Soils of high Ishim river terraces and the marginal part of Tobolsk Continent upland conjunction area

We studied the main features of the topological sequence of soils formed on the flat undifferentiated complex of III-IV Irtish river terraces with elevation varying between 75 and 80 meters above sea level in the western part of the denudation-accumulative poorly dissected Tobolsk Continent upland (58°56'37" N, 69°10'45" E). Soils are presented by various humus-accumulative types with a weak degree of manifestation of podzolization processes and different hydromorphism degree. Humus-accumulative soils with a relic humus horizon and a weak eluvial-illuvial profile differentiation (Umbric Geysols and Umbric Albeluvisols) are formed on the well-drained flat watershed surfaces of high terraces with a predominance of silver fir-birch and silver fir-rowan forests with an admixture of cedar and silver fir in the undergrowth. Umbric Albeluvisols with a relic humus horizon are formed on the upper gentle slopes of high terraces under the spruce-silver fir forests with an undergrowth of cedar, spruce and silver fir. Transeluvial-accumulative positions - small flat wetlands in the bottom part of the slope are presented by thixotropic soils - Histic Gleysols formed under spruce-silver fir green moss forests. The lower part of the sloping hillside slowly turning into terraces of modern Turtas river terrace surface is characterized by predominance of Haplic Gleysols under high humidity conditions in a mostly spruce-fir green moss forest. Soils are characterized by the presence of the second humus horizon and thin layering in the bottom of the profile, inherited from subaerial parent rocks. Preservation of relict characteristics largely depends on the position in the landscape, the extent and degree of soil hydromorphism and intensity of eluvial-illuvial differentiation of soil profile. The relict humus horizon is absent in the most drained areas of autonomous positions with Umbric Geysols, as well as in hydromorphic soils in the flat lower parts of the slopes. We observed the highest degree of relic humus horizon preservation in hydromorphic soils of autonomous positions and on the upper part of the terrace slopes. The soils at the level of III-IV Irtysh high terraces fragment are characterized by the dominance of humus accumulation over the podzolization process under ongoing erosional dissection of the territory. Formation of humus-accumulative soils with poor differentiation of profile and preferably close to the neutral pH values is possible due to a combination of geomorphological conditions, history of landscape development and biochemical characteristics of dark coniferous silver fir forests with a well-developed grassy layer.

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