Hydrological Controls on Nitrate Release at the Meltwater‐Frozen Soil Interface in Seasonally Frozen Croplands

ABSTRACT The presence of low‐permeability frozen soil layer forces meltwater to flow primarily laterally within an extremely thin saturated zone at the soil surface. However, the dynamics of solute release at this interface and the underlying driving mechanisms remain poorly understood. In this study, controlled laboratory experiments were conducted using a self‐designed, unidirectional freezing apparatus to investigate nitrate transport within the thin saturated layer at the meltwater‐frozen soil interface during early spring thaw. Results indicate that nitrogen extraction efficiency at this interface is not constant but is significantly governed by inflow conditions; specifically, it exhibits a significant positive correlation with the ionic concentration of the meltwater inflow and a negative correlation with flow rate. To account for the observed concentration effects, the Electric Double Layer theory was introduced into the analysis. Analysis reveals that high NO 3 − ‐N concentration compresses the electrical double layer thickness on soil colloid surfaces, generating an electrostatic shielding effect that microscopically accelerates nitrogen desorption from the soil surface. Incorporating these key driving factors, a CSTR model coupled with reversible first‐order kinetics was developed. This model effectively integrates the effects of unsteady flow and NO 3 − ‐N concentration and successfully reproduces the non‐equilibrium solute transport processes at the meltwater‐frozen soil interface with high simulation accuracy ( R 2 > 0.85). This study improves the understanding of the dominant factors and adsorption–desorption mechanisms governing solute release in the thin saturated layer at the meltwater‐frozen soil interface, and provides a process‐based basis for modelling and risk assessment of snowmelt‐driven non‐point source pollution in cold‐region agricultural systems.