Nitric Oxide and Nitrous Oxide Production from Soil: Water and Oxygen Effects

This study was designed to determine the effects of water and O2 on the speciation of denitrification gases (NO and N2O). Nitric oxide was found to be the principal end product from soil incubated under low-moisture conditions, whereas the relative amount of N2O increased under wetter moisture regimes. The total amount of NO plus N2O produced increased with increasing water content for the Brookston clay loam (fine-loamy, mixed, mesic Typic Argiaguoll), whereas it peaked at 150 g kg(15%) water content with the Fox sandy loam (fine-loamy over sandy or sandy or sandy-skeletal, mixed, mesic, Typic Hapludalf). The decrease in NO plus N2O at higher water contents was probably the result of the subsequent reduction of N2O to N2 in the Fox sandy loam soil. The residence time of the denitrification gases in the soil increased with increasing water content, hence facilitating the subsequent conversions of NO to N2O and N2. The thickness of the water film surrounding the microbes affected both the diffusion of O2 through the water and into the microbes as well as the diffusion of denitrification gases (NO, N2O, and N2) from the microbes into the atmosphere. In the sandy loam soil, O2 content and soil water affected both the amount and species of evolved denitrification gases. Oxygen was more effective in decreasing NO production at lower than at higher water contents. N OXIDE, N2O, and N? are gaseous products of nitrification and denitrification processes and NO and N2O are important atmospheric trace gases. Nitric oxide (and NO2) in the troposhere catalyzes photochemical O3 production, contributes to formation of acid rain and controls the formation of hydroxyl radicals, the major reactant for a number if atmospheric constituents (Parrish et al., 1990). Nitrous oxide is an important greenhouse gas and the major source of stratospheric NO, one of the predominant catalysts for O3 removal. The molar ratio of NO to N2O was found to be > 1 in nitrifier cultures, whereas it has been found to be < 1 for cultures of denitrifiers (Anderson and Levine, 1986). This may be the result of the influence of O2 on denitrification. Nitric oxide production was found to be independent of O2 partial pressure (pO2) with nitrifiers, whereas N2O production was inversely related to pO2 for the denitrifiers. The primary effect of water on denitrification in aerobic and partially aerobic soils is to restrict O2 levels by reducing the air-water interfacial area within air-filled pores (Skopp, 1985) thus producing the anaerobic condition thought to be required for denitriC.F. Drury and W.I. Findlay, Research Station, Agriculture Canada, Harrow, ON, Canada NOR 1GO; and D.J. McKenney, Dep. of Chemistry and Biochemistry, Univ. of Windsor, Windsor, ON, Canada N9B 3P4. Received 21 Feb. 1991. *Corresponding author. Published in Soil Sci. Soc. Am. J. 56:766-770 (1992). fication. Oxygen diffusion rates are some four orders of magnitude slower in water than in the gas phase and hence O2 concentraton at the microbial cell is regulated by water content (Klemedtsson et al., 1988). Relatively small changes in water content in soils may directly influence rates of denitrification. Many field studies have shown that intense denitrification activity often follows heavy rainfall or flooding periods when aeration of the soil is severely restricted (Sexstone et al., 1985). Excess water not only limits O2 diffusion into the soil, but also affects the movement, distribution, and relative proportion of evolved denitrification gases. For example, Mahendrappa and Smith (1967) reported that a 10% increase in water content above field capacity in fully anaerobic soils markedly increased the amount of N2O converted to N2 with little effect on NO 3 disappearance. Water contents above these levels further decreased rates of N2O production. The objective of this study was to investigate the physical effects of water on NO and N2O production under conditions favoring denitrification, using a gasflow column approach (McKenney et al., 1982). With this technique, NO and N2O are rapidly scrubbed from the water and pore space in soil columns by a continuous flow of an inert carrier gas, enabling us to determine the production rates of these denitrification gases. In addition, the combined effect of O2 and soil water content on NO production was investigated. MATERIALS AND METHODS Anaerobic Nitric and Nitrous Oxide Production at Varying Water