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 (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 (NO À 2) and N 2 O with respect to oxygen (O 2), ammonium (NH þ 4) and NO À 2 concentrations were evaluated using 15 N À NH þ 4 tracer incubations in the Eastern Tropical North Pacific. Within the oxycline, additions of NH þ 4 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 NH þ 4 (12-28 nM) and O 2 (460 AE 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 15 NH þ 4 and one unlabeled nitrogen (N) source, e.g., NO À 2 , NO) accounted for $ 90% of the N 2 O production from NH þ 4 and was robust across the different O 2 , NO À 2 , and NH þ 4 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 NH þ 4 were key modulators of N 2 O production rates, N 2 O yield (N 2 O produced per NO À 2 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 NO À 2 (< 1.4 μM), the range where NO À 2 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.