Rheological Properties of Boreal Semihydromorphic Soils: Relationship with Physico-Chemical Properties and Temperature Conditions

The results of rheological studies of soils on the modular rheometer MCR 302 (Anton Paar, Austria) by the amplitude sweep method (oscillatory method) are considered. Study objects are represented by semi-hydromorphic soils of taiga zone in the Northeast of European part of Russia. We found that the strongest interactions between soil particles are developed in the horizons with high content of mobile humus compounds (fulvic acids) and Al/Fe-humus complexes (ELhi,g–ELg–CRM horizons). Increased structural durability is due to the cementation of soil particles resulting from the intake of humus substances and Al/Fe-humus compounds with the development of strong interparticle bonds in the soils. Another important factor is the freezing-thawing processes. The impact of seasonal freezing on the soil rheology is clearest in the profile of semihydromorphic variants of light soils (Histic Gleyic Stagnosols), in their cryometamorphic (CRM) horizons, where increased rigidity of soil bonds is due to condensation compaction of soil particles as a result of development of freezing veil with long period of temperatures about 0С (“zero curtain”). High values of structural interrelations – Integral Z in the soil profile may be due to weak aggregation of mineral mass caused by a constant surface moisture stagnation in the soil accompanied by an intensive gley process. Disaggregated soils are most at risk of erosion and washout processes, and yet thick moss-peat horizon forming in the upper part of the profile of semihydromorphic soils protects them from negative deformation phenomena. In the northward direction from the textural-differentiated soils of southern taiga to the cryomethamorphic soils of forest-tundra we observe an increasing of the strength or rigidity of interpartical soil bonds. At the latitudinal scale, the strength or durability of soil bonds in the cryometamorphic soils of northern, far northern taiga and forest-tundra is higher than that in the semihydromorphic texture-differentiated soils of southern and middle taiga. This pattern may be due to more active intake of fulvic acids, including complex Al-FA-humic substances, as well as longer freezing of northern soils. It is shown that rheological parameters can be used as additional indicators in the diagnosis and classification of taiga soils.