An inventory of nitric oxide emissions from soils in the United States

A national inventory of soil nitric oxide (NO) emissions with county spatial resolution and monthly time resolution has been developed from empirical relationships that were derived from soil NO emissions data. The empirical relationships used to develop the inventory relate the emission of NO to soil temperature and a factor which is representative of an ecosystem or type of land use. Most of the ecosystem/land use factors were derived from analyses of NO emission data from seven different sites in the United States that are representative of three ecosystems and two agricultural land use types. Other agricultural (crop) land use factors were estimated from the two calculated factors based on differences in fertilization associated with the crops. This soil emission inventory exhibits three general features. First, croplands, especially where corn is grown, appear to be significant sources of NO. This has important implications because agricultural activities tend to be localized to certain regions in the United States (i.e., the Midwest) and are highest during the summer, when photochemistry is also most active. Second, undisturbed areas, such as grasslands, forests, and wetlands, do not appear to contribute substantially to atmospheric NO. Third, extensive areas in the western United States have not been included because soil NO emission data are lacking.