Nitrous oxide is a powerful, long-lived greenhouse gas, but we know little about the role of estuarine areas in the global N2 O budget. This review summarizes 56 studies of N2 O fluxes and associated biogeochemical controlling factors in estuarine open waters, salt marshes, mangroves, and intertidal sediments. The majority of in situ N2 O production occurs as a result of sediment denitrification, although the water column contributes N2 O through nitrification in suspended particles. The most important factors controlling N2 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 N2 O flux to the atmosphere (range 0.15-0.91; median 0.31; Tg N2 O-N yr(-1) ). Global changes in macrophyte distribution and anthropogenic nitrogen loading are expected to increase N2 O emissions from estuaries. We estimate that a doubling of current median NO3 (-) concentrations would increase the global estuary water-air N2 O flux by about 0.45 Tg N2 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 N2 O emissions of 0.002 Tg N2 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 N2 O-N yr(-1) . Decreased oxygen concentrations may inhibit production of N2 O by nitrification; however, sediment denitrification and the associated ratio of N2 O:N2 is expected to increase.

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Water and sediment methane emissions have the potential to offset “blue carbon” burial in mangrove coastal ecosystems.

Laser spectroscopy is an emerging technology for measuring nitrous oxide (N 2 O) dynamics in the environment, but most studies have focused on atmospheric applications. We have coupled a commercially available cavity ring-down spectroscope (CRDS) (Picarro G5101-I isotopic N 2 O analyzer) to an air/water gas equilibration device to collect continuous in situ dissolved N 2 O molar concentration and bulk nitrogen isotopic (d 15 N-N 2 O) data. The d 15 N-N 2 O values measured by the CRDS unit were found to be significantly affected by changes in the mixing ratios of O 2 , CO, CH 4 , and CO 2 . There was also an effect of N 2 O mixing ratio on d 15 N-N 2 O. A series of equations was developed to correct for the matrix effect of O 2 and the spectral interference by CH 4 . Chemical traps effectively prevented interferences by CO and CO 2 . The maximum corrections required for N 2 O mixing ratio and O 2 matrix effects, were 1& (at a mixing ratio of 1.2 ppmv), and 11& (at 0% O 2 content), respectively. The CH 4 correction only became important at mixing ratios greater than 500 ppmv (>0.5&). Measurements of N 2 O molar concentration and d 15 N-N 2 O from the CRDS isotopic N 2 O analyzer were similar to those measured with isotope ratio mass spectrometry. We demonstrated the utility of the laser-based system with field deployments in three estuarine tidal creeks in subtropical Australia. Future work in this field should focus on the application of the laser-based system to the measurement of N 2 O isotopologues in aquatic habitats, allowing for further constraints to be placed on the pathways of N 2 O cycling in aquatic system.

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