Oxidative and reductive microbial consumption of nitric oxide in a heathland soil

An acidic heathland podzol exhibited almost zero rates of NO production but high rates of NO consumption. The NO uptake rate constants decreased with soil depth. They were about two times higher and exhibited different kinetic characteristics under oxic than under anoxic incubation conditions. Under anoxic conditions, NO uptake followed Michaelis-Menten kinetics with Km values around 1 ppmv NO (1 μl NO 1−1 gas phase). Similar kinetics are known from the literature for NO reduction by denitrifying bacteria. Anoxic NO uptake was stimulated by addition of nitrate and glucose. When the podzol was gassed with nitrogen containing 9.5 ppmv NO, most (94%) of the anoxically-consumed NO was recovered as N2O demonstrating that NO was consumed by denitrification to N2O. Under oxic conditions, on the other hand, NO uptake followed apparent first-order kinetics up to NO mixing ratios of >4ppmv, suggesting a NO consumption pathway different from denitrification. Uptake of NO was abolished by autoclaving and exhibited a temperature optimum at 40°C demonstrating that it was due to biological activity. The activity exhibited a sharp optimum at the in-situ pH of 3.3, and a broad optimum at soil moistures between 10 and 60% of the maximum water holding capacity. Oxic NO uptake was not stimulated by addition of nitrate plus glucose. When the podzol was gassed with air containing 9.5 ppmv NO, only a small proportion (5%) of the consumed NO was recovered as N2O. Instead, labelling experiments with 15NO showed that most (90±17%) of the oxically consumed NO was converted to 15NO3−. Our results show that NO is consumed in soil both by oxidation to nitrate and by reduction through the denitrification pathway depending on oxic or anoxic conditions, respectively.