Effects of waterlogging and salinity increase on CO₂ efflux in soil from coastal marshes

Coastal marshes play a notable role in sequestering carbon in plants and soil; however, coastal ecosystems are vulnerable to global change in terms of sea-level rise and saltwater intrusion. The effects of independent and interactive hydrological treatments of waterlogging and elevated salinity on soil CO₂ effluxes in Spartina alterniflora marshes were investigated based on a mesocosm approach. This study highlighted the importance of respective soil respirations during inundation (nondrainage, R_s.ND) and reaeration (drainage, R_s.D) in the marshes soils. We found that waterlogging treatments heavily suppressed R_s.ND but increased the R_s.D, regardless of salinity levels. Light salinity enhanced soil respiration, whereas high salinity inhibited soil CO₂ efflux, regardless of the water table level. Waterlogging strengthened the negative effects of salinity on R_s.ND and offset the negative effects on R_s.D. The variations in soil respiration under the hydrological treatments can be mainly attributed to changes in root biomass; indigenous soil microbial biomass; and activities of sucrase, cellulase and dehydrogenase, as well as ionic concentrations and oxidation-reduction potential. The temperature sensitivity (Q₁₀ value) of soil respiration (only for R_s.D) after drainage showed a notable decline under the conditions of waterlogging and high salinity. The effect degree of coupled hydrological treatments on soil respiration and Q₁₀ values was similar to a single waterlogging factor before drainage and more strongly than a single factor after drainage. We suggest that the soil CO₂ efflux under water flooding and reaeration, inundation duration, and salinity levels need to be considered to understand the impacts of future hydrological changes on coastal marshes.