Role of biogeochemical and hydrodynamic characteristics in simulating nitrogen dynamics in river confluence

• Published in: Water research (Oxford) 2025-01, Vol.268 (Pt A), p.122647, Article 122647
• Main Authors: Hui, Cizhang, Li, Yi, Yuan, Saiyu, Tang, Hongwu, Zhang, Wenlong
• Format: Article
• Language: English
• Subjects: Geologic Sediments - microbiology; Hydrodynamics; Microbial community; Nitrogen; Nitrogen dynamics; Reactive transport model; River confluence; Rivers
• ISSN: 0043-1354; 1879-2448; 1879-2448
• DOI: 10.1016/j.watres.2024.122647

•Different hydrodynamic zones of confluence shape varied microbial communities.•Microbial communities in flow separation zone significantly promote NO3− removal.•Reaction rates are consistent with abundances of associated functional genes.•Simulation method coupling biogeochemical and hydrodynamic data fit well with actual reactions. The confluence area is the link of different river systems, whose specific hydrodynamic characteristics can significantly influence mass transport and distribution, which can further make a difference to microorganism growth and biogeochemical processes. However, the specific influences of hydrodynamic characteristics in confluence on formation processes of microbial communities and the biogeochemical processes remain unclear. To this end, the present study established an indoor self-circulation confluence flume and conducted 28-day culture experiment to thoroughly investigate the characteristics of microbial communities and nitrogen dynamics in sediment of confluence area. Results illustrated that the initial homogenous microbial communities gradually emerged differences among varied hydrodynamic zones with experiment going on. Concentrations of nitrogenous materials also changed at different experiment period, NO3− concentrations peaked at day 14, and then exhibited significant downtrend. The mean NO3− concentrations decreased the most in flow separation zone, with a 62 % decrease from day 14 to day 28. A numerical model was further established following the thermodynamics of enzyme catalysis reactions to simulate nitrogen transformation rates based on abundances of associated functional genes (gene-centric model). The average relative deviation between simulated and measured N2 production rates was 32 %. To further investigate the influence of hydrodynamic characteristics on nitrogen dynamics, DamKöhler numbers were calculated as the ratio of characteristic residence time to reaction time. DamKöhler numbers were better fitted with measured N2 production rates than simulated results of gene-centric model, signifying the importance of hydrodynamic characteristics in simulating nitrogen dynamics in confluence area. [Display omitted]