Environmental variables controlling nitric oxide :emissions from agricultural soils in the southeast united states

Abstract

Fluxes of nitric oxide (NO) were measured during the summer of 1994 (12 July to 11 August) in the Upper Coastal Plain of North Carolina in a continuing effort to characterize NO emissions from intensively managed 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 more diverse weather pattern and as a result represented a set of measurements more typical of soil temperature and soil moisture relationships for the southeastern United States. In order to ascertain NO flux response to fertilization and crop type, measurements were made on fields 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 −2 s −1, respectively, for corn, cotton, and 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 flux 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 lack of data in this regime). Below 30% WFPS, increasing soil moisture increased NO production and lower soil moistures led to decreased NO flux. Increased nitrogen fertilization rates led to higher NO fluxes. However, differences in physiological growth stages between crops confound extractable nitrogen values as decomposing root biomass in the mature 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 variations of NO flux from our data. There appears to be no simple relation between NO flux and the environmental variables measured in Clayton, NC during the summer of 1994.