Microbial Heterogeneity Outweighs Sediment Variability in Regulating Hyporheic Nitrogen Removal

The hyporheic zone serves as a critical hotspot for nitrogen attenuation, driven by flow in streambed sediments, biogeochemical reactions, and enhanced microbial activity. It has, however, yet to be determined how the interaction of heterogeneity in sedimentary physical (e.g., permeability) and chemical (e.g., organic matter content) properties influences nitrogen cycling in complex hyporheic environments. Here we developed numerical models coupling porous flow, reactive transport, and microbial dynamics for realistic heterogeneous streambed scenarios. Simulations reveal that small-scale spatial variations in sediments physical and chemical properties exert negligible effects on nitrogen removal, whereas the spatial heterogeneity in functional microbial biomass dominates nitrogen removal dynamics. This is caused by biofilm-induced bioclogging that drastically reduces hyporheic exchange, thereby weakening the role of sedimentary heterogeneity. This study represents the first quantitative assessment of how sedimentary and microbial spatial heterogeneities jointly regulate nitrogen removal in hyporheic systems, offering critical insights for predictive modeling of bedform interfaces.