Nitrous oxide production by nitrification and denitrification in the Eastern Tropical South Pacific oxygen minimum zone

The Eastern Tropical South Pacific oxygen minimum zone (ETSP-OMZ) is a site of intense nitrous oxide (N₂O) flux to the atmosphere. This flux results from production of N₂O by nitrification and denitrification, but the contribution of the two processes is unknown. The rates of these pathways and their distributions were measured directly using ¹⁵N tracers. The highest N₂O production rates occurred at the depth of peak N₂O concentrations at the oxic-anoxic interface above the oxygen deficient zone (ODZ) because slightly oxygenated waters allowed (1) N₂O production from both nitrification and denitrification and (2) higher nitrous oxide production yields from nitrification. Within the ODZ proper (i.e., anoxia), the only source of N₂O was denitrification (i.e., nitrite and nitrate reduction), the rates of which were reflected in the abundance of nirS genes (encoding nitrite reductase). Overall, denitrification was the dominant pathway contributing the N₂O production in the ETSP-OMZ.