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

Abstract

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.

Introduction

Bisequal soils are rather common in the Russian Taiga where Glossocryalfs border on Spodosols, and thus at the interface between the two soil types podzolisation is frequently found (Deckers, 2001). The Orthod profile typically develops in the eluvial horizon of the loamy Glossocryalf profile. Therefore, in addition to the transformations that have occurred during Alfisol development, the soil solid phase in the top horizons is affected also by the transformations which are typical of podzolisation.

Clay minerals are frequently used as indicators of pedogenesis as the mineralogy of the clay fraction is related to the pedogenic processes that have occurred in the soil, but clay minerals arising from the most recent process may have formed from those originated during the previous ones.

During Alfisol formation, the transformations of the clay phases are governed by the mobility of the elements released by mineral dissolution (Chesworth, 1992). Illite typically weathers to vermiculite through the release of potassium from the interlayer; the intercalation of Al hydroxyl-polymers often occurs, giving rise to hydroxy-interlayered vermiculites (HIV), and to pedogenic chlorites as the end members of the transformation (Allen and Hajek, 1989). Detrital chlorite tends to disappear by weathering to vermiculite (Banfield and Murakami, 1998), or smectite (Arocena and Sanborn, 1999) and, in both cases, the formation of mixed layer clay minerals between the two end members is common (e.g. Churchman, 1980, Środoń, 1999). Kaolinite has been reported as originating from almost all primary minerals (Wilson, 2004), but is typically pedogenic only in Oxisols and Ultisols, while in Cryalfs and Udalfs, its distribution is generally uniform with depth (Allen and Hajek, 1989), suggesting inheritance from the parent material.

In addition to the mineral transformations occurring via leaching of cations, some specific transformations occur during podzolisation. In the E horizons of sandy Spodosols swelling minerals are commonly present, but their abundance increases with increasing intensity of podzolisation (Egli et al., 2004). The strong acidity favours chlorite dissolution (Ross, 1969, Bain and Duthie, 1984) and the presence of complexing organic agents enhances the removal of the Al polymers from the interlayer of vermiculites and smectites (e.g. Egli et al., 2003). In the Bs horizons, the mineralogical transformations are normally less marked (Righi et al., 1999), and both swelling and non-swelling minerals have been reported (e.g. Egli et al., 2004, Kitagawa, 2005).

Besides podzolisation and lessivage, additional processes may be present. In the taiga soils, the texture of the Spodosol sequum is typically finer than in the sandy Spodosols. Thus, an additional pedogenic process that often induces an even more complex soil profile is linked to hydromorphic conditions. They may develop in small depressions because of climatic conditions, but also on vast areas with impeded drainage, and lead to additional soil features and to transformations of the clay phase caused by redox processes. The reductive processes associated to hydromorphism have a direct effect on the solubilisation of Fe and Mn oxides and hydroxides, but may affect also layer silicates through a variation in their charge, thus influencing the CEC, the swelling behaviour and all properties associated to electrostatic interactions (Stucki, 2006).

In unmanaged situations waterlogging may in turn induce the development of Sphagnum layers and a general tendency to the formation of peatland. Sphagnum layers release leachates with high dissolved organic carbon contents, and a high proportion of organic acids (David and Vance, 1991). As organic acids affect silicate dissolution mainly through complexation (e.g. Robert and Berthelin, 1986), an enhancement of weathering of all aluminium silicates is expected.

The purpose of this work was to evaluate the pedogenic processes, and their intensity, in three bisequal soils of the Russian Southern Taiga zone, and to assess the transformations that have occurred to clay minerals because of soil processes.