Salt Movement during Soil Freezing Events in Inner Mongolia, China

About 100 million ha of land in China suffers from soil salinization, mostly in the north where the soil is seasonally frozen. Although the freezing process is regarded as a major cause of severe salinization in spring, the process and mechanism of salt movement during soil freezing is still largely unknown. Soil salt content was observed at five sampling points in the Hetao Irrigation District, Inner Mongolia, China, during three winters from 2009 to 2012. Results showed that, on average, salt content of the frozen layer (0-100 cm) decreased by about 10.7% during the freezing period from early November to late February. Specifically, the salt content of the 0-10-cm layer increased by 14.6%, while salt content of the 10-30, 30-50, 50-70, and 70-100-cm layers decreased by 4.9%, 6.0%, 12.7%, and 12.9%, respectively. Salt moved out of the 0-100-cm soils only during early stages of freezing from early to late November, which should be ascribed to salt leaching because massive irrigation was applied in late October. If the rate of salt leaching was set to zero, total salt content of the 0-100-cm soils should have increased by 6.5% during the freezing period. During freezing, salt under the unfrozen soils moved toward the freezing front and was engulfed into the frozen soils as the freezing front penetrated deeper. However, in frozen soils relatively far from the freezing front, salt tended to move downward, and desalinization occurred. Therefore, in this study, salt in the 0-30-cm layer started to move downward when the freezing depth was 40 cm or deeper starting in late December, and the desalinization zone extended to the 0-70-cm soils later, when the freezing front was deeper than 80 cm.