The soils of the river-plain ecosystem are an important component of the habitat for many different plants and animals, as well as human beings. Northern Mongolian river-plain ecosystems are typically characterized by alluvial boggy soils, alluvial meadow soils and soddy soils. The geographical distribution of these soils directly influence the distribution of specific plant communities in these river-plain ecosystems.
The results of our investigations show that biological, chemical, agrochemical and water-physical properties as well as the hydro-termical regime of these soils is very different and these qualities depend upon soil formation factors and the cryogenesis of long seasonally freezing grounds. As a result of biogenic accumulation of chemical elements the total content of such elements as phosphorus (0.24%), manganese (0.11%), sulphur (0.23%), sodium (3.5%), titanium (0.7%) are highest in boggy soils.
As a result of hydrogenic accumulation the total content of calcium (3.5- 8%), magnesium (3.0%) and iron (6-7%) are highest in meadow soils.
According to the data of stationary observation conducted on the flood plain soils of the Orkhon River, the period of it's freezing lasts from mid- October to the begin-March, (ranging from about 150 to 200 days). Accordingly, the duration of soil thawing ranges from 160 to 219 days.
The depth of soil freezing in these areas ranges from 1.5 m to 4.5 m, depending on its location in the surrounding landscape. The thawing of alluvial soils can vary considerably. These variations are dependent upon soil type, soil texture, soil moisture level, as well as soil formation conditions; e.g., the depth of thawing for boggy soil is 10 cm in May, 30 cm in June, and 60 cm in July. In contrast, alluvial meadow soils thaw at higher rates than boggy soils. The maximum temperature totals, (> 10 oC, at a depth of 0.2 m), are: 100oC to 320oC for boggy soils; 400oC to 500oC for meadow soils; and 1150oC to 1250oC for soddy soils. Soil moisture is highest in boggy soils, distributed in the periphery and lower part of the flood plain. Soil moisture is optimal in meadow soils, located primarily in the central part of the flood plains. Soil moisture is fairly deficit in soddy soils, distributed in the scroll parts of the flood plains. The results of these studies underlie the importance of the cryogenesis of long seasonally freezing soil in Mongolia, specifically, the territory bordering the Southern boundary of the permafrost region of the Northern hemisphere and Northernmost boundary of the Gobi Desert. Because of the knowledge of these soils is necessary for resolution of theoretical and practical problems related to these soils. As results of establishment of optimal moisture and nutrient elements regime in these soils the plant productivity increased 2-4 times, depending on soil types and plant community.
Dr. R. Baatar Senior Researcher Institute of Geography Mongolian Academy of Science Ulaan-Baatar-210620 MONGOLIA