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• The freeze–thaw cycle affected the soil hydrological process in active layer; • There were significant differences in soil hydrothermal processes at the slope scales and profile scales; • The redistribution of soil water during the freezing and thawing periods is affected by vegetation." The Qinghai-Tibet Plateau (QTP) is a region with an extensive area of permafrost that is very sensitive to climate change. In this study, soil water and thermal dynamic processes in the active layer of the QTP permafrost area at the profile and slope scales were investigated. The results showed that the hydrothermal process in the active layer soil was strongly affected by freezing and thawing processes and external weather conditions. The temperature of the active layer fluctuated in tandem with air temperature. The soil water content was more stable during the freezing period and showed a "double-hump" trend during the thawing period. The correlation coefficient between soil and air temperature decreased with depth, from 0.896 in the surface soil layer to 0.082 in the deep soil layer; and the correlation coefficient between unfrozen water content and soil temperature during the freezing period showed an overall increasing trend with depth. The soil layers at different depths at the top and bottom of slope profiles differed in their hydrothermal processes due to the physicochemical properties and texture of the soil and vegetation types. The water-heat exchange of the surface soil is more frequent than that of the deep soil, and the frequency is more at the bottom than at the top of the slope. The soil water content at a depth of 0.25 m was the highest in the profile in association with a higher organic matter content and the blocking effect of dense roots. The changes of soil hydrothermal process in the active layer accelerated the hydrological cycle and the spatial–temporal variability in water resources in the frozen soil area. These effects might lead to a series of ecological and environmental problems, such as permafrost degradation and desertification in the QTP alpine meadow ecosystem. [ABSTRACT FROM AUTHOR]

• Heterogeneity of soil hydrothermal processes and soil structure in undegraded meadows and degraded bare ground. • Vegetation degradation in concert with freeze–thaw affects soil structure in the active layer. • Changes in soil structure in the active layer reveal the response of soil hydrological processes to the external environment. The soil structure is the key parameter that regulate soil water–heat–solute migration. To date, much remains unknown about the interaction mechanism between soil structure and the external environment in the permafrost regions. In this study, the undegraded meadow and degraded bare land in the permafrost region of the Qinghai–Tibet Plateau were used as research objects, and the soil structure characteristics and their influencing factors of the two sample sites were studied based on in situ hydrothermal observation data and CT scan. The results showed that the macropore system of the undegraded meadow was dominated by connected pores, with a small number of macropores, but the large volume, the shape mainly was elongated, and the connectivity was good. The degraded bare ground was dominated by isolated pores, with more large pores but smaller volume, more regular pore shape, and less pore connectivity than undegraded meadow. The macropore structure in both samples was affected by vegetation, freeze–thaw action, and soil moisture content. The undegraded meadow formed connected pores with mainly tubular pore aggregates in the soil layer above 23 cm due to the protective effect of vegetation roots, and only a few isolated pores existed in the deep soil layer below 23 cm due to weak freeze–thaw action and low moisture content. The daily freeze–thaw cycle days were long, and the complete freezing days were short for the degraded bare land. The frequent freeze–thaw action produced connected pores dominated by fractures, and many isolated pores, while the high water content of the deep soil intensified the freeze–thaw action, producing a large number of isolated pores and also increasing the depth of occurrence of macropores. The difference in the soil pore deepens the understanding of the relationship between the soil macropore and the soil hydrothermal process. [ABSTRACT FROM AUTHOR]