Responses of nitrogen cycling and related microorganisms to brackish wetlands formed by evapotranspiration

Elevated evapotranspiration due to warmer air temperature could raise salinity and nutrient levels of some inland wetlands, potentially impacting nitrogen cycling. To characterize the impact of high evapotranspiration on soil microbial nitrogen cycling in inland wetlands, we compared freshwater and brackish marsh (or non-marsh) wetlands in terms of sediment ammonia-oxidizing rate (AOR), denitrifying rate (DR), and related microbial communities in a typical inland basin, the Hulun Lake basin, in China. Results showed that marsh ecosystems (ME) exhibited 31% higher AOR and 65% higher DR than non-marsh ecosystems (NE). For NE, freshwater non-marsh wetland exhibited 12% higher AOR than brackish non-marsh wetland. This was probably due to the inhibitory effects of high NH₄⁺ and salinity levels on ammonia-oxidizing archaea in brackish non-marsh wetland. Conversely, DR in brackish non-marsh wetland was 23% higher than that in freshwater non-marsh wetland, with total organic carbon (TOC) significantly influencing this difference, suggesting that the higher DR in brackish non-marsh wetland was mainly due to its higher TOC level. For ME, due to the direct and indirect interference of salinity, brackish marsh wetland displayed 26% lower AOR and 19% lower DR than freshwater marsh wetland. Besides, brackish wetlands harbored distinct ammonia-oxidizing and denitrifying microbial communities compared to freshwater wetlands. The assembly of these communities was dominated by stochastic processes, while brackish wetlands exhibited more prominent deterministic processes than freshwater wetlands. Overall, high evapotranspiration altered activities and community characteristics of ammonia oxidizers and denitrifiers in inland brackish wetlands by enhancing salinity and nutrient levels, while emergent plants occurring in ME could mitigate the adverse effects of salt stress of inland brackish wetlands on nitrogen cycling.