<p>This study investigates how biological processes, particularly riparian vegetation, regulate nitrogen (N) dynamics under varying redox conditions in the riparian zone of an agricultural fluvial island. Groundwater characterized by high nitrate (NO₃⁻) concentrations and oxidizing conditions flows toward adjacent surface waters through the riparian zone. Along this subsurface pathway, NO₃⁻ concentrations declined significantly, whereas bicarbonate (HCO₃⁻) concentrations increased, reflecting enhanced reducing conditions. The strong positive relationship between the molar ratio of HCO₃⁻ to total anions and dissolved organic carbon (DOC), together with the δ¹³C-DIC signatures consistent with CO₂ derived from C₃ vegetation, indicates that root and microbial respiration are major contributors to biogenic carbon production. Depth-resolved hydrochemical monitoring further revealed that areas with tall vegetation sustained reducing conditions and low NO₃⁻ concentrations at both shallow and deep depths, while deeper zones with short vegetation exhibited oxidizing conditions and elevated NO₃⁻ concentrations. These findings demonstrate that vegetation-driven biological activity influences vertical and lateral redox heterogeneity, thereby influencing NO₃⁻ attenuation efficiency within the riparian zone. Overall, the riparian zone functions as a significant biogeochemical filter mediating N and C fluxes across groundwater–riparian–surface water interfaces, offering important implications for nutrient management and the sustainability of agricultural watersheds.</p>