Response of gaseous carbon emissions to low-level salinity increase in tidal marsh ecosystem of the Min River estuary, southeastern China

Although estuarine tidal marshes are important contributors to the emission of greenhouse gases into the atmosphere, the relationship between carbon dioxide (CO₂), methane (CH₄) emission, and environmental factors, with respect to estuarine marshes, has not been clarified thoroughly. This study investigated the crucial factors controlling the emission of CO₂ and CH₄ from a freshwater marsh and a brackish marsh located in a subtropical estuary in southeastern China, as well as their magnitude. The duration of the study period was November 2013 to October 2014. Relevant to both the field and incubation experiments, the CO₂ and CH₄ emissions from the two marshes showed pronounced seasonal variations. The CO₂ and CH₄ emissions from both marshes demonstrated significant positive correlations with the air/soil temperature (p < 0.01), but negative correlations with the soil electrical conductivity and the pore water/tide water Cl⁻ and SO₄^2 − (p < 0.01). The results indicate no significant difference in the CO₂ emissions between the freshwater and brackish marshes in the subtropical estuary, whereas there was a difference in the CH₄ emissions between the two sites (p < 0.01). Although future sea-level rise and saltwater intrusion could reduce the CH₄ emissions from the estuarine freshwater marshes, these factors had little effect on the CO₂ emissions with respect to an increase in salinity of less than 5‰. The findings of this study could have important implications for estimating the global warming contributions of estuarine marshes along differing salinity gradients.