increase with residue application was lower with plow till then other tillage treatments, and the rate of SOC increase with residue application was comparable in ridge till and no-till treatments. The range of SOC contents in the 0±10 cm layer varied from 7.5 to 18.6 g kgÿ1
.
More detailed measurements of SOC content made in 1997 in the 0±10 cm layer are presented in Fig. 2. Analysis of variance indicated signi®cant effects of tillage methods, residue application rate and interactions between them. With plow till and no-till, signi®cant linear relationships were observed in the 0±5 cm layer. However, no signi®cant relation-ship was observed between residue application rates and volumetric SOC content with ridge till. The most signi®cant linear regression equations relating SOC content in the top 0±10 cm depth to residue application rates are presented in Fig. 3. A reasonable linear relationship was observed for plow till and no-till. No signi®cant relationship was observed with ridge till, only the intercept was highly signi®-cant. These regression equations indicate that the rate of SOC sequestration for each Mg of residue applied was more for no-till than for plow till. The results obtained in 1997 are different from those in 1996, partly because bulk density of no-till treat-ments (average 1.49 Mg mÿ3
) was higher than that of plow till and ridge till treatments (both 1.34 Mg mÿ3
).
The different results for ridge till treatments remain unexplained. For carbon sequestration purposes, how-ever, preference should be given to the 1997 data because sampling was more detailed and repeated and because results are expressed on a volumetric basis.
Results of this experiment are comparable with reported SOC increases on a Typic Paleudalf (silt loam) in Kentucky (Blevins et al., 1983), on a Mollic Ochraqualf (silty clay loam) in northwest Ohio (Dick et al., 1986a), on Typic Fragiudalfs (silt loam) in Northeast Ohio (Dick et al., 1986b; Bajracharya et al., 1998) and on an Aeric Aqualf in Ohio (Bajra-charya et al., 1998). In these studies, SOC content in the top 5±7.5 cm of the soil was higher with no-till compared with plow till. Therefore, the results of the present study can be extrapolated with reasonable con®dence to determine effects of changes in soil and crop residue management on carbon sequestration in silt loams of the eastern Corn Belt.
To calculate the conversion ef®ciency of carbon applied in the residue into SOC, conversion of residue in the equations into carbon is necessary (see Sec-tion 2.3). Carbon can be obtained by dividing the slope of the regression line by 0.4 giving the conver-sion ef®ciency of C applied into SOC. This needs to be divided by the number of years of application (eight years) to obtain the ef®ciency on an annual basis. Following this procedure, the conversion ef®ciency was 0% with ridge till, 8% with plow till and 10% with no-till. Most studies ®nd conversion ef®ciencies in the range of 14% to 21% (Rasmussen and Collins, 1991). Low conversion ef®ciencies in this experiment may be the result of a lack of nutrients necessary for decom-position, since no fertilizer was applied (Himes, 1998).
Table 1
Tillage and residue rate effects on SOC (g kgÿ1) at 0±10 cm depth, 1996 data
Tillage Residue rate (Mg haÿ1yearÿ1)
0 2 4 8 16 significance
Ridge till 9.4 11.1 10.7 14.1 14.9 La
Plow till 8.3 10.3 9.9 11.2 11.5 La
No-till 9.1 11.5 10.3 13.3 15.4 Lb
Significancec ns ns ns ns ns
aSignificant atp0.05 level, Llinear. bSignificant atp
0.001 level, Llinear. cComparison of means using Tukey's test (p
3.2. Water stable aggregation
The %WSA data for the 0±10 cm layer are pre-sented in Table 2. There were no treatment effects on %WSA for soil below the 10-cm depth (data not shown). Even in the 0±10 cm layer, the data are variable and differences often not statistically signi®-cant. The range of %WSA was 30±60%, and that of MWD 0.23±1.12 mm. There was a linear increasing trend in %WSA with increase in residue rate for plow till and ridge till treatments. In contrast, increase in residue application with no-till produced no consistent increase in %WSA. There were no signi®cant effects of tillage methods on the observed %WSA (averaging across all residue treatments: 41.4% for PT, 44.7% for
RT, and 47.1% for NT). The MWD increased linearly with residue rate in ridge till treatments only. Average MWD was 0.45 mm with plow till, 0.55 mm with ridge till and 0.66 mm with no-till. The data show that residue application had a small positive effect on %WSA with plow till and ridge till, and on MWD only with ridge till. Residue application had no effect on %WSA with no-till.
