Date of Award
Master of Science
Removal of topsoil from glacial till soils exposes unproductive subsoil that is low in soil organic carbon (SOC). The objective of this study was to determine if soil carbon stocks of exposed subsoil could be improved with topsoil addition and cropping systems. In experiment one, a corn (Zea mays)-soybean (Gfycine max L. Merr.) rotation was established on exposed subsoil and topsoil that had been placed over exposed subsoil. In experiment two, corn-soybean rotation (CS), switchgrass (Panicum virgatum L.) burned annually (SA), and switchgrass burned every five years (S5) cropping systems were established on exposed subsoil. To examine soil carbon dynamics, we measured soil CO₂C emissions, total carbon (TC) inputs from crop residues, microbial biomass carbon (MBC), and soil carbon fractions. Corn grown in topsoil produced 7.14 Mg ha⁻¹ more aboveground biomass and 0.8 Mg ha⁻¹ more root biomass and had greater potential C inputs than corn grown in subsoil. Topsoil had cumulative soil CO₂₋C emissions 46% greater than subsoil and a 57% larger MBC pool. Topsoil had greater SOC contents than exposed subsoil, including the 30 to 45 cm soil depth, which was below any added topsoil. Switchgrass cropping systems were the most productive in the exposed subsoil, producing 3.47 and 2.33 Mg ha⁻¹ more aboveground biomass than soybeans and corn, respectively. Switchgrass cropping systems also had a root biomass 15 Mg ha⁻¹ greater than corn or soybeans. As a result potential carbon inputs from switchgrass residues were 6.08 and 6.71 Mg ha⁻¹ greater than corn and soybeans, respectively. The switchgrass burned annually cropping system had the greatest cumulative soil CO₂₋C emissions, followed by the S5 and CS cropping systems, respectively. The MBC pool of exposed subsoil was on average 200% greater in the switchgrass cropping systems than the corn-soybean rotation. There were no differences in SOC fractions between all cropping system treatments in the exposed subsoil. These findings suggest that topsoil addition or switchgrass establishment on exposed subsoil will result in greater potential carbon inputs, greater soil CO₂ emissions and larger MBC pools. However, improving carbon stocks of exposed subsoil appears to be a slow and long-term process.
Jesse Benjamin Grote
Grote, Jesse Benjamin, "Management alternatives for improving soil carbon stocks of exposed subsoil in Iowa" (2005). Retrospective Theses and Dissertations. 18806.