Dams shift microbial community assembly and imprint nitrogen transformation along the Yangtze River.

Dams are important for flood control, water storage, irrigation, electric generation, navigation, and have been regarded as the largest anthropogenic disturbance in aquatic ecosystems. However, how dams impact nitrogen transformation on a large watershed scale remained less studied. To explicitly address the impact of dams on nitrogen transformation, we used 16S rRNA gene sequencing to investigate the microbial dynamics and ecological processes under different dam conditions along the Yangtze River, as microbial communities are playing a key role in aquatic nitrogen transformation. Compared with landforms, dams exerted a more significant impact on the distribution patterns of microbial communities along the Yangtze River. The results showed that, by controlling suspended sand concentration, dams filtered keystone species, reshaped distribution of metacommunities, and mediated ecological assembly processes of microbial communities. Moreover, direct causal relationships between dams and nitrogen transformation were chained via microbial communities. To summarize, by combining knowledge in hydrology, microbial ecology, and biogeochemistry, this research exhibited the impact of different dams on the nitrogen transformation along a large river, and the key roles of suspended sand and microbial communities were emphasized. We anticipate a more precise modelling and prediction of nitrogen transformation in large watersheds, which may provide new perspectives for controlling the nitrogen in aquatic environments.