Responses of soil respiration and microbial activity in Phragmites australis and Spartina alterniflora marshes to hydrological changes

• Phragmites species showed higher sensitivity to combined stresses than Spartina species. • Drainage altered species-specific responses to environmental stresses. • Higher microbial activities explained better adaptation of Spartina species. • Combined stresses revealed non-additive effects on marsh soil carbon processes. Coastal wetlands face unprecedented challenges from sea-level rise; however, the effects of increased waterlogging and salinity on soil respiration remain unclear, particularly given the contrasting stress tolerance strategies of dominant marsh species such as native Phragmites australis and invasive Spartina alterniflora . We conducted a mesocosm experiment to compare soil respiration responses to waterlogging (control: low water level; treatment: high water level) and salinity (control: 0 ppt; treatments: 5, 15, and 30 ppt) between these two species, while also examining the underlying mechanisms and the role of drainage. Under waterlogged conditions, high water levels significantly suppressed soil respiration, with P. australis exhibiting a greater reduction than S. alterniflora (78.9 % vs. 74.4 %). The species showed distinct responses to salinity: high salinity (30 ppt) caused a more pronounced reduction in P. australis than in S. alterniflora (56.4 % vs. 28.9 %). Drainage conditions fundamentally altered these responses; P. australis showed a greater post-drainage enhancement under high water level (42.4 % vs. 12.9 %) but consistently exhibited higher sensitivity to salinity. These species-specific responses were mediated by differential changes in root biomass, microbial biomass carbon, and carbon-cycling enzyme activities. Principal component analysis revealed clear ecological niche separation along the stress gradients, with S. alterniflora maintaining higher biological activity and P. australis displaying greater stress sensitivity under severe combined conditions. The combined effects of waterlogging and salinity revealed non-additive impacts on soil respiration, with S. alterniflora demonstrating superior tolerance through sustained microbial activity and root functioning. Our findings underscore the importance of considering species-specific adaptive strategies and tidal dynamics when predicting coastal wetland carbon cycling under future sea-level rise scenarios, as the differential stress responses of these ecologically significant species may substantially influence ecosystem carbon dynamics.