Nitrous oxide fluxes in estuarine environments: response to global change

Nitrous oxide is a powerful, long-lived greenhouse gas, but we know little about the role of estuarine areas in the global N2O budget. This review summarizes 56 studies of N2O fluxes and associated biogeochemical controlling factors in estuarine open waters, salt marshes, mangroves, and intertidal sediments. The majority of in situ N2O production occurs as a result of sediment denitrification, although the water column contributes N2O through nitrification in suspended particles. The most important factors controlling N2O fluxes seem to be dissolved inorganic nitrogen (DIN) and oxygen availability, which in turn are affected by tidal cycles, groundwater inputs, and macrophyte density. The heterogeneity of coastal environments leads to a high variability in observations, but on average estuarine open water, intertidal and vegetated environments are sites of a small positive N2O flux to the atmosphere (range 0.15-0.91; median 0.31; Tg N2O-N yr(-1)). Global changes in macrophyte distribution and anthropogenic nitrogen loading are expected to increase N2O emissions from estuaries. We estimate that a doubling of current median NO3- concentrations would increase the global estuary water-air N2O flux by about 0.45 Tg N2O-N yr(-1) or about 190%. A loss of 50% of mangrove habitat, being converted to unvegetated intertidal area, would result in a net decrease in N2O emissions of 0.002 Tg N2O-N yr(-1). In contrast, conversion of 50% of salt marsh to unvegetated area would result in a net increase of 0.001 Tg N2O-N yr(-1). Decreased oxygen concentrations may inhibit production of N2O by nitrification; however, sediment denitrification and the associated ratio of N2O:N-2 is expected to increase.