A review of the freeze-thaw cycling effect on arable soil nitrogen and phosphorus leaching

Excessive agricultural fertilization has caused nutrient leaching and severe surface and groundwater pollution in recent years. Soil freeze-thaw cycling (FTC) is common at middle and high latitudes, high altitudes, and partial temperate regions. FTC plays an important role in soil biogeochemical processes in cold regions and may be complicated by climate change. Understanding the effects of FTC on soil nitrogen (N) and phosphorus (P) leaching is critical for effective mitigation. This study reviewed the involvement of FTC on soil nutrient leaching based on soil physical, chemical, and biological properties and found that FTC affects soil nutrient concentrations, leachate forms, and nutrient leaching pathways. FTC damages soil aggregates, microbial cells, and plant root residues, leading to the release of organic matter and various N and P forms into the soil, subsequently stimulating soil mineralization and increasing the mineral nutrient concentrations. Soil hydrothermal regime variations and soil structure changes during the FTC period promote preferential flow, thereby increasing the nutrient leaching potential. FTC affects the soil microbial biomass and the microbial community composition and structure, which changes the nutrient cycling processes. Soil chemical properties, including organic matter, pH, and cation exchange capacity, indirectly influencing soil aggregate stability, microbial resistance, and nutrient holding capacity changed during the FTC period. Soil properties (e.g., soil texture, organic matter content, and soil moisture) and climate (e.g., air temperature and snowpack) determine the nutrient leaching degree during the FTC period. The relationships between nutrient leaching and existing agricultural practices were also analyzed. Mineral fertilizer application is the primary source of nutrient leaching on farmlands. Therefore, fertilizing for the efficient use of nutrients by plants is crucial for mitigating nutrient leaching. Other practices, such as biochars, cover crops, no-tillage with straw mulching, may have a role in reducing nutrient leaching. Biochars have a high sorption capacity and may increase the soil water and nutrient holding capacity, cover crop implementation may absorb excess fertilizer nutrients from the soil and reduce leachable N and P, and no-tillage with straw mulching may change FTC by avoiding exposed soil and influencing soil physicochemical and microbial properties, thereby increasing fertilizer efficiency. However, these measures have shortcomings; cover crops and crop residues are the nutrient leaching sources during FTC. Further research is needed to understand the nutrient leaching mechanisms of these practices and to establish a complete evaluation system.