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Biomes and Regions of Northern Eurasia

Steppe and Forest-steppe

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Soils

The soils of steppes and the factors involved in their formation are discussed above and here only a brief discussion follows. Two types of soils are typical of the steppe and forest-steppe biomes: chernozems or black soils and kashtanozems or chestnut soils (Table 11.1).

Table 11.1 Zonal soils and vegetation in steppe, forest-steppe, and semi-desert

Chernozem is one of the most fertile soils on Earth. In Northern Eurasia, chernozems occupy about 1.9 million km². Many prominent scientists researched the origin of chernozem and linked it to the rich herbaceous vegetation typical of steppe and meadow steppe (Rupreht, 1866; Dokuchaev, 1883; Kostychev, 1937). About 1-2 kg m² of dead phytomass is produced annually and about a half of it is contributed by the below ground parts (Alekhin, 1934; Titlyanova et al, 1983). The organic matter does not, therefore, accumulate on the surface as litter but is more deeply and evenly distributed through the soil profile due to the extensive and deep root systems of the plants. The most favourable conditions for the development of chernozems occur in the southern foreststeppe where high phytomass reserves coincide with the optimal thermal and moisture regimes. Thick chernozems dominate in this region. Northwards, a more humid climate results in leaching of primary minerals and podzolization of soils. Southwards, moisture deficit causes a reduction in phytomass and the supply of organic matter to the soil, which leads to the formation of soils less rich in humus and nutrients. The group of chernozem soils, therefore, consists of a number of subtypes which exhibit a zonal distribution.

Most fertile are thick chernozems whose humus contents can be as high as 15 per cent (Table 11.1, Figure 11.3).

Fig. 11.3 Zonal variations in properties of soils

Typical chernozems, which develop in the northern steppe, are distinguished by a less developed humus horizon and the content of organic matter does not on average exceed 6-9 per cent, although in soils with a fine grain structure it can be as low as 4-5 per cent (Chesnyak et. al., 1983). Southern chernozems develop under the Stipa-Festuca vegetation communities in the southern steppes. They are characterized by a smaller thickness of the humus horizon, which varies between 25 cm and 70 cm. Meadow chernozems develop both in the steppe and forest-steppe biome in environments with a higher moisture supply, such as poorly drained watersheds, in topographic depressions, and stream valleys. Meadow chernozems are very similar to other types of chernozems and the major distinction is that the content of organic matter in meadow chernozems is higher and, consequently, the humus horizon has a darker colour.

The changes in soil profile also occur from west to east, reflecting the increasing aridity and continentality of climate. Thickness of soil cover decreases substantially. In the Black Sea steppes, soil profiles are about 200 cm deep while eastwards they become progressively thinner, reaching 50-60 cm in the Transvolga region and 30-40 cm in Kazakhstan. The occurrence of solonetz-like soils also increases from west to east.

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