Climate warming has significantly changed the near-surface soil freeze state, significantly impacting terrestrial ecosystems and regional agroforestry production. As Northeast China (NEC) is highly sensitive to climate change, this study introduces the concept of velocity to analyze the spatial pattern of frozen days (F-DAY), onset date of soil freeze (F-ON), offset date of soil freeze (F-OFF), and number of soil freeze/thaw cycles in spring (F-TC) in NEC from 1979 to 2020. We observed that the velocity changes of F-DAY, F-ON, and F-TC in croplands were significantly higher than those in forests (difference > 1 km yr(-1)), with the fastest velocity changes found in the cropland of the Songnen Plain. The highest velocity of FOFF was found in the forests of the Greater Khingan Range. In most study areas (> 60%), the isoline of F-DAY/F-ON/F-OFF/F-TC showed a northward movement. The isoline of F-DAY/F-ON/F-OFF/ F-TC moved in the cold direction in each cropland region (Sanjiang, Songnen, and Liaohe River Plains) and forest regions (Greater Khingan and Lesser Khingan Ranges, and the Changbai Mountains). The results of the quantitative analysis indicate that air temperature (T-A) had a more significant effect on the velocity change of F-DAY and F-ON in cropland, whereas snowpack is the dominant factor in forests. In both forests and croplands, the main factor affecting the velocity of F-OFF was snowpack, and T(A )mainly affected the F-TC. This study is significant for formulating regional climate change countermeasures and maintaining ecological security in cold regions.

The near-surface soil freeze state is affected by global warming, and its changes have profound effects on landscapes, ecosystems and hydrological processes. On the basis of daily soil freeze observational data at 476 meteorological stations over 50 freezing years from September 1, 1961 to August 31, 2011, the spatial distributions and temporal variations of the near-surface soil freeze state were estimated using five freeze variables (first date, last date, maximum seasonally frozen depth, duration and actual number of freeze days) across China, which was divided into three regions (eastern China, northwestern China and the Qinghai-Tibetan Plateau (QTP)). The near-surface soil freeze state varied greatly across China. The QTP has an earlier freeze, later thaw, longer freeze days and deeper seasonally frozen depth than the other two regions. The spatial distributions of the near-surface soil freeze state can be explained largely by altitude in northwestern China and on the QTP, whereas they can be explained by latitude in eastern China. The near-surface soil freeze state has changed significantly over 50 freezing years. On average across China over the study period, the first date of freezing was delayed by approximately 10 +/- 1 days with a rate of 0.20 +/- 0.02 days/year, the last date advanced by approximately 18 +/- 2 days with a rate of 0.36 +/- 0.04 days/year, the duration and the number of freeze days decreased by 28 +/- 2 and 23 +/- 2 days with rates of 0.56 +/- 0.04 and 0.45 +/- 0.04 days/year, respectively, and the maximum seasonally frozen depth decreased by 20 +/- 3 cm with a rate of 0.41 +/- 0.06 cm/year. The change in the freeze variables is relatively large in high-latitude and high-altitude regions.