GENESIS AND MORPHOLOGICAL CHARACTERISTICS OF MOLLISOLS FORMED IN A CATENA UNDER WATER TABLE INFLUENCE IN SOUTHERN IRAN

Abstract

A Mollisol catena in highly calcareous parent material under semiarid conditions of southern Iran was studied to determine the effects of water table depth and its fluctuations on the organic carbon content of mollic epipedons, genesis of subsurface horizons, and mineralogical variations in these horizons. The soils formed on depressions (microlows), have the shallowest water table (about lm in July and 25 cm in January), the longest time of saturation, greatest organic carbon content, and have 50 cm thick mollic epipedons. Grayish brown matrices (10YR 5/2) and yellowish brown (10YR 5/6) mottles are also apparent in B horizons. Subsurface horizons have characteristics of cambic horizon. No calcic horizon have formed in these soils, mainly due to the lack of wetting and drying cycles due to the permanent saturation. The soils with a water table depth of deeper than 1 m, have cyclic saturated conditions. Organic matter content and thickness of mollic epipedons of these soils are less than that of the soils on microlows. They show developed calcic horizons. Secondary carbonates present in B horizons of these soils are related mainly to discharge from a shallow water table, which have precipitated as secondary carbonates in the sola due to evapotranspiration and precipitation from upper horizons. The soils formed on the higher landscape positions with very deep water tables (deeper than 2m in winters) show lower amounts of organic carbon and very thin mollic epipedon. They are not saturated and do not show redoximorphic features. Only a cambic horizon has formed in these soils. as a result of organic matter addition and transformation, and translocation of CaCO3. Mineralogical study of the B horizons of these three studied soils. showed that there is a little difference in type of clay minerals between different members of the hydrosequence, but the relative amount of clay minerals are different due to weathering conditions, which are affected by internal drainage. The major mineral in the well drained soils (Barab series) is palygorskite, while in poorly drained soils (Pole Bahadoran series), smectite is dominant. In very poorly drained soils of the depressions (Dehnow series), almost permanent saturation has inhibited weathering processes of minerals. In these soils chlorite and illite are the dominant minerals. A new category for Soil Taxonomy, Fluvaquentic Calciaquolls, is proposed to accommodate Calciaquolls which have fluventic origin (Pole Bahadoran soil series).