Artificial cultivation of soil salt crust and effects of its damage rate on soil evaporation

Abstract: Soil salt crust is widely developed in arid and semi-arid areas, which has great impacts on earth surface soil hydrological processes, and interference of external forces on mechanical damage of salt crust is common. Shifting aeolian sandy soil in the Taklimakan Desert was taken as an example, different salt solutions (NaCl, Na2SO4, CaCl2, KCl) of different salinities (5, 10, 20, 30 g/L) were used to simulate the formation and development of salt crust under saline water irrigation. The air temperature and humidity during the salt crust cultivation were recorded. The optimum artificial cultivation conditions were determined based on the physiochemical properties analysis results of salt crust. Furthermore, the soil evaporation process under the influence of different damage degree of salt crust (damage rates of 100%, 50%, 25% and 0) was monitored. The results showed that the hardness, shear resistance, pH and EC of the salt crust generally increased with the increasing salinity of irrigation water. The thickness and hardness of the crust were the largest for 30 g/L NaCl solution, and its hardness was 24.17 kg/cm2, shear resistance was 6.73 kg/cm2, pH value was 7.45, and EC was 3.77 mS/cm. Daily soil evaporation in micro-lysimeters (MLS) increased with the increasing damage degree of salt crust and decreased with the days after irrigation. Soil accumulative evaporation increased with the days after irrigation and the degree of damage. During the soil moisture evaporation, soluble salts gradually crystallized on the soil surface, which not only blocked soil capillary action, but also inhibited diffusion of water vapor into the atmosphere. Meanwhile, it changed and lowered surface soil temperature, thus further reduced soil evaporation. Thus, the evaporation rate of water gradually slowed down and the amount of evaporation per day reduced. Daily evaporation and cumulative evaporation with 100% soil salt crust were significantly different from the other treatments. Cumulative evaporation inhibition efficiency of salt crust on soil evaporation increased with the increasing time after irrigation, and decreased with the increasing damage degree, which indicated that soil cumulative evaporation would change due to the change of salt crust damage. The cover of salt crust had positive effect on maintaining high soil moisture and plant growth in arid regions. Cumulative evaporation inhibition efficiency of salt crust increased with the days after irrigation, and decreased with the increasing damage rate. When the crust was damaged by 50% or 0, the cumulative evaporation inhibition efficiency reached the maximum on the 7th day. When the crust damage rate was 25%, the maximum evaporation inhibition efficiency was on the 4th day.The maximum cumulative evaporation inhibition efficiency of salt crust without damage was 58.84%, while it was only 30.20% under damaged rate of 50%. In summary, soil salt crust could be artificially cultivated, and it has obvious inhibitory effects on soil evaporation, and its damage degree has significant impacts on soil evaporation process. It is of great significance to reveal the hydrological processes of saline soils and to guide the rational utilization of soil and water resources in arid and semi-arid areas.