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 K _m 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 K_m 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.