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Physical Geography of Northern Eurasia

Soils of Northern Eurasia

<<< Azonal Soils | Physical Geography Index | Rivers, Lakes, Inland Seas, and Wetlands: Introduction >>>

The Holocene History of Soil Cover

The time factor is of particular importance in the formation of soils (Dokuchaev, 1948). Soils evolve together with changing environment and their evolution depends on sedimentological and bioclimatic specifics of the environment (Kovda, 1946,1947; Rode, 1947; Aleksandrovsky, 1983; Ivanov, 1992). The youngest soils, whose age is of the order of hundreds of years, are those that are developing on lower floodplains of large rivers covered by recent alluvial sediments, areas of recent volcanic activity (e.g., Kamchatka and the Kurils), mountains with active denudation-accumulative processes (e.g., the Caucasus), and young aeolian sandy plains in Central Asia (Sokolov, 1973; Romashkevich, 1988; Zamotaev and Targulian, 1994; Glazovskaya and Gennadiev, 1995). An important period in the development of soils in periglacial areas (e.g., taiga and mixed forests) is the last stages of the late Pleistocene glaciation, when the surface was covered by glacial and fluvioglacial deposits and modern soil formation sensu stricto began during the Holocene. fn the extraglacial loess regions (modern forest-steppe and steppe), the age of soils exceeds 10 000 years (Ivanov, 1992).

The Holocene history of soil formation in the tundra zone is still poorly researched. Apparently, it was distinguished by the formation of the peaty gley soils and peats in poorly drained localities and cryogenic taiga soils with weak surface gleying under the conditions of free drainage (Gubin, 1988; Grabetskaya and Chigir, 1988).

After the retreat of the ice sheet, cryogenic sod-forest, sod-gley, and other soils with a weakly developed profile formed in the forest zone of the East European plain. On loams, the accumulation of organic matter occurred while in the soils underlain by sands podzolization developed. During the Atlantic climatic optimum, the thick soddy-podzolic soils developed on both loams and sands. During the following stages of the Holocene, the eluvial part of the profile degraded because of the accumulation of sesquioxides on the surface of the illuvial horizon and dislocation of the boundary between horizons A and B. Thus, evolution occurred from the soddy-podzolic soils with very strong podzolization to the weakly podzolic illuvial-ferrogenous soils while the depth of the profile did not change. On loams, the depth of the eluvial part of the texture-differentiated profile might even have increased. The main characteristic of the soddy-podzolic soils which developed on loams (that distinguishes them from the soils which developed on sandy rocks) is the presence of a dark humic horizon associated with the forest-steppe vegetation of the late Atlantic and, possibly, Subboreal (Aleksandrovsky, 1983, 1996). Later this horizon degraded because of the climatic cooling and the development of the taiga vegetation. The humic and podzolic horizons were restored and a light-coloured layer developed within the podzolic horizon. The lower part of the dark horizon survived locally and in the modern soddy-podzolic soils it forms an additional humic horizon. Soils with two humic horizons occur both on the East European plain and in Western Siberia (Karavaeva, 1982). The development of a light-coloured horizon above the eluvial part of the profile is a common feature of soils formed both on loamy and sandy substrata. According to Ilyichev (1975), this horizon is an illuvial horizon of the poorly expressed modern Al-Fe-humic profile which develops within the thick profile of the middle Holocene forest texture-differentiated soils.

In the middle Holocene, in the East European plain natural zones occurred further north in comparison with their present positions. The nature of zonal soils was also changing. Thus, in the Baltic region, the soddy-podzolic illuvial-ferrogenous soils evolved in the weakly podzolized soils. The major changes in the soil-forming processes, associated with the changing boundary between the tundra and the taiga biomes, occurred in the late Holocene. During the climatic optimum, on the plains the taiga vegetation spread over most of the modern tundra biome. The thick buried northern taiga podzols, discovered in the region of Malozemelskaya Tundra in the extreme north of the European Russia, and inherited differentiation of profile in the loamy tundra soils in the European Russia testify to this.

In Western Siberia, the Holocene history of soil formation was distinguished by the autonomous acidic gleying (Karavaeva, 1982). During the Boreal climatic optimum and the second part of the Atlantic, gleysols with differentiated profiles developed on watersheds and semi-hydromorphic and organogenous soils formed in topographic depressions. During the more severe climatic conditions that followed, these soils did not experience any considerable change. Thus the main proccess, which controlled the development of soils in Western Siberia in the Holocene, was bogging. The underlying rocks, characterized by low water permeability, and flat topography resulted in poor drainage which restricted the development of differentiated profiles with a podzolic horizon.

The soils of the forest-steppe and deciduous forests have a complex history. On the boundary between the Subboreal and Subatlantic (about 2500 years BP), as climate was becoming more humid, forest vegetation advanced into the steppe, the proportion of woodlands increased in the forest-steppe and coniferous species advanced in the deciduous forest zone. Affected most strongly by this change were soils in the north-western Caucasus and in the Carpathian region. In these areas, chernozems were completely transformed in the soddy-podzolic or grey forest soils with strongly differentiated profiles in less than 2000-3000 years (Aleksandrovsky, 1983). Under the forests, the humic horizon of the middle Holocene chernozems was largely destroyed. Whereas its lower part survived, it forms an additional humic horizon in the clay-differentiated soils. Under the oak forests, which dominated the modern forest-steppe in central European Russia in the Subboreal, forest-meadow soils developed in the grey forest soils (Aleksandrovsky, 1983; Akhtyrtsev and Akhtyrtsev, 1986).

The development of soils in what is now the chernozem zone began in the early Holocene in the cryoarid tundra-steppe environments. Soils were calcareous, saline in the upper part of the profile; they were characterized by the presence of cryogenic features and had a weakly developed humic horizon. The formation of chernozems on the East European plain began during the Atlantic (about 8000 years BP) when the climate became warmer and more humid. In southern Western Siberia and Kazakhstan, the amelioration of climate occurred 1500-2000 years earlier (Khotinsky, 1977) and the formation of chernozems began 9500-10000 years BP (Ryskov and Demkin, 1997). The development of soil cover continued into the Subboreal. It was distinguished by the further expansion of the humic horizon and a decrease in the contents of easily soluble salts.

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