Chemical characteristics of salt migration in frozen soils during the freezing-thawing period

Soil water and salt movement in freezing-thawing periods play an important role in the agricultural ecological environment. Field experiments show a non-synchronized movement of soil water and salt during the freezing-thawing period with no definite mechanism. In this study, the non-synchronization of water and salt movement in the frozen layer (0–1 m) was analyzed from the perspective of soil salt composition combining with the convection–diffusion theory. Field experiments were carried out during two freezing-thawing periods from 2017 to 2019 in Yonglian experimental station of Hetao Irrigation District in Inner Mongolia, China. The contents of soil moisture, salt and its components (Na, K, Ca, Mg, CO3, HCO3, Cl and SO4) were measured. The correlation analysis between storage increments of total salt and its components was carried out and the FREZCHEM model was used to calculate the soluble and solid salt components. During the freezing period, the soil water storage increased by 5.42% on average, while the soil salt storage decreased by 18.69%, showing the different migration directions of soil water and salt. The migration direction of each component of salt was not exactly the same with that of the total salt. The main components causing the storage increment of soil salt were different in the three saline soils. The solute concentration gradient of soluble salt reached 3.07 mol/(L·m), which caused stronger diffusion effect to result in downward salt migration, while the convection effect drove the soil water and salt to move upwards. This is the major reason for the non-synchronization movement of soil water and salt during the freezing-thawing period. This study provides new data and perspective to understand the soil water and salt movement during the freezing-thawing period in agricultural areas with shallow groundwater table depth.