Seasonal nitrous oxide and methane emissions across a subtropical estuarine salinity gradient

Abstract

Currently, there is a lack of knowledge about GHG emissions, specifically N2O and CH4, in subtropical coastal freshwater wetland and mangroves in the southern hemisphere. In this study, we quantified the gas fluxes and substrate availability in a subtropical coastal wetland off the coast of southeast Queensland, Australia over a complete wet-dry seasonal cycle. Sites were selected along a salinity gradient ranging from marine (34 psu) in a mangrove forest to freshwater (0.05 psu) wetland, encompassing the range of tidal influence. Fluxes were quantified for CH4 (range −0.4–483 mg C–CH4 h−1 m−2) and N2O (−5.5–126.4 μg N–N2O h−1 m−2), with the system acting as an overall source for CH4 and N2O (mean N2O and CH4 fluxes: 52.8 μg N–N2O h−1 m−2 and 48.7 mg C–CH4 h−1 m−2, respectively). Significantly higher N2O fluxes were measured during the summer months (summer mean 64.2 ± 22.2 μg N–N2O h−1 m−2; winter mean 33.1 ± 24.4 µg N–N2O h–1 m−2) but not CH4 fluxes (summer mean 30.2 ± 81.1 mg C–CH4 h−1 m−2; winter mean 37.4 ± 79.6 mg C–CH4 h−1 m−2). The changes with season are primarily driven by temperature and precipitation controls on the dissolved inorganic nitrogen (DIN) concentration. A significant spatial pattern was observed based on location within the study site, with highest fluxes observed in the freshwater tidal wetland and decreasing through the mangrove forest. The dissolved organic carbon (DOC) varied throughout the landscape and was correlated with higher CH4 fluxes, but this was a nonlinear trend. DIN availability was dominated by N–NH4 and correlated to changes in N2O fluxes throughout the landscape. Overall, we did not observe linear relationships between CH4 and N2O fluxes and salinity, oxygen or substrate availability along the fresh-marine continuum, suggesting that this ecosystem is a mosaic of processes and responses to environmental changes.