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

Abstract

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 N2O and its isotope (15N:14N ratio in N2O or δ15N) and isotopomer composition (intramolecular distribution of 15N in the N2O or δ15Nα and δ15Nβ), 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 (δ15Nα and δ15Nβ) 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 (107-468 µmol m−2 d−1).