Denitrification and nitrous oxide cycling within the upper oxycline of the eastern tropical South Pacific oxygen minimum zone

One of the shallowest, most intense oxygen minimum zones (OMZs) is found in the eastern tropical South Pacific, off northern Chile and southern Peru. It has a strong oxygen gradient (upper oxycline) and high N₂O accumulation. N₂O cycling by heterotrophic denitrification along the upper oxycline was studied by measuring N₂O production and consumption rates using an improved acetylene blockage method. Dissolved N ₂O and its isotope (¹⁵N: ¹⁴N ratio in N ₂O or δ¹⁵N) and isotopomer composition (intramolecular distribution of ¹⁵N in the N₂O or δ¹⁵N^α and δ¹⁵N ^β), dissolved O₂, nutrients, and other oceanographic variables were also measured. Strong N₂O accumulation (up to 86 nmol L^-1) was observed in the upper oxycline followed by a decline (around 8-12 nmol L^-1) toward the OMZ core. N₂O production rates by denitrification (NO₂^- reduction to N₂O) were 2.25 to 50.0 nmol L^-1 d^-1, whereas N₂O consumption rates (N₂O reduction to N₂) were 2.73 and 70.8 nmol L^-1 d^-1. δ¹⁵N in N₂O increased from 8.57%‰ in the middle oxycline (50-m depth) to 14.87%‰ toward the OMZ core (100-m depth), indicating the progressive use of N₂O as an electron acceptor by denitrifying organisms. Isotopomer signals of N₂O (δ¹⁵N^α and δ¹⁵N^β) showed an abrupt change at the middle oxycline, indicating different mechanisms of N₂O production and consumption in this layer. Thus, partial denitrification along with aerobic ammonium oxidation appears to be responsible for N₂O accumulation in the upper oxycline, where O₂ levels fluctuate widely; N₂O reduction, on the other hand, is an important pathway for N₂ production. As a result, the proportion of N₂O consumption relative to its production increased as O₂ decreased toward the OMZ core. A N₂O mass balance in the subsurface layer indicates that only a small amount of the gas could be effluxed into the atmosphere (12.7-30.7 μmol m^-2 d^-1) and that most N₂O is used as an electron acceptor during denitrification (107-168 μmol m^-2 d ^-1).