The Xiamen Symposium on Marine Environmental Sciences (XMAS 2025)

Nitrous oxide (N₂O) is a potent greenhouse gas and ozone depletion agent, with a significant natural source from marine oxygen-deficient zones (ODZs). Open questions remain, however, about the microbial processes responsible for this N₂O production, especially hybrid N₂O production when ammonia-oxidizing archaea are present. Using ¹⁵N-labeled tracer incubations, we measured the rates of N₂O production from ammonium (NH₄⁺), nitrite (NO₂^-), and nitrate (NO₃^-) in the eastern tropical North Pacific ODZ and the isotopic labeling of the central (α) and terminal (β) nitrogen (N) atoms of the N₂O molecule. We observed production of both doubly and singly labeled N₂O from each tracer, with the highest rates of labeled N₂O production at the same depths as the near-surface N₂O concentration maximum. At most stations and depths, the production of ⁴⁵N₂O^α and ⁴⁵N₂O^β were statistically indistinguishable, but at a few depths there were significant differences in the labeling of the two nitrogen atoms in the N2O molecule. Implementing the rates of labeled N₂O production in a time-dependent numerical model, we found that N₂O production from NO dominated at most stations and depths, with rates as high as 1600 ± 200 pM N₂O  d⁻¹. Hybrid N₂O production, one of the mechanisms by which ammonia-oxidizing archaea produce N₂O, had rates as high as 230 ± 80 pM N₂O  d⁻¹ that peaked in both the near-surface and deep N₂O concentration maxima. Based on the equal production of ⁴⁵N₂O^α and ⁴⁵N₂O^β in the majority of our experiments, we infer that hybrid N₂O production likely has a consistent site preference, despite drawing from two distinct substrate pools. We also found that the rates and yields of hybrid N₂O production were enhanced at low dissolved oxygen concentrations ([O₂]), with hybrid N₂O yields as high as 20 % at depths where [O₂] was below detection (880 nM) but nitrification was still active. Finally, we identified a few incubations with [O₂] up to 20 µM where N₂O production from NO was still active. A relatively high O₂ tolerance for N₂O production via denitrification has implications for the feedbacks between marine deoxygenation and greenhouse gas cycling.

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