Effects of Land Use and Slope on Soil Physical, Mechanical and Hydraulic Quality in Heyran Neck, Ardabil Province

 The quality of soil physically, hydraulically, and mechanically are not similar in various land uses, slope aspects and positions in the steep lands. There is extensive information around the world about the land use effects and how to lessen the extent of the slope position effects on soil properties. Although several studies have been conducted regarding the land use effects on various soil mechanical properties, little is known about the effects of land use and slope positions on the shear strength and penetration resistance. The objective of this study was to compare some soil quality parameters in different land uses, slope aspects and slope positions in the steep lands of Heyran Neck, Ardabil Province, Iran.

The study was conducted in the Heyran Neck, Ardabil Province located at the 40 km of Ardabil-Astara road, northwest of Iran (48° 36ʹ 7ʺ E and 38° 23ʹ 37ʺ N). The factorial experiment was carried out based on completely randomized design with four replicates. The first factor was two land uses (forest and rangeland), the second factor was two slope aspects (north and south) and the third factor was three slope positions (shoulder, footslope, toeslope). Totally, 48 disturbed and undisturbed (using steel cylinders with 5 cm diameter and height) samples were taken from 0-10 cm soil depth. Sand, silt and clay (hydrometer method), organic carbon (Walkley–Black method), bulk density (cylinder method), saturated water content (gravimetric method), field capacity water content (equivalent 30 kPa suction) and permanent wilting point water content (equivalent 1500 kPa suction) using pressure plates method were measured in the collected soil samples. A shear vane was used to make shear strength measurements in saturation condition. The procedure used in this study was pushing the vane into the soil surface until the blades were covered (about 8 mm depth); a clockwise rotation rate was then applied to ensure that failure developed within 5 to 10 sec. The maximum stress value was recorded on a dial at the top of the vane driver. Vanes with a stress range between 0 and 100 kPa were used in all cases to induce shear failure. A non-return pointer assisted in readings. Penetration resistance was measured in the undisturbed soil samples (steel cylinders) using automatic micro penetrometer equipment (with three replicates) in the water content equivalent 1 bar suction. Data’s normality was assessed through Kolmogorov–Smirnov test; also, the analysis of variance and comparison of means by Duncan test and Pearson correlations were done using SPSS software. Figures were prepared using Excel software.         

The textural classes of studied soils were sandy loam (n= 8), loam (n= 10) and clay loam (n= 6) in the forest land and clay loam (n= 16), loam (n= 6) silty clay loam (n=1) and clay (n= 1) in the rangeland according to American textural triangle. There were found significant correlations between soil organic carbon and shear strength (r= 0.74**), bulk density (r= -0.59**), penetration resistance (r= -0.30*), saturated (r= 0.58**), field capacity (r= 0.42**) and permanent wilting point (r= 0.37**) water contents. Shear strength significantly correlated with clay (r= 0.35*), bulk density (r= -0.59**) and penetration resistance (r= -0.33*). The mean values of organic carbon, sand, saturated and permanent wilting point water contents in forest lands were significantly higher than range lands. The mean values of clay, bulk density, penetration resistance and field capacity water content in range lands were significantly higher than forest lands. The mean values of organic carbon, silt, clay, shear strength, saturated, field capacity and permanent wilting point water contents in northern slope were significantly higher than southern slope. The mean values of sand, bulk density and penetration resistance in southern slope were significantly higher than northern slope. The highest value of bulk density (1.46 g/cm3) was found in southern aspect and toeslope position. The highest values of field capacity (30.45 % g/g) and permanent wilting point (20.83 % g/g) water contents were found in shoulder position and had significant difference with footslope and toeslope positions.

In general, the results showed that from forest land use to rangeland in Heyran Neck of Ardabil Province, The quality of soil physically, hydraulically, and mechanically were reduced. Also, soil quality of northern aspect was higher than southern aspect because of intensive plant cover and high soil moisture. Soil quality of toeslope was less than shoulder and footslope position in the studied region.