Seasonal variations drive microbial community structure and nitrogen cycling in sediments of tributary pumping station forebays

Discharge from tributary pumping stations often impacts mainstream water quality, yet microbial communities and nitrogen metabolism in pumping station forebays remain poorly understood. Therefore, this study investigated the microbial community structure and nitrogen cycling mechanisms in sediments of tributary pumping station forebays within the Qinhuai River Basin using 16S rRNA and metagenomic sequencing. Results showed significant seasonal variations in the diversity and structure of sediment microbial communities, with higher diversity in spring than in winter. Genes associated with denitrification (e.g., narG, nirS, and nosZ) showed the highest abundance, suggesting that denitrification may be a key nitrogen transformation pathway. Co-occurrence network analysis revealed tighter associations between microbial taxa and nitrogen-cycling genes in spring, indicating more complex potential interactions during this season. The shift of network hubs across seasons suggested a seasonal succession of potential core functions related to nitrogen cycling. Redundancy analysis revealed that nitrate nitrogen (NO₃^--N), water temperature (WT), and ammonium nitrogen (NH₄⁺-N) were the factors most strongly associated with microbial community variation, with WT showing the strongest association with functional gene distribution. Partial least squares path modeling revealed that seasonal variation had a significant positive association with denitrification gene abundance and a significant negative association with genes related to assimilatory nitrate reduction to ammonium and anaerobic ammonium oxidation. These findings improve our understanding of microbially mediated nitrogen cycling in pumping station forebays and provide a scientific basis for water quality management in river networks influenced by pumping station drainage.IMPORTANCEThis study is important because it reveals that pumping stations, which are key infrastructure in managed river systems, are not just hydraulic structures but dynamic bioreactors where microbial communities actively transform nitrogen. By demonstrating seasonal variations in microbial diversity and revealing a high denitrification potential, the research provides a mechanistic understanding of how nitrogen pollution is naturally mitigated in these engineered environments. Crucially, it pinpoints temperature as a primary regulator of these microbial functions. These insights allow water managers to proactively optimize pumping operations and design interventions that harness microbial activity, ultimately protecting downstream water quality from nutrient pollution in a changing climate.