Ecosystem context modulates salinization impacts on diffusive greenhouse gas fluxes in coastal wetland soils

Saltwater intrusion is accelerating salinization in coastal wetlands, altering soil ionic strength and sulfate availability with potential consequences for soil diffusive greenhouse gas (GHG) fluxes. Despite numerous local studies, large-scale response patterns and associated mechanisms remain insufficiently resolved. Here, we combined a global meta -analysis of chamber-based soil diffusive GHG fluxes (76 studies; 813 paired observations) with a year-long mesocosm experiment to quantify salinization effects on soil diffusive CO 2 , CH 4 , and N 2 O fluxes. In tidal marshes, salinization suppressed CO 2 and CH 4 fluxes more strongly than in mangroves, by 31% and 42%, respectively, whereas N 2 O declined modestly by 14%. In mangroves, suppression was dominated by N 2 O, which declined by 38%, with smaller reductions in CO 2 (7%) and CH 4 (10%). Across studies, CO 2 and CH 4 responses were associated mainly with ambient salinity and temperature seasonality, whereas N 2 O responses were more closely associated with tidal range. By contrast, Δsalinity, the magnitude of salinity change, showed weaker explanatory power for soil diffusive GHG responses. The mesocosm experiment provided process-related support for these patterns. In Cyperus -planted mesocosms, salinization reduced labile carbon pools and acetate availability and was associated with lower functional potential for organic matter breakdown ( cex , cdh , xylA ) and methanogenesis ( mcrA , acsB ), consistent with lower soil diffusive CO 2 and CH 4 fluxes. In Kandelia -planted mesocosms, salinization reduced dissolved inorganic nitrogen availability, increased C:N ratios, and was associated with lower functional potential for nitrogen fixation, nitrification, and denitrification ( nifH, amoA, narG, nirS ), consistent with stronger N 2 O suppression. These findings suggest that salinization effects on coastal-wetland soil diffusive GHG fluxes are shaped by environmental gradients and soil biogeochemical conditions rather than by Δsalinity alone. Incorporating ecosystem context into process-based models may improve predictions of short- to medium-term GHG responses to salinization in coastal wetland soils.