Yoo, Gayoung (University of Illinois, Dept of Natural Resources, Urbana, IL, 61801; Phone: 217-333-4912; Email: gayoo@uiuc.edu)
G. Yoo *, M.M. Wander
Use of No-tillage (NT) practices fails to reliably increase soil C sequestration in fine-textured soils. Here we report on a study conducted to explain differences in C dynamics at two sites in Illinois where long-term use of NT practices has increased SOC storage at one site, Monmouth, but not at the other, DeKalb. Efforts emphasized SOC mineralization because there was no difference in average crop yield from 1989-2002 in NT or conventional tillage (CT) plots and soil erosion was assumed to be negligible because fields were quite level. Soil CO2 evolution rates were measured in 2000, 2001, and 2002 along with soil temperature, gravimetric water content, bulk density, penetration resistance, and pore size distribution. In DeKalb (silty clay loam soil), there was no difference in the mean (μmol m-1 s-1) or specific (μgCO2 s-1 / μg SOC) C mineralization rates of NT and CT soils. In Monmouth (silt loam soil), both mean and specific SOC mineralization rates were greater from soils under CT than NT management. Differences in SOC mineralization rates were consistent with previously observed differences in SOC sequestration. Correlation among physical parameters and nonlinear regression of mean CO2 evolution rates with soil temperature and water content indicated that soil water content interacted with soil structure to influence SOC mineralization. Pore size distribution helped explain differences in soil water-structure interactions at these two sites. Macroporosity (>30 μm) was relatively reduced in the NT soils at Monmouth but not in DeKalb; this suggests soil compaction limits SOC mineralization in NT soils at Monmouth. The least limiting water range (LLWR), an index that integrates clay content, bulk density and soil moisture, predicted observed mean soil CO2 evolution patterns better than any individual physical parameter. The LLWR in DeKalb was lower than in Monmouth and did not differ as a result of tillage practices and indicated in DeKalb, high clay content limits soil water availability for SOC mineralization regardless of tillage practices. Whereas in Monmouth, the significantly lower LLWR in the NT soil indicated that SOC mineralization was limited by higher soil strength. By increasing our understanding of structural controls over SOC mineralization, we should be able to better predict whether or not the adoption of NT practice will increase SOC sequestration at individual sites. Structural parameters, and not just soil texture, should be included in simulation models to accurately predict SOC dynamics.