Covariation of DOM molecular characteristics and bacterial carbon metabolism potential along a reservoir-agricultural river nutrient gradient.

Agricultural nutrient loading is associated with variation in dissolved organic matter (DOM) composition and microbial communities across reservoir-agricultural river continuums, yet the covariation among nutrient conditions, DOM molecular heterogeneity, and bacterial metabolic potential remains insufficiently understood. This study integrated optical spectroscopy, ultrahigh-resolution mass spectrometry, nitrate dual-isotope tracing, end-member mixing analysis (EMMA), 16S rRNA sequencing, and metagenomic functional profiling to characterize DOM sources and composition and their associations with bacterial communities in a typical reservoir-agricultural river continuum. Our results showed that nitrogen and phosphorus enrichment within agricultural river reaches systematically rearranges DOM source apportionment, with relative abundance of humic-like components (C1, C3) rising alongside elevated abundances of heteroatomic molecular formulas (CHOS, CHONS) relative to oligotrophic reservoirs. Nutrient loading reshaped DOM via exogenous input of heteroatom-rich anthropogenic organics and boosted in-situ microbial humification. EMMA modeling further indicated that anthropogenic sources contributed 37.9% to riverine DOM. Principal coordinates analysis showed that reservoir-river community differentiation coincided with nutrient and fluorescent DOM gradients. Reservoir samples were dominated by Acinetobacter, Exiguobacterium, and Clade_III and showed higher relative abundances of several genes associated with carbohydrate and aromatic compound degradation. In contrast, river communities featured humic-adapted genera (Mycobacterium, hgcl_clade, Limnohabitans, and unclassified_Sporichthyaceae) and showed higher relative abundances of genes associated with lipid utilization and terrestrial-polymer degradation. This study highlighted coordinated spatial variation in nutrient conditions, DOM characteristics, bacterial community composition, and genomic carbon metabolism potential across the reservoir-agricultural river continuum, supplying theoretical guidance for pollution control and carbon cycling regulation in anthropogenic-disturbed inland water catchments.