Galina I. Chernousenko, V.V.Dokuchaev Soil Science Institute, Pyzhevskii per. 7, Moscow, Russia
The Baikal region is characterized by the sharply continental cryoarid climate, under which one could expect the widespread development of soil salinization. However, this is not the case. It is known that soil salinity is influenced by three groups of factors: (1) sources of the salts; (2) mechanisms of their accumulation that, in turn, are dictated by the climatic, hydrological, and hydrogeological regimes of particular areas; and (3) physical properties of underlying rocks (texture, presence of aquifers and aquicludes, fissures and faults, etc). The time factor is also important. What are the major factors controlling the development of soil salinization in the Baikal region? According to the genesis and chemical composition of soil salts, this region is subdivided into three parts: the western part (Cis-Baikal region), the central part, and the eastern part (Trans-Baikal region). The Cis-Baikal region is a plain composed of Cambrian salt-bearing rocks — limestone, dolomite, rock salt, anhydrite, and gypsum — overlain by a thin mantle of Quaternary sediments. The Cambrian bedrocks outcrop to the surface on the slopes of valleys. Salt-affected soils are found in the valleys and depressions and are formed due to the washout and redeposition of salts from the salt-bearing rocks by geochemical flows, or due to the springs of hypogene mineral groundwater. The so-called gypsum solonchaks (gazha soils) are widespread; the gypsum content in them reaches 50-80% of the soil mass, and the content of calcium and magnesium carbonates is about 10-30%. Among gypsum solonchaks, both the soils without toxic salts, and the saline soils enriched in sodium and magnesium sulfates occur. In the latter group, the content of sodium and magnesium ions in the water extract (1:5) is from 1 to 90 meq/100 g soil, and the salt content (as measured in the filtrates of soil pastes) is from 0.5 to 10%. The area of soils affected by saline groundwater is small, but still they occur throughout the region. These soils are mostly characterized by a predominance of sodium chlorides; the content of chlorine ions is from 6 to 55 meq/100 g soil, the salt content is 0.1-3.5%. The central region is represented by the Baikal-Stanovoe Plateau, which is the youngest tectonic area with numerous faults and thermal water springs. The soil salinization is mainly due to the discharge of hypogene mineral waters. Sodium sulfates predominate. The gypsum salinization rarely occurs; soda-saline soils can be found. Sodium and magnesium cations predominate; their content in the water extracts is from 1 to 440 meq/100 g soil; the salt content varies from 0.1 to 56%. Among anions, sulfates prevail (0.1-800 meq/100 g soil); the contents of chlorides (1-4 meq/100 g soil) and carbonates (up to 2-7 meq/100 g soil) are generally lower. The Trans-Baikal region includes the Trans-Baikal Mountains and the Aginsk-Kerulen montane-plain territories. Salt-affected soils are found in the depressions of the Mesozoic age occupied by insular steppes and in river valleys. Soil salinization is the result of the long-term continental weathering under dry climatic conditions; in some cases, it is caused by mineral groundwater. The chemical composition of salts differs: in the northern and northeastern parts, magnesium and sodium sulfates and chlorides predominate (the content of sodium and magnesium ranges from 2 to 200 meq/100 g of soils, the salt content is 0.2-15%). Soda-saline soils occupy larger areas, especially in the southern and southeastern parts of the region; the total alkalinity reaches 9-10 meq/100 g soil; the sodium content is lower than that in the sulfate-saline soils and reaches 20 meq/100 g soil; the salt content does not exceed 1.5%. The main reasons of the different geneses and chemical compositions of soil salts in the Baikal region are as follows: (1) the presence of salt-bearing rocks of the Cambrian age in the western (Cis-Baikal) part and their absence in the central and eastern parts of the region; (2) the specificity of hydrogeological regimes in the tectonically active areas with numerous faults and the discharge of saline hypogene groundwater in the central part (the tectonically youngest part of the region); and (3) the cryoarid climate and long-term continental weathering in the presence of permafrost (as an impermeable layer) and the specific topography with numerous intermontane depressions and valleys in the eastern (Trans-Baikal) part of the region. This study was supported by the Russian Foundation for Basic Research, project no. 04-04-48197.