Soils of Mountain Floodplains of the Upper Kerulen Basin (Mongolia)

New data on the morphology, chemical and agrochemical properties, classification, and rational use of alluvial soils of floodplains of the mountain rivers of the Pacific Ocean basin (Mongolia, Upper Kerulen Depression) have been obtained. A dense river network in the northern part (subtaiga zone, 1500–1395 m a.s.l.) is formed by the Kerulen River and its tributaries Khoit Khorot, Dund Khorot, Teyun-Gol, Dzor-Gol, and Uvkhiriin-Gol rivers. Thin, rich in humus alluvial soils are formed on the floodplains of the island-braided type with a gravelly bouldery substrate. They are characterized by a poorly pronounced stratification. Soil salinization is is seen in the areas of discharge of chloride–bicarbonate magnesium–sodium groundwater. The differences between soil types within this area are related to the mucky–humus and humus horizons. The types of alluvial humus soils (Fluvisols Humic) and alluvial mucky–humus soils (Folic Fluvisols) have been specified. In the central part of the depression (the forest-steppe zone, 1395–1357 m a.s.l.), the Kerulen River forms an epigenetic valley with poorly developed floodplains. The main floodplains are formed by the Dzun-Burkhiin-Gol, Barun-Burkhiin-Gol, and Delenziin-Gol rivers. Alluvial deposits are of gravelly sandy composition. The soils are characterized by a high humus content, pronounced gleying, and salinization. The types of alluvial dark-humus soils (Eutric Fluvisols Humic) and dark-humus gley soils (Gleyic Fluvisols Humic) have been diagnosed. In the southern part of the basin (the steppe zone, 1357–1306 m a.s.l., floodplain soils are formed mainly on alluvial sands. They are characterized by a slightly alkaline reaction and are saline. The type of alluvial dark-humus gleyed saline soils (Sodic Gleyic Fluvisols Humic)) is formed there. A low fertility of the floodplain soils in the studied region is related to unfavorable water-physical properties and low content of nitrate nitrogen and available phosphorus. A grouping of soils according to their use, mainly for plowing, hayfields, pastures, and poorly available or unavailable for agriculture is proposed. Potential destructive agrogenic processes are described, when these soils are involved in intensive agricultural production. A set of measures is recommended for environmentally safe management of alluvial soils and floodplain ecosystems in the upper reaches of the Kerulen River and its tributaries in grassland farming and agriculture, including the use of various types of manure, composts, and mineral fertilizers (mainly, nitrogen and phosphorus fertilizers).