Simulating vertical soil nitrate migration induced by freeze–thaw cycles

Soil nitrate (NO₃-N) is a major contributor to groundwater contamination, posing significant risks to environmental health. This study investigated the critical role of freeze–thaw cycles in driving deeper soil hydrothermal dynamics and NO₃-N vertical migration within the 0–2 m soil profile under future climate change. Results showed that Soil and Water Assessment Tool (SWAT) predicted lower winter surface soil temperatures and less fluctuations in deeper soil layers than SWAT-FT, which equipped a physically-based freeze–thaw cycles module. Since the original SWAT model did not consider the phase transition of water and ice, the simulated soil water content was higher in winter than SWAT-FT. NO₃-N losses were greatest in May with more than 30.9 kg ha⁻¹ but were confined to surface soils. Losses in November, March, and April extended to deeper soils as influenced by legacy N and infiltration. Additionally, risk would increase with more severe emission scenarios, after freeze–thaw period, NO₃-N losses peaked at 100 cm in April of 7.0 kg ha⁻¹ and further migrated to 200 cm under high emission scenario for SWAT-FT. As for NO₃-N losses below soil profile, April comprised nearly 50% of the annual total, highlighting this critical month for mitigating losses. These findings revealed a “dual risk” of NO₃-N losses caused by instantaneous risk after fertilizer application and long-term risk from soil legacy N, highlighted the need to consider effects of freeze–thaw cycles and design targeted nitrogen management during high-risk periods.