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 (N2O), yet the metabolism responsible for N2O production is unclear. High-resolution profiles identified peaks and troughs of N2O and NO2- in the top 500 m of the water column. The first peak in N2O was not in the oxycline, but deeper at the oxic-suboxic interface. Peaks and troughs were targeted with a suite of 15N incubations (15NO3-, 15NO2-, 15NH4+) to identify pathways of N2O and N2 formation. With 15NO2-, 15N-N2O was produced at all depths with a binomial distribution with respect to the NO2- pool. With 15NO3-, the 15N was not binomially distributed. NO3- is first reduced to NO2- before reduction to N2O, and NO2- - > N2O is the dominant metabolism responsible for N2O production. The N2O produced from 15NH4+ represented 2-5% of that from 15NO2- at the top of the OMZ. In addition, the production of 45N2O, but no 46N2O, at some depths with 15NH4+, suggested a novel source akin to Nitrosomonas spp. under O2 limitation. Unlike N2O, the production of N2 with 15NO2- or 15NO3- was not binomially distributed and therefore was not entirely derived from the same source as N2O. Although indicative of an alternative N2 source to denitrification, the lack of significant production of labeled N2 with 15NH4+ 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 N2 production. [ABSTRACT FROM AUTHOR]