The impact of atmospheric oxygen supply on the nitrogen cycle during hyporheic exchange in the river bank aquifer

Hyporheic exchange between surface water and groundwater governs the biogeochemical cycling of groundwater in riparian zones and plays a crucial role in the spatiotemporal dynamics of watershed aquatic environments. The spatiotemporal dynamics of oxygen during this exchange determine the redox zonation of groundwater, thereby controlling the activity of reactions such as aerobic respiration, nitrification, and denitrification. However, most current studies assume that oxygen-rich river water is the sole source of oxygen in riparian aquifers, overlooking the significant oxygen supply process via atmospheric diffusion and dissolution into the unsaturated zone. Therefore, this study developed a numerical model of gas–liquid two-phase flow and reactive solute transport in the phreatic aquifer of a riparian zone to investigate nitrogen migration and transformation processes under the influence of atmospheric oxygen diffusion and dissolution. By comparing the classical model with an oxygen diffusion model under varying hydraulic conductivities, river stage fluctuation amplitudes, and rainfall infiltration rates, we found that: (1) Oxygen diffusion from the atmosphere increases dissolved oxygen (DO) concentrations in the aquifer by over 200%, leading to a 220% increase in nitrification rates and a 40% increase in denitrification rates; (2) The influence of oxygen diffusion on nitrogen cycling in riparian zones is positively correlated with hydraulic conductivity. oxygen is more readily supplied under high-permeability conditions, and then accelerating nitrogen cycling reactions; (3) Although oxygen-rich rainfall infiltration provides a direct DO input, it weakens the dissolution–diffusion process of oxygen. Under the “competitive supply” of these two processes, the DO flux into the riparian zone is positively correlated with infiltration rate. These results highlight the critical role of atmospheric oxygen diffusion in shaping subsurface redox conditions and nitrogen dynamics, underscoring the need to incorporate unsaturated zone processes into future riparian biogeochemical models.