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

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.