Rising water levels increase <scp>CH₄</scp> emissions and decrease <scp>CO₂</scp> exchange in a temperate salt marsh

Abstract Saline wetlands play a crucial role in climate regulation through their robust cooling effect, attributed to rapid carbon sequestration and minimal methane production. However, a comprehensive understanding of the mechanisms controlling their greenhouse gas (GHG) balance is lacking, particularly in salt marshes that are fully or partially submerged due to rising sea levels. We conducted a controlled manipulative experiment to test the effect of water levels on GHG emissions, including four water table levels: ‐10, 0, +5 cm and a fluctuating water table. We used soil cores from a Spartina anglica ‐dominated salt marsh and examined the CO 2 and CH 4 fluxes over a growing season. Daylight CO 2 uptake and dark CO 2 emission were highest at the ‐10cm water table, while CH 4 emissions were lowest at this water table. CO 2 and CH 4 fluxes were primarily driven by air and water temperature and solar irradiance. Our results indicate that salt marshes with near‐surface water levels (‐10 to 5 cm) function as potent CO 2 sinks and minor sources of CH 4 during the growing season. The high photosynthetic carbon assimilation combined with low CH 4 fluxes resulted in a Global Warming Potential value of ‐326 g CO 2 eq m −2 on a 100‐year scale. Our study accounted for CH 4 fluxes, CO 2 uptake and emission together, and identified the mechanisms controlling CO 2 and CH 4 exchange. This approach is crucial for evaluating the potential of saline tidal wetlands as net carbon sinks and for developing scientifically sound climate mitigation policies.