Reports in the literature on the effect of residue on WSA are also mixed. Skidmore et al. (1986) did not observe a positive effect of residue incorporation on %WSA in the top 5 cm of a Kansas silty clay loam. In Denmark, Christensen (1986) reported an increase in mass of dry aggregates of 1±20 mm after 11 years of straw incorporation compared with that of no straw of
a loamy sand, but no effect on aggregation of a sandy clay loam. Unger (1997b) found no difference in MWD of WSA between no-till and cultivated Torrer-tic Paleustolls in Texas. However, he observed that in
some cases the percentage of small aggregates was larger in the no-till than in the plow till treatment. In a study on a high clay Orthic Humic Gleysol in Canada, Angers et al. (1993) observed a decrease in MWD of WSA after four years of plow till compared with no-till. Beare et al. (1994) reported that plow till reduced the size of WSA compared with no-till on a well-drained sandy clay loam in Athens, Georgia. In their study, tillage effects on WSA disappeared below the 5 cm depth, except that the stability index of large aggregates (>2 mm) was higher in the 5±15 cm layer of the no-till compared with the plow till treatments. Multiple regression equations showing the in¯u-ence of clay, silt and SOC on WSA are presented in Table 3. The SOC content was a determinant of %WSA in the topsoil, but not in the subsoil. The SOC, clay and silt contents explained only 50% of the variation in %WSA. These results differ from those reported by Chaney and Swift (1984) for the surface layer of 26 soils in Britain. In their study, organic matter content explained almost 100% of the variation in MWD of WSA. However, they report soils with a wide variation in SOC contents (0.25±5.8% SOC). Unger (1997a) reported a highly signi®cant, positive relationship between SOC and %WSA >0.25 mm. However, Unger (1995) did not observe a positive relationship between SOC and WSA, and attributed this to the fact that differences between treatments were small, while differences between soils were large (Unger, 1997a). For a uniform Crosby silt loam of the present study, there was a signi®cant effect of texture on aggregation. However, the range of SOC contents was limited, which may be an explanation of the lack of differences in %WSA. The results of the present and other studies also indicate a lack of understanding of the factors determining aggregation. Biological factors like micro- and macro-¯ora and fauna are not included in most studies but may play a critical role. Additionally, organic matter quality (e.g. lignin and nitrogen concentrations) may also be an important determinant of WSA.
