Microbial Community Dynamics in the Surface Water-Groundwater Interaction Zone of the Upper Miyun Reservoir Basin

Nitrogen (N) fluxes delivered from river inflows to reservoirs are critical for regulating nutrient dynamics and sustaining water quality in drinking-water source regions. In this study, we investigated the surface-water-groundwater interaction zone within the Chao-Bai River catchment, the primary inflow area feeding the Miyun Reservoir, which is the main water supply for Beijing. Our objective was to characterize the spatiotemporal variability of N species, microbial community compositions, assembly processes, and functional taxa, and to clarify their respective contributions to N cycling. Nitrate-N, which represented 88 ± 9% of total N, was consistently higher in the Chao River (wet season: 13.7 ± 12.6 mg/L; dry season: 11.5 ± 3.8 mg/L) than in the Bai River (wet season: 6.9 ± 5.0 mg/L; dry season: 7.0 ± 4.2 mg/L). Microbial assemblies differed markedly across hydrological compartments and seasons. During the wet season, Novosphingobium dominated surface waters, whereas Limnohabitans and unclassified_Xanthobacteraceae were prevalent in groundwater, and Flavobacterium was abundant in sediments. In the dry season, surface waters were co-dominated by Flavobacterium and Limnohabitans, while Limnohabitans, Acidovorax, and Hydrogenophaga were characteristic of groundwater communities. Temperature, dissolved oxygen, and pH emerged as the principal environmental drivers structuring microbial interactions. Stochastic processes primarily governed microbial community assembly in surface waters and sediments, whereas deterministic selection exerted stronger control in groundwater. Overall, the results demonstrate that hydrological connectivity and microbial dynamics interactively regulate N fluxes and transformation pathways at the river-reservoir interface. These insights provide a mechanistic foundation for improving nutrient management and protecting drinking-water sources in reservoir ecosystems.