The competition between heterotrophic denitrification and DNRA pathways in hyporheic zone and its impact on the fate of nitrate

Abstract

Dissimilatory nitrate reduction to ammonium (DNRA) and heterotrophic denitrification (DEN) are recognized as two key nitrate reduction pathways competing for both nitrate and dissolved organic carbon (DOC). However, the mechanisms and influencing factors governing this competition in the dynamic hyporheic system remain unclear. In this study, a numerical model that coupled flow, reactive solute transport and microbial activities was developed to investigate the competition between DEN and DNRA in the hyporheic zone. The modeling results indicate that DNRA hotspots are distributed along the hyporheic flow path. The greater the flow velocity, the farther the DNRA hotspots are from the water-sediment interface. In the simulation scenario involving river stage fluctuations, the relative contribution of DNRA resulting from these fluctuations under a small initial hydraulic gradient (0.001) of the hyporheic zone increased by 60% compared to the range observed under a large initial hydraulic gradient (0.01). The C/N ratio in river water does not dictate the prevailing microbial group at specific locations within the hyporheic zone. Instead, the river C/N ratio governs the competition outcomes across the entire hyporheic system, demonstrating a three-phase pattern. The nitrate removal efficiency in the hyporheic system approaches 100% at C/N = 3. As the stream C/N ratio increases, the contribution of DNRA gradually rises and can reach 80% (C/N = 10). At this point, the contribution of the heterotrophic denitrifying bacteria and their corresponding nitrate removal efficiency decreases to only about 20%. Disregarding the DNRA reaction might lead to an underestimation of nitrate removal efficiency of a hyporheic zone. Our investigation improves the understanding of nitrate fate in hyporheic zones and provides a scientific foundation for watershed management and pollution control.