The effect of slope aspect and position on soils particle size distribution in Chelgerd region, Chaharmahal-va-Bakhtiari province

Topography, as a soil-forming factor, influences soil physical characteristics through aspect and slope position. Soil particle size distribution (PSD) is one of the most important soil physical characteristics influencing other soil characteristics directly or indirectly. This study investigates the effect of aspect and slope position on soil particle size distribution, grain-size curves and soil fractal dimension in an area, 35 km distance from Chelgerd, Chaharmahal-va-Bakhtiari province. For this purpose, two slope aspects with the same parent material were selected. Three slope positions (top, middle and base) in seventeen parallel transect along north and south-facing aspects were sampled. Also representative pedons located on three positions of each aspect were classified. Particle size distribution was analyzed by a combination of the sieve and the hydrometer method. The grain-size distribution in all three positions of the north and south-facing aspects were fitted by unimodal and biomodal Fredlund models. The fractal dimension of soil particle size distribution was determined in different aspects and slope positions. Soils in the north-facing aspect are classified as Vertisols, whereas soils in south-facing aspect are classified as Entisols and Inceptisols. Results showed that although the dominant soil textural class in both aspects was the same, the soil particle size distribution in different positions of north-facing aspect and base position of south-facing aspect followed a sigmoidal curve and in top and middle position of south-facing aspect followed two sigmoidal curves. Therefore, PSD in north-facing aspect was better described by Fredlund unimodal equation while PSD in top and middle positions of south-facing aspect was better described by Fredlund biomodal equation. Grain size distribution values generally remained similar in north-facing positions but were larger in top and middle south-facing positions. However, coefficient of curvature was independent of aspect and position. In the north-facing aspect, fractal dimension was significantly larger than that of the south-facing aspect and in the base slope position was significantly larger than that of the two other positions. It seems that in mountain area PSD and its shape, PSD-derived values and fractal dimension might better differentiate between the effects of soil forming processes, than some simple texture evaluation.