Hunt, E. Raymond (USDA-ARS, Beltsville, MDUSDA-ARS, Hydrology and Remote Sensing, Beltsville, MD, 20705; Phone: 301-504-5278; Fax: 301-504-8931; Email: erhunt@hydrolab.arsusda.gov)

Simulation of Erosion and Soil Carbon Sequestration Over an Agricultural Landscape in Iowa

E.R. Hunt *, P.C. Doraiswamy, C.S.T. Daughtry, G.W. McCarty, J.L. Hatfield, R.C. Izaurralde

Agricultural soils can be a source or a sink of CO₂ depending on management. The impacts of erosion depend on topography, hence landscape position affects the amount of soil carbon sequestration. We used a site in central Iowa (50 km by 100 km, 41.6946 ⁰to 42.7323 ⁰ N and 93.8416 ⁰ to 93.1610 ⁰ W), which was the site of the Soil Moisture Experiment 2002. The soils are recently derived from calcareous glacial tills from the Des Moine Lobe landform, and are dominated by the fine-loamy Clarion-Nicollet-Canisteo series, which are dependent on landscape position. This study examines the interaction of landscape and management on soil carbon sesquestration. The EPIC-CENTURY biogeochemical model was used to simulate the baseline level of soil carbon from soil survey data and project changes in soil organic carbon (SOC) under different tillage practices for corn and soybean crops. Areas that were not agriculture (cities, towns and lakes) were masked out for the computer simulations. There were two periods of model simulations. First, a 26-year period (1970-1995) was simulated using conventional tillage practices. Fertilizer inputs used the state-wide average as recorded by the USDA NASS. Actual weather data from NOAA NWS were used to drive the model. Initial SOC were obtained from the average soil organic matter concentration and average soil bulk density from the STATSGO and SSURGO geographic databases. Then, four different modeling scenarios were applied to the region for the next 25 years (1996-2020): A, conventional tillage; B, mulch tillage; C, no-till method; and D, no-till with a rye cover crop. Weather data were generated by the model using statistics from the climate record (1970 to 2000); the same weather file was used for each simulation. After 26 years of conventional tillage in a two-year maize-soybean rotation, the average carbon content was 60 Mg C/ha, which is a reduction of 22 Mg C/ha from the initial conditions. In 2020, conventional tillage is predicted to reduce the average SOC another 10 Mg C/ha to establish an expected baseline. With erosion turned off in the model, simulated SOC in 2020 under continuous conventional tillage was 27% higher than the baseline. SOC simulated using mulch till is 59 Mg C/ha in 2020, which is not significantly different from the starting conditions in 1995. However, when compared to the expected baseline loss of carbon over time, mulch till sequestered 9 Mg/ha of carbon in the simulations, which is 300 kg C ha^-1year^-1. No-tillage practices with and without a winter cover crop averaged 76 and 78 Mg C/ha, respectively, which represents an absolute gain in soil carbon over the 1996-2020 time period, almost up to the original amounts in the STATSGO and SSURGO databases. The extra operations planting the winter cover crop the reason for the EPIC-Century model predicting slightly lower carbon gain. The amount of soil carbon sequestered in these simulations is from both increased inputs of residue and reduced loses from erosion. The residue amount on the ground can be determined using remote sensing, which will show the overall change and type of conservation tillage practices adopted by farmers. A more important fact is the location of these practices has a strong interaction with other landscape variables such as erosion, so area-wide averages will not give a reliable indication on the yearly rate of carbon sequestration