The changes in pore space in humus aggregates of soddy podzolic soils in conditions of multiple freezing and thawing process

The paper discuss the literature data on clay mineralogy of vertisols and presents the investigation of clayey soils formed in ultra-continental climate in the Eravna depression (Buryatia). Soils are formed on the watershed and shoulder positions of the local ridge and are underlined by permafrost. Morphological analysis of soils revealed a combination of cryogenic features (permafrost wedges and cracks, cryogenic structure, above permafrost gleyization) and vertic properties (slickensides, wedge-shaped aggregates). According to the morphology, the soils were classified as Dark compact soil or Gleyic Vertisol Glossic Gelistagnic on the shoulder position, and as Chernozem-like weakly compacted cryoturbated soil or Vertic Gleyic Phaeozem Glossic Pachic Clayic Gelistagnic on the watershed where vertic features were weakly expressed. According to physical properties and clay mineralogy, the shrinking-swelling potential of the soil on the watershed is comparable to the soil of the shoulder: almost the same, and in some horizons even higher content of clay (up to 76%), fine clay (up to 54%) and swelling components in the fine clay (<0.001 mm) fraction (93-98% of montmorillonite). However, in the ultra-continental climate of Buryatia and close-lying permafrost (depth to permafrost at the end of August was about 250-280 cm), the most contrasting hydrothermic regime and the most favorable conditions for the implementation of shrinking-swelling are created in the upper steep part of the south-facing slopes. Due to the fact that soils on the watershed receive more moisture and less heat, this results in less contrasting hydrothermic conditions and less pronounced shrinking-swelling processes. The lack of conditions for full realization of shrinking-swelling potential in clayey soils of watersheds is the reason for identification of vertic features at a lower taxonomic level.