Effects of emergent aquatic plants on nitrogen transformation processes and related microorganisms in a constructed wetland in northern China

Aims Wetland plants affect ammonia-oxidation rate (AOR) and nitrogen-fixation rate (NFR), resulting in changes in plant-available nitrogen and wetland nitrogen-cycling. This study aims to investigate the underlying mechanisms and influencing factors associated with ammonium oxidation and nitrogen fixation in response to wetland-emergent aquatic plants. Methods A field experiment was conducted to study the response of activity, abundance, and community structure of ammonia-oxidizers and diazotrophs to three wetland-emergent aquatic plants in summer and autumn. The root exudates and soil physicochemical properties were determined due to their important role in rhizosphere interactions. Results AOR and NFR were significantly higher in rhizosphere than in bulk soils. The increases of AOR and NFR in rhizosphere were significantly positively affected by oxidation-reduction potential, additionally, NFR was also significantly affected by sugar-containing root exudates. Furthermore, NFR strongly correlated with diazotroph abundance, whereas AOR was linearly correlated with ammonia-oxidizing archaea (AOA) activity and mostly determined by a specific AOA taxon. Conclusions Emergent plant rhizosphere exhibited strong positive effects on AOR and NFR by altering community structure and abundance of related microorganisms. Release of available oxygen and carbon in plant rhizospheres is vital for promoting ammonium oxidation and nitrogen fixation and might further mediate wetlands nitrogen-cycling.