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 in 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 N-15 incubations ((NO3-)-N-15, (NO2-)-N-15, (NH4+)-N-15) to identify pathways of N2O and N-2 formation. With (NO2-)-N-15, N-15-N2O was produced at all depths with a binomial distribution with respect to the NO2- pool. With (NO3-)-N-15, the N-15 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 (NH4+)-N-15 represented 2-5% of that from (NO2-)-N-15 at the top of the OMZ. In addition, the production of (N2O)-N-45, but no (N2O)-N-46, at some depths with (NH4+)-N-15, suggested a novel source akin to Nitrosomonas spp. under O-2 limitation. Unlike N2O, the production of N-2 with (NO2-)-N-15 or (NO3-)-N-15 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 (NH4+)-N-15 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.