Zheltozems of Russia: Micromorphology, clay minerals, and pedogenetic analysis .

Abstract

Zheltozems (literally “yellow earths”) that can be classified as Acrisols, Alisols, or Ultisols, occupy very small areas in Russia; they are strongly modified by human activities. The unchanged profiles are preserved only locally, in particular, in the Sochi arboretum. When compiling the soil map of the arboretum, different Zheltozems were described on gentle and moderately steep slopes and diverse parent materials: calcareous claystone and its heavy-textured derivates. The micromorphology of three characteristic Zheltozem profiles developed on these different parent materials was also studied. The features that they have in common are the following: in all profiles, a specific heavy-textured Bw, or BM ‘metamorphic’ horizon was identified with a certain set of micromorphological properties: a massive microstructure with fine planes merging into deformed biogenic pores, a monic groundmass with few silt grains that are sometimes oriented, and impregnative iron-oxides pedofeatures. The b-fabric was defined as stipple speckled, poro- and granostriated, locally (mono)striated ressembling stress coatings. There are almost no clay coatings except for very few fine internal ones that are light yellow and homogeneous; clay and iron pseudomorphs (alteromorphs) occur over some skeleton grains. Many of these properties are related to the clay mineral composition. There are varying quantities of smectites and kaolinite-smectites, whereby the former increase with depth. The presence of expanding minerals is known to contribute to the disintegration of coatings. Remnants of these are observed in typic iron-manganic nodules. In terms of WRB, this horizon is an intergrade between cambic and argic, which is not accounted for in soil diagnostics. As three different Zheltozems are studied in the same area, the following genesis of this specific intergrade can be given. The saprolite in one of the profiles is composed of fragments of weathered calcareous argillites (subangular) and sandstones (angular) separated by planes with infillings. The micromass in the argillites is locally depleted of carbonates; some fragments have rusty-black iron-manganic intercalations, external clay coatings and iron quasicoatings. The infillings contain fine carbonate nodules and fragments of clay coatings. The sequence of soils studied enabled us to identify the pedoplasmation phenomena, as described by Stoops and Schaefer (2010). This process starts with the decarbonatization, fragmentation and ferrugination of rock fragments, which are gradually assimilated by the groundmass, so that only a trend to linear arrangement of grains is preserved. The pedoplasmation starts in the saprolite of in situ argillite, next follows a transformation of the lithogenic fabric into a pedogenic one with the development of a Bw horizon. These phenomena are more obvious in the profile on the oldest parent material – clayey colluvium. The low porosity and dense fabric of the Bw horizon hinder the removal of carbonates from the underlying layers (saprolite), and this may be a reason for the incompatibility between abundant rainfall inherent to the wet subtropical climate in Sochi region and the high position of the line of effervescence with hydrochloric acid in Zheltozems on calcareous argillite.