Nitrite kinetics during anoxia: the role of abiotic reactions versus microbial reduction

Anoxic spells in soil induce denitrification, i.e. the sequential reduction NO3-→NO2-→NO→N2O→N2, catalysed by the four enzymes NAR, NIR, NOR and NOS, respectively. Transient accumulation of all intermediates is inevitable, but the concentrations depend on the regulation of gene expression and the physical/chemical properties of the soil. Nitrite is chemically unstable at low pH, decomposing via a conglomerate of abiotic reactions with metals and organic compounds which can result in production of NO, N2O, N2 and nitrosated organic compounds (R-NO). There is evidence that acidic soils accumulate less nitrite than neutral soils, but it is unclear if this is due to high abiotic decomposition rate (VADEC) or fast enzymatic reduction of nitrite (VNIR) at low pH. To investigate this, we monitored the kinetics of NO2-, NO, N2O and N2 during anoxic incubations of three organic soils with pHCaCl2 ranging from 3.4 to 7.2, taken from a long-term liming experiment. In parallel, we determined the rate of abiotic nitrite decay (VADEC) and its product stoichiometry (NO, N2O and R-NO) in gamma-irradiated soils. VADEC was clearly first-order with respect to HNO2 (kHNO2 = 1.4 h-1), N-gas production (NO, N2O and N2) accounted for only ~50% of VADEC, the rest was ascribed to nitrosation (R-NO). During denitrification (live soil incubation), the nitrite concentrations reached 2-3 mM in the soils with pH 4.9 and 7.2, while the soil with pH 3.4 kept nitrite concentrations at 20-50 µM , except for a short spike reaching 160 μM. Estimated rates of nitrite scavenging by the two competing sinks (NIR and ADEC) showed that NIR was the strongst nitrite sink in soil with pH 3.4 (VNIR>VADEC), while VNIR ≈ VADEC in the soil with pH 5.9. In the soil with pH 7.2, VADEC was insignificant. Thus, the regulation of denitrification (high VNIR relative to VNAR) played a crucial role in determining nitrite kinetics, hence the fate of nitrite in acid soils. High nitrite reductase activity effectively minimized abiotic nitrite decomposition and nitrosation of soil organic matter. The results shed light on regulation of denitrification in acid soils, and its implications for the fate of nitrogen during denitrification events.