Variations in Soil Salt Ions and Salinization Degree in Shallow Groundwater Areas During the Freeze–Thaw Period

In shallow groundwater areas, the freeze–thaw process can easily exacerbate soil salinization. The variations and migrations of Na+, K+, Ca2+, Mg2+, Cl−, SO42−, and HCO3− at the depth of 0–100 cm under shallow groundwater depth (2.63–2.87 m) during the freeze–thaw period were analyzed. And a multi-index comprehensive evaluation method based on factor analysis was employed to investigate the soil salinization degree. The results show that K+, Mg2+, and HCO3− exhibited surface enrichment during the freeze–thaw period, while Na+, Cl−, and SO42− accumulated in the frozen layer during the freezing stage. However, there is no surface enrichment of Ca2+. During the freezing stage, Mg2+ and Cl− exhibited the strongest migration capabilities among cations and anions, respectively. During the thawing stage, K+ and HCO3− were the cation and anion with the highest ionic migration capabilities, respectively. Total salinity (TS), Cl−, SO42−, HCO3−, Na+, K+, Mg2+, and residual sodium carbonate (RSC) were identified as the dominant factors influencing the salinization degree during the freeze–thaw period. During the freezing stage, soil salt ions predominantly migrated from the unfrozen to the frozen layer, and the salinization degree in the frozen layer increased with the development of the frozen layer. In the thawing stage, soil salt ions migrated upward from the thawing front, and the salinization degree at the depth of 0–30 cm increased. This study provides insights for the prevention and control of soil salinization in arid regions.