A role for nitrite in the production of nitrous oxide in the lower euphotic zone of the oligotrophic North Pacific Ocean

Understanding the role of the oceans in the Earthʹs changing climate requires comprehension of the relevant metabolic pathways which produce climatically important trace gases. The global ocean represents one of the largest natural sources of nitrous oxide (N2O) that is produced by selected archaea and/or bacteria during nitrogen (N) metabolism. In this study, the role of nitrite (NO2−) in the production of N2O in the upper water column of the oligotrophic North Pacific Subtropical Gyre was investigated, focusing primarily on the lower euphotic zone where NO2− concentrations at the primary NO2− maximum reached 195 nmol L−1. Free-drifting sediment trap arrays were deployed to measure N cycle processes in sinking particulate material and the addition of selected N substrates to unpreserved sediment traps provided an experimental framework to test hypotheses regarding N2O production pathways and controls. Sinking particles collected using NO2−-amended, unpreserved sediment traps exhibited significant production of N2O at depths between 100 and 200 m. Subsequent stable isotope tracer measurements conducted on sediment trap material amended with 15NO2− yielded elevated δ15N values of N2O, supporting N2O production via a NO2− metabolism pathway. Experiments on seawater collected from 150 m showed N2O production via NO2− metabolism also occurs in the water-column and indicated that the concentration of NO2− relative to NH4+ availability may be an important control. These findings provide evidence for the production of N2O via nitrifer-denitrification in the lower euphotic zone of the open ocean, whereby NO2− is reduced to N2O by ammonia-oxidizing microorganisms.