Urbanization intensifies deterministic selection of pathogenic bacteria in river networks: Nitrogen-driven niche partitioning and cross-scale risk forecasting through DOM-bacteria interplay.

Urbanization modifies the composition of dissolved organic matter (DOM) and nitrogen nutrients, profoundly affecting river microbial communities. However, the mechanisms driving pathogenic and non-pathogenic bacteria remain unclear. In this study, we analyzed the river system in Shijiazhuang, a northern Chinese megacity, using high-throughput sequencing, spectroscopy, and machine learning to explore how DOM and nitrogen influence bacterial communities. Results show that downstream urban rivers have elevated nitrogen levels (TN = 8.85 ± 2.14 mg L). DOM fluorescence peaks here, showing low humification and strong autochthonous characteristics (FI > 1.9, HIX <4). Both pathogenic (Shannon = 5.34) and non-pathogenic bacteria (Shannon = 6.43) reach maximum diversity downstream, with species replacement driving community differences. Nitrogen is the key factor increasing deterministic selection pressure on pathogenic bacteria from 1.51 % upstream to 25.76 % downstream. Midstream urban sewage contributes 41.39 %-89.26 % to downstream pathogenic bacteria. In winter and downstream, bacterial networks become more complex. DOM explains up to 60.22 % of downstream pathogenic bacterial variation. Protein-like DOM (C3) promotes summer pathogen proliferation but inhibits winter diversity. This study highlights nitrogen's role in pathogen niche differentiation and DOM's spatiotemporal regulation of bacterial interactions, providing a framework for health risk warnings in urbanized rivers.