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

The ocean is a major source of atmospheric nitrous oxide (N₂O), an important greenhouse gas and ozone-depleting agent. The oceanic flux of N₂O varies regionally, and in the midlatitude North Atlantic, the production of N₂O is poorly constrained. Incubation experiments with ¹⁵N-ammonium and ¹⁵N-nitrite revealed active N₂O production from ammonium oxidation and nitrite reduction in the surface ocean, suggesting the midlatitude North Atlantic could be a net source for N₂O, with a flux density of 0.06 µmol-N₂O m⁻² d⁻¹ in the top 120 m. The peak of N₂O 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 N₂O oversaturation and low in situ N₂O production. The apparent N₂O yield, i.e., the molar ratio of N₂O-N production over nitrite production was 1.7% at the peak N₂O production depths in the surface layer and decreased to less than 0.1% at peak ammonium oxidation depths. The majority of N₂O production was apparently through “hybrid formation,” in which ammonium and nitrite each contribute one nitrogen atom to N₂O formation, a process that is proposed to be mediated by ammonia oxidizing archaea.