NO_x emissions from soil: Implications for air quality modeling in agricultural regions

Attaining the ambient standard for tropospheric ozone has been difficult in many metropolitan areas, despite efforts to reduce anthropogenic sources of the ozone precursors, including the nitrogen oxides (NO_x). Until recently, NO_x emissions from biogenic sources in soils were not considered in simulations of air quality and emissions reductions scenarios, yet they may be significant, especially in agricultural regions where nitrogen fertilizers are applied. Soil NO_x is produced primarily by microbial processes; production and emissions from soils are controlled by a suite of environmental variables, including inorganic nitrogen availability, water-filled pore space, and soil temperature. Agricultural management practices such as fertilization and irrigation affect these environmental variables and thus have the potential to dramatically alter soil NO_x emissions. Although current models incorporate some of these variables, accurate regional estimation of soil NO_x emissions requires modeling approaches that explicitly incorporate the spatial and temporal patterns of management practices, especially fertilization, as well as other environmental controlling variables such as water-filled pore space and soil temperature.