Technical Abstract: Nitrous oxide (N2O), carbon dioxide (CO2), and methane (CH4) emissions from cropped soils totaled 154, 34, and 8 Tg CO2 eq., respectively, in 2008. However, that amount was offset by a storage, or carbon sequestration, of 42 Tg CO2 eq. in cropped soils in 2008. When carbon sequestration is taken into account, net emissions of GHG from cropland agriculture amount to approximately 154 Tg CO2 eq. Greenhouse gas emission from agricultural soils, primarily N2O, were responsible for the majority of total emissions, while CH4 and N2O from residue burning and rice cultivation caused about 4% of emissions in 2008. Soil CO2 emissions from cultivation of organic soils (15%) and from liming (2%) are the remaining sources. Nitrous oxide emissions from soils are the largest source in the U.S. because N2O is a potent greenhouse gas and due to the large amounts of nitrogen added to crops in fertilizer that stimulate N2O production. Emissions from residue burning are minor because only ~3% of crop residue is assumed to be burned in the U.S. Cropped soils in the U.S. are a net CO2 sink mainly because reduced tillage intensity has become more popular in recent years and lands used for perennial hay cropping, as well is idle cropland enrolled in the Conservation Reserve Program (CRP), continue to store carbon. Nitrous oxide emissions were largest in areas where a large portion of land is used for intensive agriculture. For example, 90% or more of the land in many counties in the Corn Belt is intensively cropped. Corn is the leading crop for N2O emissions followed by soybean and hay. Emissions from corn cropping are high because large amounts of nitrogen (N) fertilizer are routinely applied and the land area used for corn production is the most extensive. Although little N fertilizer is applied for soybean cropping, N2O emissions are high because soybeans supply large amounts of N to the soil from biological fixation of atmospheric nitrogen (N2). In general, N2O emissions are highly correlated with crop areas and nitrogen inputs. Biological fixation makes up about half of total N additions, followed by synthetic fertilizer addition and manure.