3.3. Water retention characteristics
Statistical analyses of the data showed that water retention of soil for 0±10 cm depth at some matric suctions was signi®cantly in¯uenced by the residue rate, but not by tillage treatments, and no consistent
interaction effects were observed between tillage and residue rate. Below the 10-cm depth, however, no consistent treatment effects on water retention were observed (results not shown). Soil water retention increased with increasing residue rate for soil water
suction >30 kPa for depths of 0±10 cm (Table 4). Using the relationship 2/gr (where is the matric suction,the surface tension,the density of water,gthe acceleration due to gravity andrthe radius of largest pores still ®lled with water), the water
Table 2
Tillage and residue rate effects on water stable aggregation in top 0±10 cm depth
Till Residue
(Mg haÿ1yearÿ1)
%WSA (mm) MWD
(mm)
Total %WSA
5±8 2±5 1±2 0.5±1 0.25±0.5
Plow till 0 0.3 0.8 3.0 9.8 15.5 0.23 29.5
2 0.5 3.4 5.0 9.0 14.9 0.35 32.8
4 1.1 4.2 7.3 12.2 18.9 0.49 43.8
8 0.3 5.3 11.2 16.9 17.9 0.57 51.6
16 1.1 6.4 10.6 14.1 16.9 0.62 49.1
Significance Ca Lb Lb nsd nsd Lb Lb
Ridge till 0 1.9 3.5 4.7 7.7 14.1 0.43 31.9
2 0.7 5.6 7.4 13.5 18.6 0.53 45.8
4 0.9 5.3 7.4 12.4 17.5 0.52 43.6
8 0.6 5.5 9.1 9.5 24.9 0.53 49.5
16 2.1 7.1 9.7 16.4 17.4 0.72 52.7
Significance nsd nsd Lb nsd Qc nsd La
No-till 0 0.5 2.3 5.6 12.6 18.5 0.36 39.4
2 6.3 9.9 11.4 17.6 15.6 1.12 60.6
4 1.5 4.2 6.3 7.8 19.2 0.47 39.0
8 2.6 7.6 7.7 11.8 16.7 0.70 46.4
16 2.9 5.7 7.3 7.8 26.5 0.65 50.1
Significance nsd nsd nsd nsd Lb nsd nsd
aSignificant atp0.1 level, Ccubic. bSignificant atp0.05 level, Llinear. cSignificant atp0.001 level, Qquadratic. dnsnot significant
Table 3
Stepwise regression analysis of WSA vs. clay, silt and SOC content
Depth (cm) Regression equation r
0±30 WSA0.3060.0039 clay 0.26
WSA ÿ0.6880.016 clay0.013 silt 0.59
WSA ÿ0.8520.126 SOC0.018 clay0.013 silt 0.57
0±10 WSA0.1510.267 SOC 0.50
WSA ÿ0.0580.260 SOC0.005 silt 0.57
WSA ÿ1.0390.296 SOC0.015 clay0.015 silt 0.68
10±30 WSA0.2190.0060 clay 0.40
WSA ÿ0.8630.0192 clay0.014 silt 0.65
WSA ÿ0.823ÿ0.069 SOC0.019 clay0.015 silt 0.66
retention curve can be used to assess pore size dis-tribution. At 208C, the relationship becomes:r0.15/ (where both, and r are in cm; Marshall and Holmes, 1979). The higher water content at suction ranges from 30 to 300 KPa indicates that residue application increases the amount of macropores with diameters from 1 to 10mm. Increased macroporosity
is a result of higher SOC content and possibly of higher microbial and earthworm activity.
4. Conclusions
A linear relationship was observed between the residue application rate and volumetric SOC content for plow till and no-till, but not for ridge till. The conversion ef®ciency of residue carbon into SOC was lower for plow till (8%) than for no-till (10%), con-®rming the ®rst hypothesis that carbon sequestration rates under no-till are higher than under plow till. The SOC levels increased with residue level in both, no-till and plow till, con®rming the second hypothesis. No increases in SOC contents were observed for ridge till (1997 measurements), the reasons for which remain obscure. There was no increase in %WSA with residue rate in case of no till, but a slight increase with the other tillage treatments. Water retention was not sig-ni®cantly different between tillage treatments, but residue application rate had a positive effect on water retention at matric suctions between 30 and 300 KPa, indicating an increase in macropores (1±10mm) due to
residue application. There was no increase in aggre-gation and porosity with residue application under
no-tillage compared with the other no-tillage treatments, thus leading to rejection of the third hypothesis. There was a positive effect of residue application on aggregation in plow till and ridge till treatments and on porosity in all treatments.
References
Angers, D.A., Bolinder, M.A., Carter, M.R., Gregorich, E.G., Drury, C.F., Liang, B.C., Voroney, R.P., Simard, R.R., Donald, R.G., Beyaert, R.P., Martel, J., 1997. Impact of tillage practices on organic carbon and nitrogen storage in cool, humid soils of eastern Canada. Soil Tillage Res. 41, 191±201.
