Soil water and salt transport in seasonally frozen cropland: isotopic tracing method with geochemical modelling

Understanding the dynamics of water and salt migration in seasonally frozen agricultural soils is critical for effective arid land management. This study combined field monitoring with a three-method extraction strategy for stable isotope analyses (direct vapor equilibration, centrifugation, and cryogenic vacuum distillation). By integrating resulting isotope signatures into an Isotope Mass Balance (IMB) model, we quantitatively differentiated phase-state water pools. The results confirmed that freezing induces significant Rayleigh fractionation, enriching ice in heavy isotopes relative to mobile water. In contrast, the bound water fraction remains hydraulically isolated and isotopically distinct, requiring its exclusion from phase-change calculations. Coupling with geochemical modelling (FREZCHEM) revealed that salt migration is controlled by the interplay between thermally driven convective fluxes and concentration-driven diffusive fluxes, although individual ion exhibited distinct redistribution pathways.Cryogenic precipitation regulates soil salt transport regime: extensive surface crystallization reduces dissolved ion concentrations, thereby maintaining the steep upward gradient required for continuous salt accumulation. The model demonstrated that crystallization accounted for up to 40.5 % of the total salt load incorporated into solid phases during freezing. These solid salts create a "geochemical trap" in which re-dissolution lags behind the initial spring meltwater pulse, significantly reducing leaching efficiency. Consequently, sustainable salinity management cannot rely on hydraulic regulation alone. Effective irrigation strategies must integrate groundwater management with the specific composition of the salt load to overcome these persistent geochemical constraints.