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 N 2 O budget. This review summarizes 56 studies of N 2 O fluxes and associated biogeochemical controlling factors in estuarine open waters, salt marshes, mangroves, and intertidal sediments. The majority of in situ N 2 O production occurs as a result of sediment denitrification, although the water column contributes N 2 O through nitrification in suspended particles. The most important factors controlling N 2 O 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 N 2 O flux to the atmosphere (range 0.15–0.91; median 0.31; Tg N 2 O‐N yr −1 ). Global changes in macrophyte distribution and anthropogenic nitrogen loading are expected to increase N 2 O emissions from estuaries. We estimate that a doubling of current median NO 3 − concentrations would increase the global estuary water–air N 2 O flux by about 0.45 Tg N 2 O‐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 N 2 O emissions of 0.002 Tg N 2 O‐N yr −1 . In contrast, conversion of 50% of salt marsh to unvegetated area would result in a net increase of 0.001 Tg N 2 O‐N yr −1 . Decreased oxygen concentrations may inhibit production of N 2 O by nitrification; however, sediment denitrification and the associated ratio of N 2 O:N 2 is expected to increase.