Freezing and thawing characteristics of saline farmland soils: analysis of hydrothermal parameters and driving mechanism

Soil physicochemical and hydrothermal properties are reshaped by freeze–thaw, yet dynamic responses of key hydrothermal parameters and the mechanisms governing water–salt evolution remain insufficiently validated under natural conditions. A representative saline farmland in northern China was investigated combining hourly in-situ monitoring with laboratory freeze–thaw experiments. Key periods were identified, and the spatiotemporal evolution of coupled water–heat–salt dynamics and parameter responses was systematically quantified. Air temperature and snow cover jointly induced vertical differentiation in heat and mass transfer. Water retention by frozen soil and snow increased shallow soil water content by 32.20% relative to early winter, whereas snowmelt infiltration reduced salt content by 15.44–34.02%. During freezing, relative permeability decreased by up to eight orders of magnitude in surface layers and by approximately three orders of magnitude in deeper layers. After thawing, permeability exceeded pre-freeze level and facilitated snowmelt infiltration and downward salt leaching. Thermal conductivity increased by 20.76–56.78%, whereas volumetric heat capacity decreased by 22.30–39.41%, leading to faster cooling than warming. The heat absorbed during thawing could not offset freezing losses. Soil temperature gradient (STG) and matrix potential gradient (MPG) dominated water–salt migration and intensified toward the cold end. Maximum STG (0.775 °C/cm) and MPG (134.9) in shallow soils were 9–10 and 3–4 times higher, respectively, than those in deeper layers. The mechanisms underlying moisture recovery and salt redistribution during freeze–thaw were elucidated, providing a reference for parameterized characterization and quantitative assessment of water–salt processes in comparable saline soils. • Water–salt evolution was clarified using a field-laboratory verification framework. • The quantitative response of hydrothermal parameters was characterized. • The primary-secondary contributions of driving forces were quantified.