Analysis of the pore structure characteristics of saline soil in the profile within the frozen depth

Soil pores play a critical role in the complex physical coupling process and are closely associated with the mechanism of soil salinization. This research used mercury intrusion porosimetry (MIP), scanning electron microscopy (SEM), and laser particle size analysis on a soil profile about 230 cm deep in Qian'an, where soda saline–alkali soil is distributed and the deepest underground freezing depth is 180 cm. The material composition of the soil in active layer was identified, and the soil pores were classified as micropores (<0.04 μm), small pores (0.04–0.4 μm), mesopores (0.4–4 μm), and macropores (>4 μm). The soil deposited at a depth of 0 to 80 cm was affected by freeze–thaw cycles and dry–wet cycles, developing longitudinal cracks. During the rainy season, clay particles and salt were carried downward by rainfall, whereas at a depth of 30 to 80 cm, the exchangeable sodium ions and clay particles in the soil created a thick diffused layer and hindered rainfall infiltration, resulting in salt accumulation. The small pore diameter and low porosity of the soil in this depth range caused pores degradation, further exacerbating the salinization process. The soil deposited at a depth of 80 to 120 cm had low clay content, resulting in limited upward migration of unfrozen water during freezing periods. Upon thawing, the salt and fine clay particles moved upward with capillary water, leading to a lower salt content than the upper soil. Additionally, this depth range had a high content of macropores with a large average pore diameter. The soil deposited at a depth of 120 to 180 cm had high clay content, and during short freezing periods, salt migrated upwards with unfrozen water along poorly connected longitudinal cracks with limited clay particle migration. The macropore content was low, and the average pore diameter was small due to clay particles filling the pores. The salt content in this depth range was lower than in the upper soil. The pore structure characteristics of the saline soil in the active layer provide insights into salt deposition mechanisms, and this research offers data support from a pore-scale perspective for preventing the salinization process.