Hydrodynamic forcing controls denitrification-DNRA competition and groundwater-derived nitrate contaminant attenuation.

River stage fluctuations regulate river aquifer interactions and subsurface biogeochemistry in upstream and midstream rivers. Nevertheless, how diel river-groundwater stage fluctuations impact surface subsurface mixing and the fate of groundwater derived nitrate remains poorly understood in downstream gaining rivers, critical hotspots for nitrogen accumulation and ecological risks. This study develops a coupled hydro-biogeochemical numerical model to evaluate how diel stage fluctuations govern nitrate attenuation via denitrification and dissimilatory nitrate reduction to ammonium (DNRA) competition. The model represents a typical dune-shaped streambed, where nitrate-polluted groundwater discharges into organic-carbon-rich stream. Diel surface water fluctuations propagate into the aquifer, inducing groundwater responses with 10-90% amplitude attenuation and 3-24 h time lags. Diel hydrological fluctuations expand surface-groundwater mixing zones, regulate in-situ C/N stoichiometry, alter nitrate reduction rates, and govern denitrification-DNRA competition. Under C/N ratios of 0.5-2, moderate hydraulic-head offsets (30-50%) and 6-18 h out-of-phase subsurface fluctuations favor DNRA, whereas large head differences (>70%) shift competition toward denitrification. Baseline simulations reveal that hydrological fluctuation regimes exert stronger control over this competition than C/N ratios, although biogeochemical kinetic properties may mask these fluctuation‑driven signals and limit the generalizability of such patterns. These results indicate targeted hydrodynamic‑regime regulation can enhance nitrate attenuation and mitigate nitrogen pollution risks in lowland rivers.