Mechanisms of microbial and functional gene influences on nitrogen cycle in pumping station forebays

Nitrogen pollution in rivers has long been a key environmental factor hindering ecosystem recovery. As critical nodes in water systems, pumping station projects further complicate river environmental issues. This study focused on the pumping stations in the Qinhuai River, a tributary in the lower reaches of the Yangtze River in China, and investigated the mechanisms driving the nitrogen cycle in the forebays of pumping stations through field sampling, metagenomic analysis, and statistical methods. The results indicated that the most abundant phylum of nitrogen-cycling microorganisms in the forebays was Pseudomonadota, accounting for 50.29% of the community. The nitrogen cycle processes were primarily dominated by nitrogen assimilation, nitrogen mineralization, and denitrification. The abundance of denitrification-related genes (nirKS and nosZ) and the diversity and abundance of nitrogen-functional microorganisms were higher in rural areas than in urban areas. NH₃-N exhibited a strong correlation with microbial communities in rural areas, while dissolved oxygen (DO) and MnO₄⁻ were more closely associated with microbial communities in urban areas. Results from partial least squares path modeling (LS-PM) indicated that the maximum path coefficients in rural and urban areas were 0.315 for denitrification functional genes and 0.687 for nitrogen-mineralization functional genes, respectively. The path coefficient for nitrification in urban areas was greater than those for dissimilatory nitrate reduction and denitrification, suggesting that rural conditions are more conducive to nitrogen removal, while urban conditions favor nitrogen fixation. This study provides significant insights for understanding the ecological processes involved in pumping station water diversion projects.