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

In arid and semi-arid areas, soil shrinkage and swelling has become natural phenomena that manifest as changes in soil volume in response to water content, and finally the soils generate cracks. Desiccation cracks will also destroy the internal structure of the soil and provide preferential circulation channels for rainwater and irrigation water, causing many serious consequences to agricultural production, such as nutrient loss and groundwater pollution. Meanwhile, cracks typically occur in shrink-swell soils with high contents of clay minerals, which makes it more complex to predict the fluid transport in shrink-swell soils, since the porous medium turns into a variable-solid-skeleton soil and the cracks are typically randomly distributed. Specially, for the saline soils, salts may accumulate around cracks due to water loss at cracks, and the partial salt accumulation change hydraulic properties, destabilize soil particles, and destroy soil structure. Besides, the salts flow more easily into deep soil or groundwater through preferential channels formed by cracks. Therefore, the potential threats are more serious under the influences of cracks resulting from multiple drying and wetting (D-W) cycles under cyclical and regional changes in environmental conditions. In order to reveal the evaporation and crack development characteristics of saline soil under dry-wet cycles and to explore the intrinsic influence mechanism, this study conducted constant temperature soil evaporation experiments on the Na₂SO₄, CaCl₂, and NaCl-type saline soils with five salt contents (0, 0.3%, 0.6%, 1%, and 2%), as well as dry-wet cycles experiments on the Na₂SO₄-type soils. Combining with digital image processing technology, the geometrical characteristics of soil drying and shrinkage crack network during evaporation were quantitatively analyzed, and the interactions between water evaporation and shrinkage cracking as well as the mechanisms of soil salt (including salt type and content) and dry-wet cycle were further systematically revealed. The results showed that (1) the evaporation process of different treatments is similar, that is, the evaporation process contains a linear stage and a nonlinear stage. (2) Soil salt inhibits the water evaporation, and the inhibition effect increases with the increase in soil salt content; the type of salt and the dry-wet cycles can change the evaporation rate of the soil deceleration section. (3) The salt type has significant influence on crack development. Soil salt inhibited the formation and development of surface cracks for the Na₂SO₄-type saline soil. The crack area density, total length of cracks, and average width of cracks of the saline soils decreased by 4.5%～9.4%, 0.01%～7.9%, and 10.5%～21.3%, compared to the non-saline soil, respectively. However, with the increase of soil salt content in CaCl₂-type saline and NaCl-type saline soils, compared to the non-saline soil, the crack area density increased by 2.8%～5.5% and 3.5%～8.3%, and total length of cracks increased by 17.7%～35.0% and 11.9%～36.9%, while the average width of cracks decreased by 15.5%～22.1% and 8.8%～21.5%. (4) The effect of dry-wet cycles on crack indexes is inhibited at low salinity and promoted at high salinity, and this effect increases with the increase in the number of dry-wet cycles. The mechanism analysis showed that solute potential and crystal morphology of different types of saline soils are important factors affecting soil evaporation characteristics and crack development; Sodium soil has larger diffusion bilayer than calcium soil, which reduces the tensile strength of soil. The Na₂SO₄ in soil inhibits surface cracking by promoting cementation between microaggregates and plugging soil pores. The dry-wet cycle promotes surface cracking through the swelling-induced crack healing in the case of hydrophilic clay minerals in contact with water.