Freeze-Thaw Processes of Soils in Active Layers in Northeast China

The processes of thawing and freezing and their associated complex hydrothermal coupling can significantly affect variations in mean annual temperatures and the formation of ground ice in permafrost regions. In this article, using soil temperature and moisture data in the permafrost region of the Nanweng’he River in the Da xing’anling Mountains, the freeze-thaw characteristics of the permafrost were studied. Variations in the soil temperature and the moisture were analyzed during each stage of the freeze-thaw process, and the effects of the soil moisture and ground vegetation on the freezing-thaw were discussed in this paper. The study results show that the thawing in the active layer is unidirectional, while the ground freezing is bidirectional (upward from the bottom of the active layer and downward from the ground surface). During the annual freeze-thaw cycle, the migration of soil moisture had different characteristics at different stages. In general, during of a freezing-thawing cycle, the soil water molecules will migrate downwards, i.e., soil moisture will transport from the entire active layer to the upper limit of permafrost. In the meantime, freeze-thaw in the active layer can be significantly affected by the soil moisture content and vegetation.