Effects of emergent plants on soil carbon-fixation and denitrification processes in freshwater and brackish wetlands in a watershed in northern China

•Emergent plant positively affected CO2-fixation and denitrification rates.•Emergent plant alleviated the adverse effects of salinity on denitrifying bacteria.•Rhizosphere effect promoted the dominance of niche-based process of bacteria community.•Organic carbon released by plants promoted wetland CO2-fixation and denitrification.•Plants with abundant root-derived organic carbon are suitable for wetland construction.