Kinetics of nitrous oxide production from ammonia oxidation in the Eastern Tropical North Pacific
Claudia Frey, Xin Sun, Laura Szemberski, Karen L. Casciotti, Emilio García‐Robledo, Amal Jayakumar, Colette L. Kelly, Moritz F. Lehmann, Bess B. Ward
Abstract
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.