Nitrous oxide as a function of oxygen and archaeal gene abundance in the North Pacific

Oceanic oxygen minimum zones are strong sources of the potent greenhouse gas N(2)O but its microbial source is unclear. We characterized an exponential response in N(2)O production to decreasing oxygen between 1 and 30 μmol O(2) l(−1) within and below the oxycline using (15)NO(2)(−), a relationship that held along a 550 km offshore transect in the North Pacific. Differences in the overall magnitude of N(2)O production were accounted for by archaeal functional gene abundance. A one-dimensional (1D) model, parameterized with our experimentally derived exponential terms, accurately reproduces N(2)O profiles in the top 350 m of water column and, together with a strong (45)N(2)O signature indicated neither canonical nor nitrifier–denitrification production while statistical modelling supported production by archaea, possibly via hybrid N(2)O formation. Further, with just archaeal N(2)O production, we could balance high-resolution estimates of sea-to-air N(2)O exchange. Hence, a significant source of N(2)O, previously described as leakage from bacterial ammonium oxidation, is better described by low-oxygen archaeal production at the oxygen minimum zone's margins.