Characteristic Analysis of the Spatio-Temporal Distribution of Key Variables of the Soil Freeze-Thaw Processes over Heilongjiang Province, China

The soil freeze-thaw phenomenon is one of the most outstanding characteristics of the soil in Heilongjiang Province. Quantitative analysis of the characteristics of changes in key variables of the soil freeze-thaw processes is of great scientific importance for understanding climate change, as well as ecological and hydrological processes. Based on the daily surface temperature and air temperature data in Heilongjiang Province for the past 50 years, the spatial-temporal distribution characteristics of key variables and their correlations with air temperature and latitude in the freeze-thaw process of soil were analyzed using linear regression, the Mann-Kendall test, the local thin disk smooth spline function interpolation method, and correlation analysis; additionally, the spatial-temporal distribution of key variables and the changes in the surface temperature during the freeze-thaw process are discussed under different vegetation types. The results show that there is a trend of delayed freezing and early melting of key variables of the soil freeze-thaw process from north to south. From 1971 to 2019 a, the freezing start date (FSD) was delayed at a rate of 1.66 d/10 a, the freezing end date (FED) advanced at a rate of 3.17 d/10 a, and the freezing days (FD) were shortened at a rate of 4.79 d/10 a; with each 1 degrees C increase in temperature, the FSD was delayed by about 1.6 d, the FED was advanced by about 3 d, and the FD was shortened by about 4.6 d; with each 1 degrees increase in latitude, the FSD was delayed by about 2.6 d, the FED was advanced by about 2.8 d, and the FD was shortened by about 5.6 d. The spatial variation in key variables of the soil freeze-thaw process under the same vegetation cover was closely related to latitude and altitude, where the lower the latitude and altitude, the more obvious the variation trend; among them, the interannual variation trend of key variables of soil freeze-thaw under meadow cover was the most obvious, which varied by 9.65, 16.86, and 26.51 d, respectively. In addition, the trends of ground temperature under different vegetation types were generally consistent, with the longest period of unstable freeze-thaw and the shortest period of stable freeze in coniferous forests, compared to the shortest period of unstable freeze-thaw and the longest period of stable freeze in meadows. The results of the study are important for our understanding of soil freeze-thaw processes and changes in Heilongjiang Province, as well as the evolution of high-latitude permafrost; they also promote further exploration of the impact of soil freeze-thaw on agricultural production and climate change.