High‐resolution profiles and nitrogen isotope tracing reveal a dominant source of nitrous oxide and multiple pathways of nitrogen gas formation in the central Arabian Sea

The oxygen minimum zone (OMZ) of the Arabian Sea is a significant source of nitrous oxide (N 2 O), yet the metabolism responsible for N 2 O production is unclear. High‐resolution profiles identified peaks and troughs of N 2 O and NO 2 ‐ in the top 500 m of the water column. The first peak in N 2 O was not in the oxycline, but deeper at the oxic‐suboxic interface. Peaks and troughs were targeted with a suite of 15 N incubations ( 15 NO 3 ‐ , 15 NO 2 ‐ , 15 NH 4 + ) to identify pathways of N 2 O and N 2 formation. With 15 NO 2 ‐ , 15 N‐N 2 O was produced at all depths with a binomial distribution with respect to the NO 2 ‐ pool. With 15 NO 3 ‐ , the 15 N was not binomially distributed. NO 3 ‐ is first reduced to NO 2 ‐ before reduction to N 2 O, and NO 2 ‐ → N 2 O is the dominant metabolism responsible for N 2 O production. The N 2 O produced from 15 NH 4 + represented 2‐5% of that from 15 NO 2 ‐ at the top of the OMZ. In addition, the production of 45 N 2 O, but no 46 N 2 O, at some depths with 15 NH 4 + , suggested a novel source akin to Nitrosomonas spp. under O 2 limitation. Unlike N 2 O, the production of N 2 with 15 NO 2 ‐ or 15 NO 3 ‐ was not binomially distributed and therefore was not entirely derived from the same source as N 2 O. Although indicative of an alternative N 2 source to denitrification, the lack of significant production of labeled N 2 with 15 NH 4 + discounts anaerobic ammonium oxidation (anammox), as we understand it. Dissolved organic nitrogen or nitrate/nitrite reduction to ammonium are suggested as the additional sources of N in N 2 production.