SEASONAL-VARIATIONS OF NITRIC-OXIDE FLUX FROM AGRICULTURAL SOILS IN THE SOUTHEAST UNITED-STATES

Fluxes of nitric oxide (NO) were measured from the summer of 1994 to t he spring of 1995 from an intensively managed agricultural soil using a dynamic flow through chamber technique in order to study the seasona l variability in the emissions of NO. The measurements were made on a Norfolk sandy loam (Fine-Loamy, Siliceous, Thermic Typic Paleudult) so il located at an agricultural research station in the Upper Coastal Pl ain region of North Carolina. Soil nitric oxide fluxes from 3 crops, r epresenting 3 levels of fertilizer application (corn, 168 kg N ha(-1); cotton, 68 kg N ha(-1) and soybean, 0 kg N ha(-1)), were measured in each season (summer, fall, winter, and spring). Additional measured so il parameters included soil temperature, soil water content (expressed as percent water filled pore space, %WFPS), and extractable nitrogen. The greatest NO flux observed in each crop occurred during the summer (June to August) measurement period (corn, 21.9 +/- 18.6 ng N m(-2) s (-1); cotton, 4.3 +/- 3.7 ng N m(-2) s(-1); and soybean, 2.1 +/- 0.9 n g N m(-2) s(-1)). NO flux decreased in each crop through the fall mont hs to a minimum flux in the winter. Application of fertilizer during t he spring months once again produced substantial NO flux, but not as h igh as during the summer months. Over 80% of NO flux from the three cr ops measured occurred in the summer months with an estimated 5% of the nitrogen applied as fertilizer emitted as NO in a year's time. The co rn crop, which had the highest amount of applied fertilizer, had the h ighest average yearly NO flux (7.0 +/- 4.8 ng N m(-2) s(-1)) followed by cotton and soybean in order (1.7 +/- 1.2 ng N m(-2) s(-1) and 1.0 /- 0.3 ng N m(-2) s(-1), respectively). NO flux from soil tracked soil temperature very closely throughout the year, especially through the summer and spring months. However, NO flux measured under a cotton can opy decreased when soil temperature was >25 degrees C and soil moistur e content was <20%WFPS. Overall, the data support the assumption that in the Southeast United States, which has naturally emitted VOC's and large acreages of fertilized soils, NO emissions from agricultural soi ls may result in the formation of tropospheric ozone, especially durin g the summer months when NO emissions are highest.