Approximately 50% of soils in the Northern Hemisphere experience seasonal freezing and thawing, which influences physical, chemical, and biological processes in the vadose zone. Soil freeze–thaw drives mechanical processes, including frost heave and soil aggregate formation and breakdown, and controls snowmelt infiltration and runoff. These hydrologic processes determine the soil moisture conditions, which affect plant mortality and growth, soil microbial activities, and nutrient (e.g., C and N) cycles. Nutrients leached from the thawed soil, often with rapid infiltration of snowmelt water, may affect the quality of the groundwater and surface water, in combination with enhanced erosion and sediment load due to freeze–thaw. Nutrients released as greenhouse gases may contribute to climate feedback. With recent climate warming and changes in the extent and depth of frozen soil and permafrost, it is important to understand frozen-soil processes and their interaction with the environment. The objective of this review is to highlight important aspects of soil freeze–thaw and related processes and to point out research challenges and opportunities in the cold vadose zone.

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