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Nitrification and Nitrous Oxide Production in the Offshore Waters of the Eastern Tropical South Pacific

Marine oxygen deficient zones are dynamic areas of microbial nitrogen cycling. Nitrification, the microbial oxidation of ammonia to nitrate, plays multiple roles in the biogeochemistry of these regions, including production of the greenhouse gas nitrous oxide (N2O). We present here the results of two oceanographic cruises investigating nitrification, nitrifying microorganisms, and N2O production and distribution from the offshore waters of the Eastern Tropical South Pacific. On each cruise, high‐resolution measurements of ammonium ([NH4+]), nitrite ([NO2−]), and N2O were combined with 15N tracer‐based determination of ammonia oxidation, nitrite oxidation, nitrate reduction, and N2O production rates. Depth‐integrated inventories of NH4+ and NO2− were positively correlated with one another and with depth‐integrated primary production. Depth‐integrated ammonia oxidation rates were correlated with sinking particulate organic nitrogen flux but not with primary production; ammonia oxidation rates were undetectable in trap‐collected sinking particulate material. Nitrite oxidation rates exceeded ammonia oxidation rates at most mesopelagic depths. We found positive correlations between archaeal amoA genes and ammonia oxidation rates and between Nitrospina‐like 16S rRNA genes and nitrite oxidation rates. N2O concentrations in the upper oxycline reached values of >140 nM, even at the western extent of the cruise track, supporting air‐sea fluxes of up to 1.71 μmol m−2 day−1. Our results suggest that a source of NO2− other than ammonia oxidation may fuel high rates of nitrite oxidation in the offshore Eastern Tropical South Pacific and that air‐sea fluxes of N2O from this region may be higher than previously estimated.