Coupling behavior of natural organic matter and nitrate reduction in iron-rich riparian zones under varying hydraulic retention times

Excessive nitrate poses an ecological threat to aquatic environments. Riparian zones, serving as critical ecological buffers, can effectively facilitate its reductive removal. Elucidating the underlying mechanisms requires research grounded in representative hydrochemical compositions and key hydraulic gradients; however, relevant studies remain scarce. This study investigated the effects of fulvic acid and butyric acid—two representative components of natural organic matter prevalent in infiltrating river water—on nitrate reduction and the associated response of microbial communities in an iron-rich mineral riparian zone under simulated varying hydraulic retention times (HRT). The results demonstrated active nitrate reduction across all systems. In the fulvic acid system, mineralization and nitrification generated substantial nitrate, and shorter HRT further enhanced this process. In contrast, nitrate reduction in the butyric acid system was rapid and complete. Nitrogen fixation resulted in significant ammonium production accompanied by a notable increase in iron concentration, an effect most pronounced under shorter HRT conditions simulating dry seasons. High-throughput sequencing revealed that the carbon source shapes microbial community structure, thereby determining nitrogen transformation pathways, while HRT modulates the intensity and extent of these transformations by regulating the abundance of specific microorganisms. • Organic carbon shapes functional microbial community structure, and HRT influences the abundance of dominant species. • Fulvic acid inhibits nitrate degradation. The shortening of HRT stimulates mineralization and nitrification to produce nitrate. • Nitrate degrades rapidly in a butyric acid environment, with substantial generation of ammonium and ferrous iron.