Environmental and anthropogenic factors affect bacterial community and nitrogen removal in the Yarlung Zangbo River.

Microorganisms play a critical role in the process of nitrogen removal in aquatic environment, which is regulated by multiple environmental factors. As a high-altitude region, the Qinghai-Tibet Plateau has unique composition of bacterial communities due to its unique geographical conditions, which may affect the nitrogen conversion of Plateau rivers. However, the regulation of nitrogen removal by environmental factors and bacterial community in high-altitude rivers has been rarely reported. This study investigated denitrification, anammox, and dissimilatory nitrate reduction to ammonium rates as well as the community of bacteria and denitrifiers in the Yarlung Zangbo River. The results showed that denitrification was the dominant nitrate removal process. Redundancy analysis revealed that environmental factors including suspended particulate matter, chemical oxygen demand, dissolved oxygen, nitrogen and phosphorus content, electrical conductivity, and pH explained a large amount of the variance in bacterial community. Denitrifiers carrying nitrite reductase-related gene were an important driver of denitrification in the Yarlung Zangbo River. The low water temperature brought by high altitude significantly reduced the denitrification rate. The cascade dams on the river affected the particle size distribution of sediment, changed the community composition of bacteria and denitrifying bacteria, and increased the denitrification rate in the downstream. Our findings highlight that nitrogen removal processes in high-altitude rivers are jointly regulated by environmental and anthropogenic factors through shaping denitrifier abundance.