Nitrous oxide production in marine environments with strong oxygen gradients

Abstract

Nitrous oxide (N2O) is a powerful greenhouse gas and an ozone depletion agent. The marine environment, especially where strong oxygen gradients exist, is a major source of N2O to the atmosphere. Nitrification (ammonium oxidation) and denitrification (nitrate and nitrite reduction) are the two main N2O production pathways in oxic and anoxic conditions, respectively, with overlap across oxic-anoxic gradients. To study N2O production in the coastal waters and the open ocean, incubations with 15N-labeled ammonium, nitrite and nitrate were performed. Three environmental settings spanning the full extent of oxygen concentration were investigated: A coastal salt marsh subjected to long-term fertilization in Northeastern US; the oxygen minimum zones (OMZs) in the Eastern Tropical North and South Pacific (ETNP and ETSP), and the well-oxygenated sub-Arctic North Atlantic.

Coastal salt marshes play an important role in the removal of excess nitrogen from land, involving several microbial processes that produce N2O. Decadal scale fertilization increased N2O production via nitrification and denitrification. The presence of oxygen was necessary for net N2O production because N2O consumption outpaced production under anoxic conditions.

In the sun-lit, oxygenated surface layer of mid-latitude North Atlantic, active N2O production was detected from ammonium oxidation, suggesting the mid-latitude North Atlantic could be a N2O source. Isotopic pairing analysis suggested that the majority of N2O production was through “hybrid formation”, in which ammonium and nitrite each contribute one nitrogen atom to N2O formation, a process that is proposed to be mediated by ammonia oxidizing archaea.

The OMZs in the ETNP and ETSP are regions of intense N2O efflux, primarily from denitrification across the oxygen gradient overlying the oxygen depleted zone (ODZ). Although the contribution of N2O from nitrification was small, the N2O yield during nitrification increased by two orders of magnitude under decreasing oxygen concentrations. Quantitative analysis of oxygen controls on N2O production from nitrification and denitrification were incorporated in a global biogeochemical model. Marine N2O production was ~ 50% higher in this simulation than current estimates. As the OMZs are predicted to expand in the future, larger volume of intense N2O production sites would result in increased marine N2O efflux.