Environmental factors controlling soil warming and wetting during 2000–2020 in permafrost and non-permafrost regions across the Qinghai–Tibet Plateau

The Qinghai–Tibet Plateau (QTP) has experienced rapid environmental changes, including climate warming and wetting, since the 1980s. These environmental changes significantly impact the shallow soil hydrothermal conditions, which have key roles in land–atmosphere feedback and ecosystem functions. However, the spatial variations and responses of soil hydrothermal conditions to environmental changes over the QTP with permafrost (PF) and seasonal frost (SF) remain unclear. In this study, we investigated the spatial variations in soil temperature (ST) and soil moisture (SM) changes over the QTP from 2000 to 2020 using 99 in-situ sites with observations at 4 depths (i.e. 10, 40, 100 and 200 cm). The main environmental controlling factors were further identified using a calibrated statistical model. Results showed that significant (p < 0.05) soil warming occurred at multiple soil layers during 2000–2020 with a wide variation (i.e. 0.033–0.039 °C per year on average), whereas the warming rates at PF sites were two times greater than those at SF sites. In addition, the soil wetting rate was high over the SF region, whereas the soil wetting rate was low over the PF region. Aside from air temperature, changes in thawing degree days and solar radiation (Srad) contributed most to soil warming in the PF region, whereas changes in rainfall, Srad and evaporation (EVA) have been identified as the key factors in the SF region. As for soil wetting, changes in snowfall, freezing degree days and vegetation have noticeable nonlinear effects over the PF region, whereas changes in EVA, Srad and rainfall highlighted distinct linear and nonlinear effects in the SF region. These findings can enhance our understanding of the hydrothermal impacts of future environmental changes over the QTP.