Land area planted with corn is expected to be approximately 15% higher in 2007 compared to 2006. The demand for corn-based ethanol is largely responsible for this increase. Previous studies have concluded that net greenhouse gas (GHG) emissions from crop production (including emissions from inputs, farm operations, and soil GHG fluxes), feedstock conversion, product distribution, and combustion of corn ethanol are roughly equal or somewhat (30-40%) less than emissions from burning fossil fuel to obtain an equivalent amount of energy. We investigate how previous land use impacts net GHG fluxes associated with corn grown for ethanol. Because field studies rarely measure all the components that contribute to net GHG fluxes, models are necessary to perform full greenhouse gas accounting. We demonstrate the ability of the DAYCENT ecosystem model to simulate N2O emissions (a primary source of GHG emissions from biofuel cropping), crop yields, and soil C levels for different land uses. DAYCENT was then used to estimate changes in soil GHG fluxes resulting from converting Conservation Reserve Program (CRP) land and corn/soybean rotations to continuous corn cropping. Model results are combined with estimates of GHG emission savings from fossil fuel displaced per unit of grain yield and GHG emission costs of crop production and feedstock conversion to calculate net GHG fluxes for selected sites in the US.