Rising water levels increase CH4 emissions and decrease CO2 exchange in a temperate salt marsh

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 CO2 and CH4 fluxes over a growing season. Daylight CO2 uptake and dark CO2 emission were highest at the ‐10cm water table, while CH4 emissions were lowest at this water table. CO2 and CH4 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 CO2 sinks and minor sources of CH4 during the growing season. The high photosynthetic carbon assimilation combined with low CH4 fluxes resulted in a Global Warming Potential value of ‐326 g CO2eq m−2 on a 100‐year scale. Our study accounted for CH4 fluxes, CO2 uptake and emission together, and identified the mechanisms controlling CO2 and CH4 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.