Long term effects of common Midwestern agroecosystems on soil organic carbon

Soil organic carbon (SOC) is highly sensitive to agricultural land management. As a result there is a great deal of interest in using cultivated soils to sequester atmospheric CO2. In this study we evaluate the impact of six cropping systems on SOC levels at the Wisconsin Integrated Cropping System Trial (WICST) over a 20 year period. Analysis of SOC (g kg-1 or Mg ha-1) showed a significant loss in all of the systems at WICST. While the rotationally grazed pasture system sequestered carbon (C) in the top 30 cm of the soil (9 Mg ha-1) these gains were offset by losses at depth (-9.3 Mg ha-1). Both no-till (NT) practices and inclusion of perennial crops reduced SOC loss, but neither resulted in C sequestration in the soil profile. Results from this study demonstrate the importance of i) comparing current and initial soil samples when evaluating SOC sequestration and ii) evaluating SOC changes at depth. We hypothesize that the losses of SOC observed at WICST are primarily the result of insufficient C allocation below ground to offset C losses via oxidation. While Wisconsin’s warming climate may be accelerating the mineralization of soil organic matter (SOM), the evidence for such a causal relationship is not sufficient at this point to support such a hypothesis.

Authors: Gregg R. Sanford, Joshua L. Posner, Randall D. Jackson, Christopher J. Kucharik, Janet L. Hedtcke