Response of denitrification rate and denitrifiers to human-disturbance intensity and physicochemical factors of overlying water in a subtropical stream, China

Human activities have greatly increased the availability of bioactive nitrogen and, consequently, have altered the nitrogen cycle in riverine systems. Here, we studied denitrification rates and denitrifiers in river sediment and their relationships with aquatic physicochemical variables along a gradient of human-disturbance intensity in a subtropical stream in China. We used high-throughput sequencing to characterize denitrifying assemblages (denitrification genes narG, nirK, nirS and nosZ as functional markers). We found that denitrification rates were positively correlated with water temperature, and the primary factors affecting denitrifier assemblage species richness were water temperature and NO3 2. Denitrification rate was negatively associated with narG and nirS gene-coding denitrifier species richness and associated (both positively and negatively) with the relative abundances of many of the dominant denitrifiers with narG and nirS genes. The potential active denitrifiers with narG and nirS genes predominantly occurred in the most intensely disturbed area within the catchment (which also recorded the highest temperatures), but denitrification rates in the intensely and slightly disturbed areas were similar. Our research found relationships between denitrifier assemblage alpha diversity and relative abundances of dominant species, environmental factors, and denitrification rates. In this study, we did not find human-disturbance intensity to be a direct influence on denitrification, likely because denitrification is a complex process influenced by both the environment and the microbial assemblage.