While the effect of increasing tidewater inundation caused by sea-level rise on carbon cycling had been well studied in saltmarshes, little is known about the effect of increasing tidewater inundation on CO2 and CH4 effluxes in the tidal freshwater marsh soils. Herein, the effects of tide inundation on porewater geochemistries (NH4+, NO3-, DOC, dissolved CH4, and DIC) and CH4 and CO2 effluxes were examined in the soils of tidal freshwater marshes in the Minjiang River Estuary, East China Sea. By applying "mesocosm" and a simulated tide pool, the tide inundation height increased by 15 cm and 30 cm over the control (CK). The CO2 effluxes decreased by 28.53% and 36.56%, and the dissolved CH4 concentrations increased by 47.83% and 73.91%, in treatments (CK+15 cm) and (CK+30 cm), respectively. The CH4 effluxes did not change significantly in the treatment (CK+15 cm), but increased by 29.27% in treatment (CK+30 cm). The increasing tidewater inundation had no significant impact on DOC concentrations, but increased NH4+ concentrations and decreased DIC and NO3- concentrations. Increasing tide inundation also reduced the temperature sensitivity of CH4 and CO2 effluxes. The study highlighted that the sea level rise-induced increase in tidewater inundation would decrease the annual global warming potential of tidal freshwater wetlands by 28% and 35% in the next 50 and 100 years, respectively.
Keywords: CH4 efflux; CO2 efflux; Minjiang River Estuary; freshwater tidal wetlands; increased inundation; sea-level rise.