Eluvial soils of depressions at the Ob-Tom interfluve (the subtaiga zone of Western Siberia)

Closed depressions are widespread on plains composed of loess loam in different natural zones. Questions of genesis and classification of soils with light-colored acid eluvial horizons formed in depressions are still discussed. These soils cannot be identified as solods due to the absence of carbonates or as podsolic soils due to the lack of illuvial horizon. Our article is devoted to the study of eluvial soils in depressions in the area between the Ob' and Tom' rivers (Tomsk Region, Russia). According to the Russian soil classification, the studied soils of depressions with light acid eluvial horizons and a sharp boundary of transition to bedrock were assigned to a great group of eluvial soils -eluvozems. Our aim was to clarify the characteristics of soils and their diagnostics. We studied soils in depressions typical of the territory, differing in size and duration of water stagnation. We established that modern processes in eluvozems take place against the background of deep lithogenic differentiation of the soil profile, acquired during the previous suffusion-subsidence stage of the depression formation. The soil of small depressions (sod eluvozems) can be considered as one of the initial stages of eluvozem development. The eluvial horizon is not absolutely light-colored, but in contrast to background grey soils, it is characterized by deep eluvial silt depletion, removal of exchangeable cations and acidic pH. The diagnostic feature is the accumulation of free iron and ortsteins with a high content of manganese oxides in the upper part of the eluvial profile. Metamorphic eluvozems are formed in depressions with a flat bottom in the zone of free lateral outflow of pedogenesis products on the lithologic barrier surface. The diagnostic feature for their selection is a small metamorphic horizon that characterizes the transition zone of cracking of the suffosion-textural horizon during desiccation-wetting cycles. The humus horizon is substituted by an eluvial one, and ortstein content increases in the middle of the eluvial profile up to 2-4g per 100g of soil. Concretionary eluvozems are formed with an increase in the depression size, stagnant-percolating moisture regime and, correspondingly, redox conditions. Ortstein content reaches 10-12g per 100g, and manganese oxide (MnO) content in their composition reduces. The maximum content of free iron corresponds to horizons with high content of concretions. The eluvial horizon is diagnosed as segregation-bleached. Gley formation is the leading soil process in long-term moisture depressions. Gley eluvozems are characterized by deep removal of free iron and sharp depletion of concretions by manganese oxides, indicating the duration of reductive conditions. Depressions occupied by aspen-birch forest patches play an important role in biodiversity conservation. Ploughed small depressions are unsuitable for crops since the formed aquitard causes spring stagnation of snowmelt waters and surface waterlogging.

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