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 (N 2 O), a potent greenhouse gas and ozone‐depleting agent. To investigate controls on N 2 O production, the responses of ammonia oxidation (AO) to nitrite () and N 2 O with respect to oxygen (O 2 ), ammonium () and concentrations were evaluated using tracer incubations in the Eastern Tropical North Pacific. Within the oxycline, additions of and O 2 stimulated N 2 O production according to Michaelis–Menten kinetics, indicating that both substrates were limiting, and that N 2 O production, even if the exact mechanisms remain uncertain, is mediated by predictable kinetics. Low half‐saturation constants for (12–28 nM) and O 2 (460 ± 130 nM) during N 2 O production indicate that AO communities are well adapted to low concentrations of both substrates. Hybrid N 2 O formation (i.e., from one and one unlabeled nitrogen (N) source, e.g., , NO) accounted for ~ 90% of the N 2 O production from and was robust across the different O 2 , , and conditions. Lack of response to variable substrate concentrations implies that the unlabeled N source was not limiting for N 2 O production. Although both O 2 and were key modulators of N 2 O production rates, N 2 O yield (N 2 O produced per produced) seemed to be controlled solely by O 2 . The N 2 O yield increased when O 2 concentrations dropped below the half‐saturation concentration for AO to (< 1.4 μ M), the range where production decreased faster than N 2 O production. Our study shows that O 2 control on N 2 O yield from AO is robust across stations and depths.