Angers, D.A., Samson, N., LeÂgeÁre, A., 1993. Early changes in water-stable aggregation induced by rotation and tillage in a soil under barley production. Can. J. Soil Sci. 73, 51±59. Bajracharya, R.M., Lal, R., Kimble, J.M., 1998. Long-term tillage
effects on soil organic carbon distribution in aggregates and primary particle fractions of two Ohio soils. In: Lal, R., Kimble, J.M., Follett, R.F., Stewart, B.A. (Eds.), Management of C Sequestration in Soils. CRC Press, Boca Raton, Fl, pp. 113±123.
Beare, M.H., Hendrix, P.F., Coleman, D.C., 1994. Water-stable aggregates and organic matter fractions in conventional and no-tillage soils. Soil Sci. Soc. Am. J. 58, 777±786.
Blake, G.R., Hartge, K.H., 1986. Bulk density. In: Klute, A. (Ed.), Methods of Soil Analysis. Part I. Physical and Mineralogical Methods, second ed. Agronomy Monograph No 9. ASA and SSSA, Madison, WI, pp. 363±382.
Blevins, R.L., Smith, M.S., Thomas, G.W., Frye, W.W., 1983. Influence of conservation tillage on soil properties. J. Soil Water Conserv. 38, 301±305.
Bull, L., Sandretto, C., 1996. Crop residue management and tillage system trends. Statistical Bulletin Number 930. USDA-ERS, Washington DC.
Carter, M.R., 1996. Analysis of soil organic matter storage in agroecosystems. In: Carter, M.R., Stewart, B.A. (Eds.), Table 4
Residue effects on soil water content (volume %) for 0±10 cm depth
Suction (kPa) Mulch rate (Mg haÿ1yearÿ1)
0 2 4 8 16 significance
0 45.25 43.19 43.72 45.15 45.45 nsc
0.01 43.94 42.69 41.53 41.99 42.82 Qa
3 42.70 41.65 40.17 40.60 41.89 Qa
6 43.02 40.46 39.67 39.84 41.00 nsc
30 28.55 30.09 30.09 31.25 33.57 Lb
100 26.03 28.92 28.03 29.33 31.66 Lb
300 23.83 27.06 26.18 28.55 30.03 Lb
Structure and Organic Matter Storage in Agricultural Soils. CRC/Lewis Publishers, Boca Raton, FL, pp. 3±11.
Chaney, K., Swift, R.S., 1984. The influence of organic matter on aggregate stability in some British soils. J. Soil Sci. 35, 223± 230.
Christensen, B.T., 1986. Straw incorporation and soil organic matter in macro-aggregates and particle size separates. J. Soil Sci. 37, 125±135.
Dick, W.A., 1983. Organic carbon, nitrogen, and phosphorus concentrations and pH in soil profiles as affected by tillage intensity. Soil Sci. Soc. Am. J. 47, 102±107.
Dick, W.A., Van Doren Jr., D.M., Triplett Jr., G.B., Henry, J.E., 1986a. Influence of long-term tillage and rotation combinations on crop yields and selected soil parameters. I. Results obtained for a Mollic Ochraqualf Soil. Research Bulletin 1180. OARDC, Wooster, OH.
Dick, W.A., Van Doren Jr., D.M., Triplett Jr., G.B., Henry, J.E., 1986b. Influence of long-term tillage and rotation combinations on crop yields and selected soil parameters. II. Results obtained for a Typic Fragiudalf soil. Research Bulletin 1181. OARDC, Wooster, OH.
Gee, G.W., Bauder, J.W., 1986. Particle-size analysis. In: Klute, A. (Ed.), Methods of Soil Analysis. Part I: Physical and Mineralogical Methods, second ed. Agronomy Monograph No 9. ASA and SSSA, Madison, WI, pp. 383±412.
