Venteris, Erik (Ohio Division of Geological Survey, Petroleum Geology Group, Ohio Division of Geological, Columbus, OH, 43229; Phone: 614-265-6459; Fax: 614-447-1918; Email: erik.venteris@dnr.state.oh.us)

Impact of Soil Redistribution on the Mass Balance of Soil Organic Carbon in Hummocky Glacial Topography, Iowa

E.R.Venteris *, G.W. McCarty, J.C. Ritchie, T.E. Fenton, T.C. Kaspar

The Soil Organic Carbon (SOC) budget is a function of the balance between vegetative production, oxidative losses, and mass transport by water/ tillage. Models for spatio-temporal prediction are needed that account for mass transport processes in a realistic manner. Past work has concentrated on water erosion but has neglected water deposition and soil transport by tillage. Understanding these processes is essential to successful numerical modeling and for the placement of sites to monitor SOC changes for carbon accounting. To better understand and model these processes, two high-resolution (25 m) soil sample grids were collected on separate agricultural fields (chisel ploughed, corn soybean rotation). SOC and Cs-137 (to model erosion/deposition) were measured for each soil sample point. Five meter resolution data were collected with GPS to provide topographic information (DEM). A unique characteristic of the study fields is that they contain small (100 m from bottom to top, 5 meters relief) closed depressions. Hence the mass balance of soil transport should sum to near zero. Mass balance results for four studied depressions were –2.37, -4.89, 0.941 and 0.9 Mt/ha/year. These numbers are within potential losses/ gains from wind erosion (±2.5-5 Mt/ha/year). Erosion/deposition models based on Cs-137 concentration (Walling and He method) are giving reasonable results and do not show systematic bias towards erosion or deposition. The site is especially useful for the calibration of spatio-temporal soil and carbon erosion/deposition models as large mass imbalances would expose biases in these models(RUSLE, WATEM, etc.). These results also demonstrate the need to take into account landscape redistribution of carbon when using the benchmark approach to assess temporal changes in SOC inventory.