Simulating vertical soil nitrate migration induced by freeze-thaw cycles

Authors: ZHANG, YINGQI - China Agricultural University; ZHANG, XIAOYU - China Agricultural University; DING, BEIBEI - China Agricultural University; QI, JUNYU - University Of Maryland; Marek, Gary; FENG, PUYU - China Agricultural University; LIU, DE LI - Wagga Wagga Agricultural Institute; SRINIVASAN, RAGHAVAN - Texas A&M University; CHEN, YONG - China Agricultural University

Submitted to: Journal of Hydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/3/2025
Publication Date: 5/5/2025
Citation: Zhang, Y., Zhang, X., Ding, B., Qi, J., Marek, G.W., Feng, P., Liu, D., Srinivasan, R., Chen, Y. 2025. Simulating vertical soil nitrate migration induced by freeze-thaw cycles. Journal of Hydrology. 660. Article 133451. https://doi.org/10.1016/j.jhydrol.2025.133451.
DOI: https://doi.org/10.1016/j.jhydrol.2025.133451

Interpretive Summary: Nitrate leaching in intensively managed agricultural production areas such as the Upper Mississippi River Basin (UMRB) can be affected by freeze-thaw cycles. However, limited research has addressed nitrate leaching at deeper soil depths as influenced by the timing and magnitude of freeze-thaw cycles and how future climate change may affect these processes. Researchers from USDA-ARS Bushland and university partners from the U.S., Australia, and China simulated the effects freeze-thaw cycles on nitrate leaching in the UMRB using an improved Soil and Water Assessment Tool (SWAT-FT) model with projected climate data. Fluctuations in winter surface soil temperatures were greater for the SWAT-FT than those of the native model SWAT. Although nitrate leaching was most pronounced at shallow soil depths during May, leaching at deeper soil depths was observed for transition months of March, April, and November with cumulative values representing nearly 70 percent of annual total values for future scenarios. These findings highlight the need for targeted management strategies to mitigate both short- and long-term nitrate leaching in the UMRB.

Technical Abstract: 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.