Role of nitrate and nitrite for production and consumption of nitric oxide during denitrification in soil

Abstract

Anaerobic production and consumption of NO was measured in a calcic cambisol (KBE; pH 7.3) and a forest luvisol (PBE; pH 4.4) which were incubated at 80% water-holding capacity and continuously flushed with N2. Both NO production and NO consumption were negligibly low when nitrate and nitrite concentrations in the soil were exhausted. Addition of glucose alone had no effect, but addition of nitrate ± glucose greatly stimulated both NO production and NO consumption. NO consumption followed an apparent first-order reaction at low NO mixing ratios (1–3 ppmv), but a higher NO mixing ratios it followed Michaelis-Menten kinetics. In PBE the apparent Km was 980 ppbv NO (1.92 nM in soil water). During reduction of nitrate, nitrite intermediately accumulated and simultaneously, production rates of NO and N2O were at the maximum. Production rates of NO plus N2O amounted to 20% and 34% of the nitrate reduction rate in KBE and PBE, respectively. NO production was hyperbolically related to the nitrite concentration, indicating an apparent Km of 1.6 μg nitrite-N g−1 d.w. soil (equivalent to 172 μM nitrite in soil solution) for the reduction of nitrite to NO in KBE. Under nitrate and nitrite-limiting conditions, 62–76% and 93–97% of the consumed NO-N were recovered as N2O-N in KBE and PBE, respectively. Gassing of nitrate plus nitrite-depretsu KBE with increasing mixing ratios of NO2 resulted in increasing rates of NO2 uptake and presumably in the formation of low concentrations of nitrite and nitrate. This NO2 uptake resulted in increasing rates of both NO production and NO consumption indicating that nitrite or nitrate was limiting for both reactions.