15N tracer incubations and natural abundance isotopes reveal nitrification and denitrification production of nitrous oxide in the oxygen minimum zone

Nitrous oxide (N₂O) is a powerful greenhouse gas and ozone depletion agent. The ocean is a major source of N₂O to the atmosphere. The Eastern Tropical South Pacific oxygen minimum zone (ETSP-OMZ) is a region of intense N₂O efflux, characterized by N₂O oversaturation at the oxic-anoxic interfaces above and below the oxygen deficient zone (ODZ). Nitrification and denitrification are the two main N₂O production pathways in oxic and anoxic waters, respectively, which overlap at oxic-anoxic interfaces. To investigate instantaneous N₂O production, tracer incubations with ¹⁵N-labeled substrates (NH₄⁺, NO₂^- and NO₃^-) were performed. Parallel analyses of natural abundance isotopes (δ¹⁵N, δ¹⁸O) were used to investigate long-term, integrated N₂O cycling.

The investigations focused on three depth intervals. 1) In the upper oxycline above the ODZ, the highest N₂O production occurred at the N₂O concentration peak. Tracer experiments showed that denitrification was probably the major biological process responsible for the observed low natural abundance δ¹⁵N- and δ¹⁸O-N₂O. Although the contribution of N₂O from nitrification in the upper oxycline was small, the N₂O yield from nitrification (the ratio of N₂O to NO₂^- production from NH₄⁺) increased by 100-fold as oxygen concentration decreased from 100% to ~1% saturation. 2) Within the ODZ, N₂O was produced and consumed by denitrification. Tracer experiments showed NO₂^- and NO₃^- were reduced to N₂O, which was further reduced to N₂. As a result, high δ¹⁵N- and δ¹⁸O-N₂O were associated with under-saturated N₂O concentration. 3) Below the ODZ, instantaneous N₂O production was slow. Natural abundance distributions suggest both nitrification and denitrification maintain N₂O concentration peaks of up to a few hundred percent oversaturation in this depth interval.

Overall, denitrification is the most important pathway of N₂O cycling in the ETSP-OMZ, and is responsible for this “hot spot” of marine N₂O efflux.