Contrasting Roles of Microbial Heterogeneity and Sediment Heterogeneity in Controlling Hyporheic Nitrogen Removal

Abstract. Bacteria in streambed sediments reduce nitrogen pollution by performing chemical reactions that can transform pollutants. However, how differences in sediment types (like sandy vs. clay-rich layers) or the distribution of these bacteria affect the nitrogen removal in streambed sediments remains unclear. To explore this, we built numerical models to mimic real streambed conditions and explore how small-scale variations in sediment and microbial communities influence nitrogen removal. Our results show that the spatial changes in physical and chemical properties of sediments (e.g., permeability and organic matter content) have little impact on nitrogen removal. Instead, variations in the distribution of nitrogen-processing microbes are far more critical. This is because bacteria and the sticky substances they produce can slowly clog the sediment pores. This clogging limits the movement of pollution to the very top layers of sediments, making deeper sediment differences irrelevant to overall nitrogen removal. These results mean that future numerical models aiming to predict nitrogen removal should focus on mapping where these helpful microbes live, rather than overcomplicating simulations with too much detail about sediment variations. Simplifying sediment assumptions could make models more practical while still capturing the key role of microbes in cleaning streams.