Responses of freeze-thaw process and hydrothermal variations within soil profiles to the cultivation of forest and grassland in Northeast China

The freeze-thaw process (FTP) deeply affects soil hydrothermal dynamics and agricultural productivity and sustainability, particularly in the context of land degradation under climate warming. Cultivation of forest and grassland induces changes in soil properties, which may change the freeze-thaw regimes within the soil profiles. Nonetheless, there has very limited knowledge of the response of FTP after the cultivation of forest and grassland. To reveal the impact of the cultivation, we compared the soil FTPs and hydrothermal dynamics in farmland under conventional tillage with uncultivated grassland and woodland. In this study, soil temperature (ST) and soil water content (SWC) were monitored in situ at different depths (10, 30, 50, 70 and 100 cm) in Northeast China, an important agricultural zone with frequent freeze-thaw cycles (FTCs). The results showed that compared to the undisturbed grassland and woodland, the farmland suffered from more intensive FTP involving more variation in soil hydrothermal dynamics. The FTP in the farmland was characterized by unidirectional freezing and bidirectional thawing. Temporally, the onset and the end of the freezing or thawing period in the upmost 30 cm soil layer of the farmland were 1–11 days earlier than that of the grassland and woodland. Besides, the farmland had significantly longer freeze-thaw durations (15 and 27 days longer than grassland and woodland, respectively) as well as subjected to more frequent FTCs (4 and 8 times more than grassland and woodland, respectively). Regarding the efficiency of thermal transfer between the soil and the air, it was more efficient in the farmland. Especially for the upper 30 cm soil layer, the farmland had a stronger correlation between ST and air temperature (AT). Throughout a whole freeze-thaw period, the farmland had less water migrated from the deeper layer to the upper 100 cm soil layer, with 5.38 and 32.95 mm less than grassland and woodland, respectively, in terms of incremental soil water storage. There was an interactive relationship between ST, SWC and the FTP, and overall, the cultivation altered the soil FTP and hydrothermal regimes. These findings have potential implications for a better understanding of the effect of cultivation on the soil FTP and provide a reference for parameter settings in simulated freeze-thaw experiments.