MODEL ESTIMATES OF REGIONAL NITRIC-OXIDE EMISSIONS FROM SOILS OF THE SOUTHEASTERN UNITED-STATES

Efforts to reduce tropospheric ozone (O-3) pollution in the southeaste rn United States by reducing emissions of volatile organic compounds h ave had only modest success, and attention is now being drawn to sourc es of nitrogen oxide (NOx), which is the other major precursor of phot ochemical production of O-3. Emissions of nitric oxide (NO) from the s oils of this region are poorly known. In this study, we adapt the CASA model of terrestrial-ecosystem productivity and trace-gas emissions t o make spatially gridded, monthly estimates of NO emissions from soils for a nine-state region of the southeastern United States. Gridded in put data layers to the model include temperature, precipitation, solar surface radiation, soil texture, land cover, fertilizer sales by coun ty, and normalized-difference vegetation index. Total. N gas emissions are modeled as proportional to the rates of gross N mineralization an d fertilizer N input. The relative proportional emissions of NO, N2O, and N-2 are determined by soil water content. The model estimates 126 x 10(9) g NO-N/yr emitted from soils for the entire nine-state area. A gricultural soils emit two-thirds of the total but cover only similar to 17% of the land area. Highest NO emissions occur along the cultivat ed Mississippi River corridor, southern Alabama-Georgia, and the Carol ina coastal plain. Simulated emissions fall within the range of common ly reported values for temperate forests and cultivated fields, and th e regional estimates are probably accurate within a factor of 2. Compa red to NOx emissions from industrial, transportation, and utility sect ors, soil emissions are similar to 10% of total regional emissions. Ho wever, the soil source could be a significant fraction of local NOx in rural agricultural areas, where emissions from fertilized fields usua lly range from 300 to 900 mg NO-N . m(-2).yr(-1). Fertilizer managemen t could be an effective O-3-abatement strategy in some rural agricultu ral areas, although this possibility and general uncertainties in the soil source of NO should not be grounds to delay other O-3-abatement s trategies for the much larger, nonsoil NOx sources in the region.