Coastal wetlands are considered a significant sink for global carbon in the form of SOC. However, over half of New England salt marsh has been lost due to human activities in the past three centuries. Restoration of these salt marshes needs understanding the change of soil C biogeochemical cycles after alteration of inundation levels and water salinity. This study compared the alteration of water salinity and inundation levels on SOC decomposition loss during a 60-days incubation experiment. Intact soil cores from a freshwater marsh and a salt marsh were exposed to simulated two inundation levels (flooded and drained) with seawater and freshwater. Soil and water physio-chemical properties, CO2 flux, CH4 flux, and dissolved OC (DOC) was measured. The freshwater soil cores included the new C from C3 plants and old C from C4 plants, δ13C signatures in CO2 flux and soil C were also investigated to detect the CO2 source.
The increase of salinity in freshwater marsh enhanced 50% -80% the microbial CO2 emission and impeded CH4 flux, while decreasing the salinity (from 26 ppt to 16 ppt) in salt marsh had no effect on CO2 and CH4 flux. In comparison with salinity, the change of inundation levels had more pronounced effect on the CO2 flux: drained soil cores emitted nearly 10-fold higher CO2 than its flooding reference in both marshes. Drainage of soil cores also increased DOC concentrations. Under the drained condition, CO2 emission from old C4 plant residuals was substantially increased by seawater treatment. This study suggested that restoration of salt marsh would 1) decrease the CH4 emission due to the intrusion of sea water; 2) inhibit the aerobic decomposition but increase the anaerobic oxidation, with a net effect of higher C sequestration rate.