Nitrous oxide production in surface waters of the mid‐latitude North Atlantic Ocean

Abstract The ocean is a major source of atmospheric nitrous oxide (N 2 O), an important greenhouse gas and ozone‐depleting agent. The oceanic flux of N 2 O varies regionally, and in the midlatitude North Atlantic, the production of N 2 O is poorly constrained. Incubation experiments with 15 N‐ammonium and 15 N‐nitrite revealed active N 2 O production from ammonium oxidation and nitrite reduction in the surface ocean, suggesting the midlatitude North Atlantic could be a net source for N 2 O, with a flux density of 0.06 µmol‐N 2 O m −2 d −1 in the top 120 m. The peak of N 2 O production was detected in the upper 100 m, shallower than the depth at which highest rates of ammonium oxidation to nitrite occurred. Oxygen was not depleted in the water column, but its concentration minimum corresponded to highest N 2 O oversaturation and low in situ N 2 O production. The apparent N 2 O yield, i.e., the molar ratio of N 2 O‐N production over nitrite production was 1.7% at the peak N 2 O production depths in the surface layer and decreased to less than 0.1% at peak ammonium oxidation depths. The majority of N 2 O production was apparently through “hybrid formation,” in which ammonium and nitrite each contribute one nitrogen atom to N 2 O formation, a process that is proposed to be mediated by ammonia oxidizing archaea.