Research on the distribution of soil water, heat, salt and their response mechanisms under freezing conditions

In seasonally frozen soil compartments, freezing affects hydrothermal cycles and solute transport. In order to effectively identify the variation characteristics of soil temperature, moisture and salinity during freezing and the interaction among them, a laboratory unidirectional freezing experiment was set up. During the experiment, three initial gravimetric moisture contents (N1, 18.84 %; N2, 23.43 %; N3, 28.78 %), three freezing temperatures (N4, −5°C; N5, −10°C; N6, −15°C), and three soil bulk densities (N2, 1.33 g⋅cm−3; N5, 1.45 g⋅cm−3; N7, 1.58 g⋅cm−3) were established. The spatial distribution characteristics of the temperature, moisture and salinity in the soil columns were analysed, on the basis of these results, soil water, heat, and salt response relationship functions were constructed by using three indicators, i.e., the soil temperature difference, water flux and salt flux. The study results indicated that as the moisture content increased, the water migration ability increased. In addition, as the freezing temperature decreased and the soil bulk density increased, the rapid freezing of soil inhibited water migration. Furthermore, the movement of water carried a certain amount of salt, and in the soil columns with freezing temperatures of -10 and -15°C, the total soil salt migration decreased by 10.05 and 16.41 mg·cm-2, respectively, compared with the soil column with a freezing temperature of -5°C. The response relationship among the soil water, heat, and salt improved with an increase in the soil moisture content, weakened with a decrease in the freezing temperature and an increase in the soil bulk density, simultaneously, the RMSE between the simulated and measured values of the soil water, heat, and salt response function reduced. In addition, at the spatial scale, the response relationship among soil water, heat, and salt was most significant at the critical soil freezing depth. These study results elucidate the migration and diffusion of soil water, heat, and salt in cold regions and their associated interaction mechanisms, thereby providing guidance for scientific and effective measures for soil improvement, improving the soil ecological environment and ensuring stable increases in grain production.