Havlin, J.L., Kissel, D.E., Maddux, L.D., Claassen, M.M., Long, J.H., 1990. Crop rotation and tillage effects on soil organic carbon and nitrogen. Soil Sci. Soc. Am. J. 54, 448±452. Himes, F.L., 1998. Nitrogen, sulfur and phosphorus and the
sequestering of carbon. In: Lal, R., Kimble, J.M., Follett, R.F., Stewart, B.A. (Eds.), Soil Processes and the C Cycle, CRC Boca Raton, FL, pp. 315±320.
Kemper, W.D., Rosenau, R.C., 1986. Aggregate stability and size distribution. In: Klute, A. (Ed.), Methods of Soil Analysis. Part I. Physical and Mineralogical Methods, second ed. Agronomy Monograph No 9. ASA and SSSA, Madison, WI, pp. 425±442.
Klute, A., 1986. Water retention: laboratory methods. In: Klute, A. (Ed.), Methods of Soil Analysis. Part I. Physical and Mineralogical Methods, second ed. Agronomy Monograph No 9. ASA and SSSA, Madison, WI, pp. 635±662.
Lal, R., 1995. The role of residues management in sustainable agricultural systems. J. Sust. Agric. 5(4), 51±78.
Lal, R., 1997. Residue management, conservation tillage and soil restoration for mitigating greenhouse effect by CO2 -enrich-ment. Soil Tillage Res. 43, 81±107.
Lal, R., Kimble, J., Levine, E., Whitman, C., 1995. World soils and greenhouse effect: an overview. In: Lal, R., Kimble, J., Levine, E., Stewart, B.A. (Eds.), Soils and Global Change. CRC Press, Boca Raton, FL, pp. 1±25.
Larson, W.E., Clapp, C.E., Pierre, W.H., Morachan, Y.B., 1972. Effects of increasing amounts of organic residues on continuous corn II. Organic C, nitrogen, phosphorous and sulfur. Agron. J. 64, 204±208.
Marshall, T.J., Holmes, J.W., 1979. Soil Physics. Cambridge University Press, Cambridge.
Nelson, D.W., Sommers, L.E., 1986. Total carbon, organic carbon, and organic matter. In: Page, A.L., Miller, R.H., Keeney, D.R. (Eds.), Methods of Soil Analysis. Part 2. Chemical and Microbiological Properties, second ed. Agronomy Monograph No 9. ASA and SSSA, Madison WI, pp. 539±579.
Paustian, K., Collins, H.P., Paul, E.A., 1997. Management controls on soil carbon. In: Paul, E.A., Paustian, K., Elliott, E.T., Cole, C.V. (Eds.), Soil Organic Matter in Temperate Agroecosystems, Long-Term Experiments in North America. CRC Press, Boca Raton, FL, pp. 15±49.
Rasmussen, P.E., Collins, H.P., 1991. Long-term impacts of tillage, fertilizer, and crop residue on soil organic matter in temperate semiarid regions. In: Brady, N.C. (Ed.), Advances in Agronomy 45. Academic Press, New York, pp. 93±134.
Russell, E.W., 1973. Soil Conditions and Plant Growth, tenth ed. Longman, London.
Skidmore, E.L., Layton, J.B., Armbrust, D.V., Hooker, M.L., 1986. Soil physical properties as influenced by cropping and residue management. Soil Sci. Soc. Am. J. 50, 415±419.
Unger, P.W., 1995. Soil organic matter and water stable aggregate effects on water infiltration. Soil Sci. 3, 9±16.
Unger, P.W., 1997a. Aggregate and organic carbon concentration interrelationships of a Torrertic Paleustoll. Soil Tillage Res. 42, pp. 95±113.
Unger, P.W., 1997b. Management-induced aggregation and organic carbon concentrations in the surface layer of a Torrertic Paleustoll. Soil Tillage Res. 42, pp. 185±208.
