Kinetics of nitrous oxide production from ammonia oxidation in the Eastern Tropical North Pacific

Abstract

Marine oxygen-deficient zones represent a natural source of nitrous oxide (N2O), a potent greenhouse gas and ozone-depleting agent. To investigate controls on N2O production, the responses of ammonia oxidation (AO) to nitrite ((Formula presented.)) and N2O with respect to oxygen (O2), ammonium ((Formula presented.)) and (Formula presented.) concentrations were evaluated using (Formula presented.) tracer incubations in the Eastern Tropical North Pacific. Within the oxycline, additions of (Formula presented.) and O2 stimulated N2O production according to Michaelis–Menten kinetics, indicating that both substrates were limiting, and that N2O production, even if the exact mechanisms remain uncertain, is mediated by predictable kinetics. Low half-saturation constants for (Formula presented.) (12–28 nM) and O2 (460 ± 130 nM) during N2O production indicate that AO communities are well adapted to low concentrations of both substrates. Hybrid N2O formation (i.e., from one (Formula presented.) and one unlabeled nitrogen (N) source, e.g., (Formula presented.), NO) accounted for ~ 90% of the N2O production from (Formula presented.) and was robust across the different O2, (Formula presented.), and (Formula presented.) conditions. Lack of response to variable substrate concentrations implies that the unlabeled N source was not limiting for N2O production. Although both O2 and (Formula presented.) were key modulators of N2O production rates, N2O yield (N2O produced per (Formula presented.) produced) seemed to be controlled solely by O2. The N2O yield increased when O2 concentrations dropped below the half-saturation concentration for AO to (Formula presented.) (< 1.4 μM), the range where (Formula presented.) production decreased faster than N2O production. Our study shows that O2 control on N2O yield from AO is robust across stations and depths.