Soil water dynamics in the active layers under different land-cover types in the permafrost regions of the Qinghai–Tibet Plateau, China

The mechanisms of hydrological processes, biochemical cycles, and permafrost revolution, and the potential impacts of climate change on these, are still poorly quantified in alpine regions, partly due to a lack of understanding of soil water dynamics in the permafrost active layer. In this study, soil water in the active layer was monitored in-situ at nine sites, including wet meadow (WM), alpine meadow (AM), alpine steppe (AS), transitional area-steppe (TA-S), transitional area-meadow (TA-M), bare land (BL), extremely degraded wet meadow (EDWM), alpine steppe on south-facing slope (ASSF), and alpine meadow on north-facing slope (AMNF) regions. These sites are located in the hinterland of the Qinghai–Tibet Plateau (QTP). Ground-ice distributions and isotope variations under different alpine ecosystems were also examined. The results demonstrate that soil water content was low at 0.5-m depth and high at the surface and at 1.0-m depth in the soil profiles from the TA-S, BL, EDWM, ASSF, and AMNF sites. However, denser vegetation coverage masked the effects of the freeze–thaw processes, which leading soil water content increased gradually with soil depth. Moreover, rainfall infiltration for recharging soil water in the lower layers was hampered due to the buffering action of mattic epipedons and the existence of clayed layers. Additionally, preferential flow often occurred in the degraded alpine meadows, which supplied deeper soil water. The Pearson correlation coefficients between soil water in the deeper layers and ground-ice were above 0.67 (significance <0.05), suggesting that deep soil water had a stronger effect on the formation of ground-ice near the permafrost table than soil water at the surface and middle root layers. Furthermore, results from the analysis of isotopic tracers suggest that precipitation directly recharged more ground-ice near permafrost table at the EDWM, ASSF, and AWNF sites, but less so at the WM, AM, AS, and BL sites, due to greater evapotranspiration and land cover. These results provide insights into the effect future climate warming can have on ecological succession and regional hydrological processes.