With the increased demand for corn ethanol, farmers are expected to plant the largest corn acreage in the United States since 1944. One of the main reasons for producing corn ethanol is the reduced greenhouse gas (GHG) emissions compared with gasoline. However, quantifying the offset of GHG emissions by corn is complex. Bioenergy crops offset CO₂ emissions by converting atmospheric CO₂ to organic C in crop biomass and soil, but they also emit N₂O and vary in their effects on soil oxidation of methane. Growing the crops requires energy, and so does converting the harvested product to usable fuels. Nitrous oxide and other GHG emissions vary with the climate, soil properties, and land use. With more than 40% of contracts for land enrollment in the Conservation Reserve Program (CRP) expiring over the next 5 years, significant portions of this land could be converted to corn production. This would change land that has been a sink for CO₂ to being a source, even under no-till management. Corn production is also likely to increase on lands with crops grown in rotation with corn, such as soybeans. This may result in increased soil C storage, but greater N₂O emissions from the soil and increased fossil fuel use from agricultural machinery and chemical inputs. The GHG mitigation potential from increased corn ethanol production will be very different depending on prior land use and production potential of the land.