A Process-Based and Empirical Model for Determining Soil CO2 Transport and Production: Implications for Estimating Soil Carbon Loss.

Crop residue left on the surface after harvest can be a viable source for biofuel production. However, removal may negatively impact soil organic C. Quantification of C losses relative to C inputs is one approach for assessing the impact of management practices on C sequestration in agricultural land. In C budget studies, the primary mechanism for quantifying C loss from soils is by measuring soil surface CO₂ efflux. Soil surface CO₂ efflux is a result of CO₂ production from plant roots and microbes and gas transport through the soil. These components respond differently to soil temperature, water content, and land management practices; thus a mechanistic understanding of their contributions to soil surface CO₂ efflux is needed to quantify C losses in the soil. Our objectives were to use an empirical and a process-based model to estimate soil surface CO₂ efflux from root and microbial respiration, at different soil depths and to assess the potential changes in soil C storage with variable rates of residue removal (0, 50, and 100%) under no-tillage (NT) and conventional tillage (CT), and N fertilization rates (0, 170, and 280 kg N ha^-1). Field studies were established in fall of 2008 on a Nicollet-Canisteo association soil in North-central IA, and a Marshall association soil in Southwest, IA in continuous corn. In 2010 and 2011, a C budget approach was used to estimate net ecosystem productivity (NEP) to determine if management practices had a net gain or loss of soil C storage. Initial findings from the field studies show that NT had significantly lower microbial respiration (C loss) compared to CT. There were no significant differences in microbial respiration when rates of 170 and 280 kg N ha^-1 were applied, however when no N was applied, microbial respiration was lower. Only with adoption of NT and N fertilization greater than 170 kg N ha^-1 with no residue removal, were there potential increases of soil C storage in both sites. In the poorly-draining soil in North-central IA, the potential for residue removal without a net loss in soil C storage was 44% and 22% for the southwest well-draining soil site under 280 kg N ha^-1fertilization and NT. This study demonstrates how C budgets by means of NEP with attention to microbial respiration can provide insights on whether adoption of conservation practices such as NT and N fertilization can aid in offsetting C losses from residue removal.