Technical Abstract: Agricultural lands can mitigate detrimental effects of greenhouse gases because soils can be managed to reduce carbon dioxide emissions and sequester carbon in soil organic matter. Simulation models are useful tools for studying the long-term impacts of crop and soil management practices on soil organic matter. Scientists at the ARS Hydrology and Remote Sensing Laboratory, Beltsville, MD and the National Soil Tilth Research Laboratory, Ames, IA, used the Environmental Policy Integrated Climate (EPIC) model to study the effects of tillage systems of varying intensity on soil organic matter to a depth of 20 cm in corn and soybean production areas of Iowa. The model accurately simulated state averages of corn and soybean yields during a 35 year period (1970-2005) and adequately simulated surface soil organic matter, as judged by comparison with measured data in central Iowa. The results suggested that adoption of conservation tillage impacts positively on soil organic carbon sequestration and that croplands of Iowa are acting as a net sink for atmospheric CO2. The impacts of conservation agriculture was highest in croplands receiving less rainfall, suggesting that crop residues have a positive effect on water balance through reducing soil evaporation, increasing infiltration or reducing water runoff. The information can be used by USDA-NRCS, extension agents and producers to promote conservation tillage systems on more than 55 million acres in the US Corn Belt region.