Freeze-Thaw Action Characteristics of Shallow Soil on Degraded Alpine Meadow in the Source Region of Three Rivers

The sample plots were set up in the degraded alpine meadows in the Source Region of the Three Rivers, where the regularities including occurrence of shallow soil freezing and thawing in degraded alpine meadows, spatial and temporal changes of soil temperature and correlation between soil temperature and air temperature, etc. were discussed based on the acquisition of soil temperature data of a complete freeze-thaw cycle in different degraded grasslands and different soil layers through continuous field observations. The results show that in a freeze-thaw cycle, the period of soil from freezing to thawing in the degraded alpine meadow lasted for 179-196 days, and the soil temperature showed an approximate sinusoidal trend. With the aggravation of degradation, the 0-40 cm soil layers were frozen and thawed earlier. Compared with non-degraded grassland, the freezing and thawing start dates of severely degraded grassland were 7-23 and 18-38 days earlier, respectively. The frozen front surface of the soil moved faster from the surface to the deeper soil layer, the soil temperature gradient became larger and the rate of change became faster, the soil was more likely to be warmed up and cooled down; the soil temperature's response to air temperature became stronger from non-degraded soil to severely degraded soil, and the correlation coefficients between soil temperature of all layers of soil and air temperature were 0.646-0.876, 0.751-0.901, 0.821-0.930 and 0.854-0.951, respectively. September and March were transitional periods of soil temperature in different soil layers. In the degraded alpine meadow sample plots, the degree of degradation, depth of soil and air temperature were the factors affecting the process of freezing and thawing of shallow soil. The change of soil freezing and thawing characteristics caused by degradation was not conducive to the stability of alpine meadow ecosystem and frozen soil environment.