REDISTRIBUTION OF SOIL ORGANIC CARBON IN AGRICULTURAL SOILS

It is commonly asserted that land cover conversion from native prairies to agricultural land use has resulted in broad-scale losses of soil organic carbon (SOC) through increased respiration of SOC resulting from disturbance and losses due to wind and water erosion. Recently, that conceptual framework has been challenged; in part, because downslope deposition of eroded soil can be an effective sink for SOC. In an effort to assess the size and landscape distribution of SOC pools in the agricultural Midwest, we have developed an approach to predict SOC in the top 1.5 meters of soil based on common terrain attributes. Separate regression equations were developed for surface (top 25 cm) vs. deep soils (down to 1.5 m) and for grassland vs. cropped sites. Terrain attributes that emerged as the most useful in regression equations were: % slope, profile curvature, and compound topographic index in addition to soil depth. Preliminary results show that these regression equations were able to explain 82% and 77% of the observed variability of samples collected from grassland and cropland sites, respectively. We then applied these regression equations to assess how SOC would change under alternate land use. Results were varied and showed that conversion of grassland to cropland produced SOC depletion at sites with steeper slopes and SOC accumulation at downslope depositional sites. Ongoing work will apply these regressions to determine land use effects on net SOC gain or loss at the landscape scale.