Fluvial Dissolved Organic Matter Quality Modulates Microbial Nitrate Transformation: Enhanced Denitrification under Low Carbon-to-Nitrate Ratio

Dissolved organic matter (DOM), particularly the molar ratio of dissolved organic carbon (DOC) to NO₃^- plays a crucial role in shaping fluvial microbial NO₃^- transformation processes. However, the underlying mechanisms involving microbial genes remain poorly unexplored. To address this gap, six sampling campaigns were conducted in the Qingshui River, northwest China. Samples from different temporal and spatial points were categorized into three groups based on DOC:NO₃^- values: low (L, <0.582), medium (M, 0.582 ∼ 3.13), and high (H, >3.13). Surprisingly, despite stoichiometric models suggesting that low DOC:NO₃^- suppresses denitrification, our results showed the highest median abundances of denitrification genes in group L, followed by M and H. Isotope analysis confirmed denitrification as the dominant process in group L. This enhanced denitrification was linked to the higher proportion of fulvic acid-like compounds in DOM, which promoted microbial cooperation, stabilized network interactions, and maintained elevated gene abundances related to respiration and energy metabolism (especially electron transfer). Laboratory experiments further demonstrated that fulvic acid-rich DOM, even under low DOC:NO₃^- conditions, served as an efficient carbon source, enhancing the activities of nitrate reductase and nitric oxide reductase. These findings provide new insights into how DOM composition modulates microbial nitrate transformations, offering strategies to promote fluvial denitrification.