Saturday, 15 July 2006
159-16

Soil Salinity in the Gobi (Mongolia).

Ye.I. Pankova, V.V.Dokuchaev Soil Science Institute, Pyzhevskii per. 7, Moscow, Russia

It is common knowledge that deserts throughout the world display some similar features predetermined by the similarity of natural conditions, including the presence of saline soils. At the same time, every desert proves to be specific in terms of soil salinity. The Mongolian part of the Gobi Desert is a stony desert in the center of Asia with extremely continental and dry climate. The difference between maximum and minimum temperatures reaches 90-100ºC. The annual precipitation equals only 35 mm, in some years the desert remains absolutely dry. Strong winds, particularly in the spring, and deep soil freezing in the winter are typical of the Mongolian Gobi. The specificity of soil salinity in this desert has been the subject of growing attention of many scientists. First researchers indicated a small amount of saline soils in spite of the high climatic aridity. The soil salinity in the region was thoroughly studied by specialists of the joint Russian-Mongolian Complex Biological Expedition in the second half of the 20th century (Pankova, 1992). Three different types of landscapes with specific salinization patterns were distinguished: (1) relatively flat interfluves; (2) closed intermontane basins, and (3) natural oases. Mountainous areas were not taken into consideration. Flat interfluves account for 90% of the total area in the Gobi Desert. Dry channels turning into water- and mudflows during rainfalls in the mountains dissect them. Gray-brown desert (arid) soils are developed from the skeletal Quaternary colluvium overlain by a 30- to 35-cm-thick fine earth material with inclusion of pebbles. The soils are not saline, but they may contain a small amount of salts and fine crystalline gypsum under the stones. Soils derived from the clayey red-colored deposits of the Cretaceous-Paleogene age occupy about 3-5% of the interfluves. In most cases, they are saline. The salt transfer by wind into the adjacent territories leads to soil salinization, even if the soils are developed from the nonsaline skeletal deposits. In the Trans-Altai Gobi, the so-called extremely arid soils are widespread; they are saline at the surface and are underlain by the rocks without salts. The total area of closed intermontane basins is estimated at 8-10%. Ancient and recent depressions are distinguished. The ancient depressions are deeper and are filled with a stratum red-colored saline Cretaceous-Paleogene deposits with fossil animal skeletons. Saline takyrs and solonchaks, often in combination with saline sandy soils, are formed in these depressions. In wet years, during rainfalls in the mountains, the mudflows reaching the depressions form temporary lakes. This is the zone of surface runoff accumulation. When the lakes are dried, the surface transforms into solonchak; then, the salts are removed by wind, thus exposing the takyr-like bottom of the depression. The recent (Quaternary) depressions are also the zone of runoff accumulation. However, their bottoms are composed of colluvial and alluvial deposits overlain by shallow loamy sediments. Takyr-like soils, sandy soils, and solonchaks are formed, but the soil salinity is less pronounced. Natural oases represent a specific type of landscapes. Lavrenko and Yunarov (1952) were first to describe them and show their difference from the oases formed under the impact of irrigation. In the Trans-Altai Gobi, natural oases occupy a small area (<1%) in the zones of tectonic faults with deep groundwater discharge (Timofeyev, 1985). The groundwater is slightly saline and contains boron; it has alkaline reaction. The hydromorphic strongly saline soils are formed in such places. In the Ekhiin-Gol natural oasis, the solonchaks may contain up to 40-70% of salts in the surface horizons; the salt content decreases in the deeper layers (50-200 cm). Thus, the main area of the Mongolian Gobi is occupied by nonsaline soils. The recent soil salinization is restricted to the areas of surface runoff accumulation in the closed depressions and to natural oases with the discharge of deep saline groundwater in the zones of tectonic faults. The extreme aridity and continentality of the climate are the factors limiting the intensity of weathering and pedogenesis; as a consequence, the rate of salt formation and accumulation is low. This study was supported by the Russian Foundation for Basic Research, project no. 04-04-48197.

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