Variation in soil chemical properties along toposequences in an arid region of the Levant

Detailed characterization of soils and their variation along different topography positions has not been investigated in depth for Mediterranean arid regions. There is a need to accurately understand the variation and the spatial distribution of soil properties within dry region of the Levant. Such understanding is required to optimize the use and management of scarce land and water resources. The objective of this study was to examine the effect of hillslope characteristics on the variation of selected soil chemical properties in an arid Mediterranean climate. At each of five selected transects four sites were chosen to represent four different topographic positions: summit, shoulder, backslope and toeslope. A soil profile was examined at each site and a representative sample from each horizon was withdrawn for chemical analyses. The analyses indicated that generally, the carbonate contents of the surface horizons decreased from higher to lower positions of the toposequence, the carbonate content increased with depth for profiles occupying the lower positions. This suggests more intense leaching within soil at lower positions. The effect of steepness and curvature on controlling the variation of soil properties was obvious at the summit and shoulder positions. Leaching process seems to hinder the effect of steepness and curvature for soils at lower positions. Lower positions receive runoff water and organic matter from upper positions, which complicate the relationship between landform shape and organic matter content. Continuous tillage resulted in lower organic matter contents for soils at lower positions. Higher pH values were reported for soils down along the transect due to the movement of soil material from upper hillslope positions. Electrical conductivity, exchangeable calcium and potassium contents decreased downslope due to higher moisture accumulation. However, no obvious relationship was found between the variation of pH or EC in one hand and the variation of steepness and curvature on the other. Potassium content was variable due to its greater mobility. The analyses indicated that variation in the soil CEC is governed by two factors: the leaching pattern, which is controlled by hillslope position, and the accumulation of Eolian carbonates at the soil surface. The distribution of iron oxides and types of clay minerals indicated more weathering in a descending direction and with soil depth, which is attributed to higher availability of soil moisture along the same direction. These relationships suggested systematic variation of chemical properties along toposequences in this arid environment.