Dual drivers of riverine nitrogen transformation: Microbial–plankton community assembly and environmental interactions

Nitrogen cycling is a critical process for maintaining the ecological function and water quality stability of river ecosystems. However, under increasing anthropogenic disturbances, its transformation pathways and ecological response mechanisms have become increasingly complex. The water-sediment interface, as a biogeochemically active zone for nitrogen transformations, is influenced by both environmental factors and biological processes. Yet, the drivers of its multi-pathway nitrogen cycling remain unclear, particularly under multi-modal and multi-factor interaction scenarios. This study employed co-occurrence network analysis, random forest modeling, and coupled matrix and tensor factorization (CMTF) to identify biological-environmental associations, screen key taxa and environmental factors influencing different nitrogen cycling pathways, and explore the latent core drivers and mechanisms underlying multi-path nitrogen transformation processes. The co-occurrence network indicated that nitrogen transformations in surface water are more dynamically and jointly regulated by rapid physicochemical fluctuations and multi-trophic interactions. The random forest results showed denitrification and nitrogen fixation were strongly responsive to salinity, NH₄⁺ and NO₃^-, while phytoplankton and zooplankton primarily influenced organic nitrogen transformation, assimilation, and ANRA pathways. Microbial communities mainly participated in inorganic nitrogen transformation processes. CMTF further resolved three major ecological mechanisms: (1) reductive processes associated with nitrogen fixation and the DNRA pathway primarily driven by environmental factors, (2) nitrification processes jointly governed by microbial communities, oxygenated and nutrient conditions, (3) organic nitrogen transformation and ANRA processes co-regulated by biological activity and environmental factors. This study elucidates the differential ecological drivers of nitrogen cycling pathways in river systems, enhances the understanding of multi-pathway nitrogen dynamics in complex ecosystems, and provides theoretical insights for watershed nitrogen pollution control and ecosystem management.