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 N 2 O accumulation. N 2 O cycling by heterotrophic denitrification along the upper oxycline was studied by measuring N 2 O production and consumption rates using an improved acetylene blockage method. Dissolved N 2 O and its isotope ( 15 N: 14 N ratio in N 2 O or δ 15 N) and isotopomer composition (intramolecular distribution of 15 N in the N 2 O or δ 15 N α and δ 15 N β ), dissolved O 2 , nutrients, and other oceanographic variables were also measured. Strong N 2 O 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. N 2 O production rates by denitrification (NO 2 − reduction to N 2 O) were 2.25 to 50.0 nmol L −1 d −1 , whereas N 2 O consumption rates (N 2 O reduction to N 2 ) were 2.73 and 70.8 nmol L −1 d −1 . δ 15 N in N 2 O 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 N 2 O as an electron acceptor by denitrifying organisms. Isotopomer signals of N 2 O (δ 15 N α and δ 15 N β ) showed an abrupt change at the middle oxycline, indicating different mechanisms of N 2 O production and consumption in this layer. Thus, partial denitrification along with aerobic ammonium oxidation appears to be responsible for N 2 O accumulation in the upper oxycline, where O 2 levels fluctuate widely; N 2 O reduction, on the other hand, is an important pathway for N 2 production. As a result, the proportion of N 2 O consumption relative to its production increased as O 2 decreased toward the OMZ core. A N 2 O 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 N 2 O is used as an electron acceptor during denitrification (107‐468 µmol m −2 d −1 ).