Soil NOx Emissions: Not So Innocuous?

In the U.S., the Clean Air Act Amendments of 1990 have substantially reduced nitrogen oxide (NOx = NO + NO2) emissions from industrial sources, including power generation. As a consequence, wet inorganic nitrogen deposition rates have decreased by nearly 30 percent in the eastern U.S. Given emission reductions from power generation, it has become increasingly important to understand the fluxes and fate of other NOx sources, including vehicular emissions, lightning, and emissions emanating from soils during microbial reactions. These soil emissions, also called “biogenic NOx” emissions, are released as byproducts during both nitrification and denitrification reactions and have historically been difficult to quantify due to their diffuse concentrations and temporal fluctuations. Hence, large knowledge gaps exist regarding the fluxes and dynamics of soil NOx emissions as a precursor to atmospheric nitrate formation. This presentation synthesizes several recent studies that use isotopic fingerprints of soil NOx emissions to examine spatial and temporal extents of soil NOx emissions, their influence on nitrate deposition, and implications for regional emission inventories.