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

Plants could affect wetland ecosystem functions by influencing the carbon and nitrogen cycles driven by microorganisms. However, the underlying mechanisms associated with CO2-fixation rate (CFR) by autotrophic microbes and denitrification rate (DR) in relation to wetland emergent plants remain unclear. In this study, we conducted a field experiment to analyze the effects of emergent plants on the activities and community characteristics of CO2-fixing autotrophic bacteria and denitrifying bacteria in two types of wetlands in a lake basin: freshwater and brackish water. We found that the effects of emergent plants on CFR and DR differed between freshwater and brackish wetlands. Compared to freshwater wetland, brackish wetland exhibited higher CFR in the rhizosphere, probably due to the abundance of organic carbon conducive to facultative autotrophic bacteria. However, the lower rhizosphere DR in brackish wetland compared to that in freshwater wetland was due to the salt stress. In both freshwater and brackish wetlands, rhizosphere soil increased CFR and DR. The microbial communities associated with them were distinct from those in non-rhizosphere. Besides, neutral-based processes governed the assembly of these bacterial communities, while the rhizosphere showed more prominent dominance of niche-based processes than non-rhizosphere. Statistical analyses revealed that soil organic carbon and root exudates jointly drove the differences in the metabolic activities and composition of CO2-fixation- and denitrification-associated microbial communities between rhizosphere and non-rhizosphere. Overall, our findings suggest that the organic carbon released by plants through root exudates and plant litter is vital for promoting wetland soil carbon-fixation and denitrification and alleviating the detrimental effects of salinity on denitrification. This study provides valuable information for the conservation of CO2-fixation and nitrogen removal with suitable plant species in wetlands.