Temporal and Spatial Changes of Freeze-Thaw Cycles in Ulan'aodu Region of Horqin Sandy Land, Northern China in a Changing Climate

Freeze–thaw cycles were examined using soil temperature data from 1981 to 2011 in Ulan'aodu region of Horqin sandy land in Inner Mongolia, China, to investigate the characteristics of soil freezing and thawing and its response to global changes. The results showed that soil freeze–thaw cycles mainly occurred in March and November. Soil freeze–thaw cycles at different depths showed different trends, more freeze–thaw cycles and larger freeze-thaw amplitude occurred in the upper layers than in the deeper layers. The significant trends in soil freeze–thaw cycles and soil temperatures at several depths were presented: soil freeze–thaw cycle frequencies showed a decreasing trend with time, and the highest value of freeze–thaw cycles were observed at a depth of 0 cm; the freeze–thaw cycle amplitude showed a decreasing trend over the years, and the largest freeze–thaw cycle amplitude was observed at the 0- to 5-cm depth. Both decreasing trend reached a significant level (p < 0.05) in the year 2007 and a significant abrupt decrease took place in 1997.