Pedogenic processes and clay transformations in bisequal soils of the Southern Taiga zone

In bisequal soils a complex combination of pedogenic processes has occurred and the minerals that arise from a specific soil forming process may have been modified by other processes. In this work we investigated the mineralogical composition of the clay fraction and related it to pedogenic processes in three soils of the Russian Taiga. The soils showed evidences of clay translocation, podzolisation, and gleying with different intensities and combinations. Clay translocation was present in all profiles, but better expressed in Profile 2. Evidences of podzolisation were found in Profiles 2 and 3, but in this latter hydromorphic processes have also occurred. In Profile 1 almost permanent water stagnation has lead to the development of a thick sphagnum cover. In all profiles, during the formation of the Alfisol sequa, the transformation of illite into vermiculite or smectite has occurred through the formation of mixed-layered minerals while the pH favoured Al-intercalation in the interlayer. When podzolisation started, the presence of HIV/HIS in the E horizon was no longer favoured, because of Al complexation by organic acids, but in the Bs horizons Al-intercalation in the interlayers continued, thus prolonging a mineralogical process that has started during the previous pedogenic phase. The formation of swelling minerals was moderate and related to the cheluviation phase of podzolisation. We found swelling mixed layer minerals only in the surface horizons of Profile 1 and 3, where the vegetation cover was favourable to the production of aggressive organic acids and thus Al may be removed also from the structural sites of phyllosilicates. The reducing conditions acted upon soil minerals, but affected directly only Fe oxi(hydr)oxides. The dissolution of the protective Fe-films from mineral surfaces has, however, favoured the physical disintegration of feldspar grains where the waterlogging conditions were almost permanent.