Response mechanism of soil structural heterogeneity in permafrost active layer to freeze–thaw action and vegetation degradation

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.