Mechanism explorations of salt and drying-wetting cycle affecting evaporation and crack development of loam soil

Soil shrinkage and swelling have been characterized by the variation in the soil volume in response to water content in agricultural production. The soils can then generate some cracks in arid and semi-arid areas. Desiccation cracks can also destroy the internal structure of the soil to form the preferential circulation channels for rainwater and irrigation water, leading to nutrient loss and groundwater pollution. Meanwhile, the cracks typically occur in shrink-swell soils with high contents of clay minerals. Furthermore, the porous medium can often turn into a variable-solid-skeleton soil, particularly with the typically randomly distributed cracks. As such, it is difficult to predict the fluid transport in the shrink-swell soils. Specially for the saline soils, salts can accumulate around cracks, due to the water loss at cracks. The partial salt accumulation can change the hydraulic properties, and then destabilize soil particles, leading to destroy the soil structure. Besides, the salt flow can be easily into the deep soil or groundwater through preferential channels that are formed by cracks. Therefore, there are more serious potential threats in the cracks under cyclical and regional changes in environmental conditions. This study aims to reveal the evaporation and crack development of saline soil under dry-wet cycles. The soil evaporation experiments were conducted on the Na₂SO₄, CaCl₂, and NaCl-type saline soils with five salt contents (0, 0.3%, 0.6%, 1%, and 2%) at constant temperature. Dry-wet cycle experiments were on the Na₂SO₄-type soils. Digital image processing was combined to quantitatively analyze the geometrical characteristics of soil drying and shrinkage crack network during evaporation. A systematic investigation was then implemented on the interactions between water evaporation and shrinkage cracking, in order to explore the mechanisms of soil salt (including salt type and content) and dry-wet cycle. The results showed that: 1) There was similar evaporation in the different treatments. The evaporation also included a linear and a nonlinear stage. 2) Soil salt inhibited the water evaporation. The inhibition effect also increased with the increase in soil salt content; The type of salt and the dry-wet cycles were used to change the evaporation rate of the soil deceleration section. 3) The salt type shared a significant influence on the crack development. Soil salt also inhibited the formation and development of surface cracks in the Na₂SO₄-type saline soil. The crack area density, total length of cracks, and average width of cracks of saline soils decreased by 4.5%-9.4%, 0.01%-7.9%, and 10.5%-21.3%, respectively, compared with the non-saline soil. Furthermore, the area density of cracks increased by 2.8%-5.5% and 3.5%-8.3%, respectively, and the total length of cracks increased by 17.7%-35.0% and 11.9%-36.9%, respectively, while the average width of cracks decreased by 15.5%-22.1% and 8.8%-21.5%, respectively, with the increase of soil salt content in the CaCl₂-type saline and NaCl-type saline soils. 4) The dry-wet cycles also inhibited the crack indexes at the low salinity but promoted them at the high salinity. This effect increased with the increase in the number of dry-wet cycles. The mechanism analysis showed that the solute potential and crystal morphology were the important influencing factors on the soil evaporation and crack development in the different types of saline soils; Sodium soil shared a larger diffusion bilayer than calcium soil, which reduced the tensile strength of soil. The Na₂SO₄ promoted the cementation between micro aggregates and plugging soil pores, leading to the suppression of surface cracking. Therefore, the dry-wet cycle can promote surface cracking through swelling-induced crack healing, in the case of hydrophilic clay minerals in contact with water.