Assessing the impact of freeze-thaw cycles on nitrate leaching in the Mollisol regions under global warming

Nitrate leaching is a major form of agricultural non-point source pollution and a critical source of groundwater contamination. Clarifying its occurrence characteristics is a prerequisite for formulating targeted measures to control nutrient losses from cropland. This study employed and improved a hydrological model (Soil and Water Assessment Tool-Freeze-Thaw; SWAT-FT) to investigate the driving effect of freeze-thaw cycles on soil hydrothermal dynamics and nitrate vertical migration within the 0-2 m soil layer in the Upper Mississippi River Basin under future climate change. The results showed that the fully calibrated SWAT-FT model, which considered the insulating effect of winter snow cover and water-ice phase change processes, provided more physically meaningful simulations of soil hydrothermal processes in winter compared to the original SWAT model. In the future, SWAT-FT predicted higher winter surface soil temperatures and greater fluctuations in deeper soil temperatures, along with lower soil water content than SWAT, fully demonstrating the key role of a physically based process mechanism model in simulating soil hydrological processes. Moreover, nitrate leaching peaked at approximately 33.7 kg ha^-1 in May following fertilization but was confined to the surface soil layer. Furthermore, nitrate leaching in November, March, and April, which the months associated with freeze-thaw cycles, was similarly elevated, with migration into deeper soil layers further influenced by legacy soil nitrogen. Particularly in April, the amount of nitrate finally leached out of the soil layer accounted for approximately 50% of the total annual leaching. These results reveal a “dual risk” in nitrate losses: the immediate risk post-fertilization and the long-term risk from legacy soil nitrogen. This highlights the vital role of freeze-thaw cycles on nitrate losses and underscores the necessity of developing targeted management measures.