Model Estimates of Regional Nitric Oxide Emissions From Soils of the Southeastern United States

Efforts to reduce tropospheric ozone (O3) pollution in the southeastern United States by reducing emissions of volatile organic compounds have had only modest success, and attention is now being drawn to sources of nitrogen oxide (NOx), which is the other major precursor of photochemical production of O3. Emissions of nitric oxide (NO) from the soils of this region are poorly known. In this study, we adapt the CASA model of terrestrial-ecosystem productivity and trace-gas emissions to make spatially gridded, monthly estimates of NO emissions from soils for a nine-state region of the southeastern United States. Gridded input data layers to the model include temperature, precipitation, solar surface radiation, soil texture, land cover, fertilizer sales by county, and normalized-difference vegetation index. Total N gas emissions are modeled as proportional to the rates of gross N mineralization and fertilizer N input. The relative proportional emissions of NO, N2O, and N2 are determined by soil water content. The model estimates 126 × 109 g NO-N/yr emitted from soils for the entire nine-state area. Agricultural soils emit two-thirds of the total but cover only ∼17% of the land area. Highest NO emissions occur along the cultivated Mississippi River corridor, southern Alabama–Georgia, and the Carolina coastal plain. Simulated emissions fall within the range of commonly reported values for temperate forests and cultivated fields, and the regional estimates are probably accurate within a factor of 2. Compared to NOx emissions from industrial, transportation, and utility sectors, soil emissions are ∼10% of total regional emissions. However, the soil source could be a significant fraction of local NOx in rural agricultural areas, where emissions from fertilized fields usually range from 300 to 900 mg NO-N·m−2·yr−1. Fertilizer management could be an effective O3-abatement strategy in some rural agricultural areas, although this possibility and general uncertainties in the soil source of NO should not be grounds to delay other O3-abatement strategies for the much larger, nonsoil NOx sources in the region.