Methods to Evaluate Soil and Crop Management-Induced Soil Carbon Changes.

Throughout the years, many studies evaluated the changes in soil carbon (C) mass on a fixed depth (FD) using the bulk density (BD) associated with the specific depth without considering soil thickness or soil mass. However, different management practices can influence soil BD associated with the specific depth and thus change the soil mass and associated soil C in that depth. This study evaluates the effect of different tillage practices and crop residue management on changes in soil C as calculated by a fixed depth method and by an equivalent soil mass (ESM) basis after seven years (2001 to 2008) of cropping seasons in Akron, CO. Two tillage practices were evaluated; no-tillage (NT) and a chisel plow (CP) system. The broadleaf and grass rotation in comparison with continuous corn with two irrigations (full and delay) regime were also evaluated. The soil type is Weld silt loam (fine, smectitic, mesic Aridic Argiustolls). The experiment (tillage, cropping sequence, and irrigation) was organized as split-plot design with three replications. After seven years, the gain in soil C at the 0-30 cm depth calculated on ESM basis ranged between 8.5 Mg C ha^-1 for the rotation plots to 9.9 Mg C ha^-1 for the continuous corn plots. Whereas, calculating the changes in soil C on the FD method showed a lost in soil C (-0.8 Mg C ha^-1) for the rotation plots and a 2.8 Mg C ha^-1 gained for the continuous corn plots.