Response of soil hydrothermal processes within the active layer to variable alpine vegetation conditions on the Qinghai-Tibet Plateau

Alpine vegetation plays an important role in the thermal stability of the permafrost under a warming climate, as it affects ground hydrothermal dynamics. The response of soil hydrothermal dynamics in the active layer to permafrost degradation under different alpine grassland types is unclear on the Qinghai-Tibet Plateau. In this study, long-term soil temperature and soil water content in the active layer were monitored in situ from October 2010 to December 2018 at five sites in the Kaixinling permafrost region on the interior Qinghai-Tibet Plateau along the Qinghai-Tibet Railway. The sites included an alpine steppe (AS), three alpine meadows (AM) with different degrees of degraded vegetation, and an alpine swamp meadow (ASM). Based on field-monitored data, the variations in soil temperature, soil water content, and freeze-thaw processes were examined in the active layer. The response characteristics of the soil hydrothermal processes to climate change were analysed under the different alpine grasslands. The results showed that the duration of the thawing and freezing stages of the active layer of the AMs was shorter than that of the ASM and the AS. The average mean annual soil temperature (MAST) in the active layer of the AM ((-1.25 & PLUSMN; 0.50) & DEG;C) was lower than those in the AS ((-0.71 & PLUSMN; 0.39) & DEG;C) and ASM ((-0.45 & PLUSMN; 0.57) & DEG;C), while the AM had the highest rate of soil temperature increase ((0.2 & PLUSMN; 0.06) & DEG;C per year). The annual amplitude of ground temperature in the active layer increased with the transition direction of the alpine vegetation type from ASM to AM to AS. The small surface offset (SO) and thermal offset (TO) (absolute values) indicated that the ground thermal state of the AM was more unstable, as it was more sensitive to the increase in air temperature than the ASM or the AS. Soil properties controlled the distribution of soil water content within the active layer, but vegetation improved the shallow soil structure by producing more belowground phytomass, thus, enhancing soil water content in the 0-30 cm layer. The average soil water content at depths of 0-30 cm was directly proportional ( p < 0.05) to the phytomass. Soil water contents at depths of 0-30 cm in the ASM ((37.7 & PLUSMN; 5.3)%) and the AM ((40.8 & PLUSMN; 5.9)%) were significantly higher than those in the AS ((22.7 & PLUSMN; 3.2)%). These results provide valuable insight into the hydrothermal interactions between the degradation of permafrost and alpine vegetation under a warming climate.