ENVIRONMENTAL VARIABLES CONTROLLING NITRIC-OXIDE EMISSIONS FROM AGRICULTURAL SOILS IN THE SOUTHEAST UNITED-STATES

Fluxes of nitric oxide (NO) Were measured during the summer of 1994 (1 2 July to 11 August) in the Upper Coastal Plain of North Carolina in a continuing effort to characterize NO emissions from intensively manag ed agricultural soils in the southeastern United States. Previous work during a similar time of year on the same soil type was characterized by severe moisture stress conditions. The summer of 1994 provided a m ore diverse weather pattern and as a result represented a set of measu rements more typical of soil temperature and soil moisture relationshi ps for the southeastern United States. In order to ascertain NO Aux re sponse to fertilization and crop type, measurements were made on field s with three distinct fertilizer practices and crop types, namely corn , cotton, and soybean. Average NO fluxes were 21.9 +/- 18.6, 4.3 +/- 3 .7, and 2.1 +/- 0.9 ng N m(-1)s(-1), respectively, for corn, cotton, a nd soybean. NO flux increased exponentially with soil temperature when soil water content was not limiting [> 30% Water Filled Pore Space (% WFPS)]. During conditions when soil water content was limiting, NO flu x was inhibited and had no relationship with soil temperature. Above a value of 30% WFPS, increasing soil water content had no effect on NO emissions (the upper limit of %WFPS could not be estimated due to a la ck of data in this regime). Below 30% WFPS, increasing soil moisture i ncreased NO production and lower soil moistures led to decreased NO fl ux. Increased nitrogen fertilization rates led to higher NO fluxes. Ho wever, differences in physiological growth stages between crops confou nd extractable nitrogen values as decomposing root biomass in the matu re corn crop added an undetermined amount of available nitrogen to the soil. Interactions between soil water content, fertilizer application , and soil temperature make it very difficult to predict day-to-day va riations of NO Bur from our data. There appears to be no simple relati on between NO flux and the environmental variables measured in Clayton , NC during the summer of 1994. Copyright (C) 1996 Elsevier Science Lt d