| Abstrakt: |
Nitrous oxide (N 2 O) 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 N 2 O production, especially hybrid N 2 O production when ammonia-oxidizing archaea are present. Using 15 N-labeled tracer incubations, we measured the rates of N 2 O production from ammonium (NH 4+), nitrite (NO 2-), and nitrate (NO 3-) in the eastern tropical North Pacific ODZ and the isotopic labeling of the central (α) and terminal (β) nitrogen (N) atoms of the N 2 O molecule. We observed production of both doubly and singly labeled N 2 O from each tracer, with the highest rates of labeled N 2 O production at the same depths as the near-surface N 2 O concentration maximum. At most stations and depths, the production of 45 N 2 O α and 45 N 2 O β were statistically indistinguishable, but at a few depths there were significant differences in the labeling of the two nitrogen atoms in the N 2 O molecule. Implementing the rates of labeled N 2 O production in a time-dependent numerical model, we found that N 2 O production from NO 3- dominated at most stations and depths, with rates as high as 1600 ± 200 pM N 2 O d -1. Hybrid N 2 O production, one of the mechanisms by which ammonia-oxidizing archaea produce N 2 O, had rates as high as 230 ± 80 pM N 2 O d -1 that peaked in both the near-surface and deep N 2 O concentration maxima. Based on the equal production of 45 N 2 O α and 45 N 2 O β in the majority of our experiments, we infer that hybrid N 2 O production likely has a consistent site preference, despite drawing from two distinct substrate pools. We also found that the rates and yields of hybrid N 2 O production were enhanced at low dissolved oxygen concentrations ([O 2 ]), with hybrid N 2 O yields as high as 20 % at depths where [O 2 ] was below detection (880 nM) but nitrification was still active. Finally, we identified a few incubations with [O 2 ] up to 20 µ M where N 2 O production from NO 3- was still active. A relatively high O 2 tolerance for N 2 O production via denitrification has implications for the feedbacks between marine deoxygenation and greenhouse gas cycling. [ABSTRACT FROM AUTHOR] |
