Distribution Characteristics of Denitrifying Bacteria in the Rhizosphere of Wetland Plants in Urban Rivers of the Karst Region in Southwest China

Abstract The discharge of nitrogen-rich wastewater into urban rivers often leads to water eutrophication, and the construction of river wetlands is a crucial measure to mitigate this issue. Microorganisms play a significant role in the nitrogen removal processes within river ecosystems, particularly in the rhizosphere of plants where microbial activity is intense. This study investigates the distribution characteristics of denitrifying microbial communities in the rhizosphere sediments of wetland plants in the Xiaoche River urban wetland in Guiyang. High-throughput sequencing was employed to analyze the bacterial diversity in the rhizosphere and non-rhizosphere sediments of three typical wetland plants (Acorus calamus, Cyperus alternifolius, and Echinochloa crus-galli). Additionally, the abundance of denitrifying and anaerobic ammonium-oxidizing (anammox) bacteria in the sediments was determined using real-time quantitative PCR. Sequencing results indicated that there are 16 bacterial phyla with a relative abundance greater than 1% in both rhizosphere and non-rhizosphere sediments, with Proteobacteria, Bacteroidetes, and Acidobacteria being the dominant phyla, collectively accounting for over 50% of the relative abundance. The relative abundance of Proteobacteria was higher in the rhizosphere than in the non-rhizosphere, while Bacteroidetes showed higher relative abundance in the non-rhizosphere compared to the rhizosphere. There were 24 bacterial genera with relative abundance greater than 1%, and the dominant genera varied significantly among different sampling sites. Cluster analysis revealed significant differences in genus-level populations between rhizosphere and non-rhizosphere samples, with high similarity between the populations of Acorus calamus and Cyperus alternifolius. Quantitative gene results indicated that the abundance of denitrification and anammox genes was lower in the non-rhizosphere sediments than in the rhizospheres of the three plants, with anammox 16S rRNA and nirS gene abundance levels reaching 1010 copies/g dry sediment, suggesting a high richness of anammox and denitrifying bacteria in the rhizospheres of wetland plants in the Xiaoche River. Redundancy analysis (RDA) showed that the environmental factors most influencing the abundance of these two genes were total phosphorus (TP), organic matter (OM), and ammonium nitrogen (NH4+-N).