Influence of Tillage Practices on Soil Structural Controls over Carbon Mineralization

The influence of tillage practices on soil organic carbon (SOC) dynamics is manifested indirectly through the modification of soil structure. This study was conducted at two sites in Illinois where long-term use of conventional (CT) and no tillage (NT) practices has increased SOC at Monmouth, a silt loam, but not at DeKalb, a silty clay loam soil with higher SOC contents. We evaluated whether soil structural quality could be related to observed SOC mineralization and explain the inconsistent influence of tillage on SOC stocks. Soil physical parameters and soil CO₂ evolution rates were measured in 2000, 2001, and 2002. At DeKalb, there was no difference in the mean (μmol m⁻¹ s⁻¹) or specific (μgCO₂ s⁻¹/μg SOC) SOC mineralization rates of NT and CT soils. In Monmouth, mean and specific SOC mineralization rates were greater from soils under CT than NT management. This indicates use of NT practices had increased physical protection of SOC at that site. The Q₁₀ equation, which is based on soil temperature and moisture, better explained CO₂ efflux in DeKalb than in Monmouth. The poorer fit of the equation in Monmouth reflects its reliance on gravimetric moisture content, which inadequately describes the status of soil water influencing heterotrophic activity. The least limiting water range (LLWR), which integrates the affects of clay content, bulk density, and soil moisture on biological activity and predicted observed soil CO₂ efflux patterns (R = 0.600, p = 0.0025) better than any other physical parameter, indicated use of NT practices at Monmouth increased soil compaction or strength enough to reduce C mineralization. In DeKalb, where soils have an inherently high capacity to protect SOC from decay, tillage has had no influence on SOC dynamics. The variable affect of tillage practices on C sequestration were explained by soil physical properties.