Salinity stress changed the biogeochemical controls on CH₄ and N₂O emissions of estuarine and intertidal sediments

Elevated salinity is expected to drive changes in biogeochemical cycling and microbial communities in estuarine and intertidal wetlands. However, limited information regarding the role of salinity in shaping biogeochemical controls and mediating greenhouse gas emissions is currently available. In this study, we used incubation experiment across salinity gradients of the estuarine and intertidal sediments to reveal the underlying interconnections of CH₄ and N₂O emissions, biogeochemical controls and salinity gradients. Our results indicated that sediment biogeochemical properties were significantly affected by the increasing salinity, which were attributed to the accelerated sediment enzyme activities. The increasing salinity promoted CH₄ and N₂O emission rates by stimulating organic carbon decomposition and nitrogen transformation rates. In addition, the copy number of mcrA, nirS and nirK genes increased along with the salinity gradients, which strongly mediated the CH₄ and N₂O emission rates. Stepwise regression analysis suggested that labile organic carbon and denitrification were the most crucial determinants of CH₄ and N₂O emission rates, respectively. Overall, salinity could enhance CH₄ and N₂O emission mainly by altering sediment geochemical variables, microbial activity and functional gene abundance in estuarine and intertidal environments. Furthermore, increasing salinity could enhance the carbon and nitrogen export, which may pose a threat to the ecological function of estuarine and intertidal ecosystems. This study may contribute to the knowledge about the importance of biogeochemical controls induced by salinity in mediating greenhouse gas emissions.