Nitrous Oxide Production in the Eastern Tropical South Pacific Upwelling Zone

Abstract:

The Eastern Tropical South Pacific upwelling zone, where low to undetectable oxygen concentrations exist in the water column, is a region of intense nitrous oxide (N₂O) production in the ocean. N₂O production is generally attributed to nitrification and denitrification in oxic and anoxic waters, respectively, with overlap under suboxic conditions. Seawater samples from different depths and in situ oxygen concentrations were incubated with ¹⁵N tracer labeled substrates (NH₄⁺, NO₂^- and NO₃^-) to measure potential N₂O production rates. These rates were used to determine contributions of nitrification and denitrification to total N₂O production.

N₂O reached maximum concentrations at the base of oxycline just above the oxygen deficient zone (ODZ) and nitrification was the major production pathway. The N₂O yield from nitrification, i.e., the ratio of N₂O to NO₂^- production from NH₄⁺, increased from ~0.04% to ~1% as oxygen concentration decreased from 100% to ~1% saturation. This relationship is consistent with culture studies showing increased N₂O yield from nitrification at low oxygen; and thus with high N₂O production rate from nitrification in the oxycline. N₂O production from NO₃^- was detected at the base of oxycline. Highest N₂O production rates (up to 10 nM d^-1) were detected at the top of the ODZ, with denitrification as the major pathway. At the secondary nitrite maximum within the core of the ODZ, rates of N₂O production from denitrification were relatively high despite low N₂O concentration, suggesting N₂O reduction to N₂ must be co-occurring. This implies rapid N₂O turnover by denitrification within the ODZ and a close coupling between production and consumption. These results indicate that oxygen concentrations greatly influence both the magnitude of N₂O production and the relative contributions of nitrification and denitrification. Because most N₂O production occurred in the oxycline and in the uppermost layer of the ODZ, anticipated spatial expansion of these zones could increase the extent of N₂O production and the marine N₂O efflux to the atmosphere.