EFFECTS OF LAND USE AND SLOPE GRADIENT ON SOIL ORGANIC CARBON POOLS IN PARTICLE-SIZE FRACTIONS AND SOME SOIL PHYSICO-CHEMICAL PROPERTIES IN HILLY REGIONS, WESTERN IRAN

This study was conducted to investigate the effects of slope gradient and land use changes on soil organic pools in particle-size fractions under natural forest in hilly regions in western Iran. Three common land uses in the selected site including natural forest (NF), disturbed forest (DF) and cultivated land (CL) and three slope gradients (0-10%, S₁, 10-30%, S₂, and 30-50%, S₃) were selected as a basis for soil sampling. A total of 108 soil samples were taken from the 0-10 cm surface layer in the whole studied region. Some physical and chemical properties as well as particulate organic matter (POM), particulate total nitrogen (PTN) and organic carbon (OC) and total nitrogen (TN) associated with different primary particle size fractions were measured. The results showed that the POM in the forest land use was considerably higher than the deforested and cultivated lands in all slope gradients. POM was mainly concentrated in the sand-size fraction. The values of PTN were significantly higher in the forest land use and in the down lower slopes than in the deforested and cultivated counterparts and steep slopes except for the CL land use. The C: N ratios in POM fraction were around 17-18 in the forest land and around 23 in the cultivated land. In forest land, the silt-associated OC had highest values among the primary particles. The enrichment factor of SOC, EC, was the highest for POM. For the primary particles, EC of both silt and clay primary fractions for selected land uses and slope gradients tend to be in the following orders: CL>DF>NF and S₃>S₂>S₁. Slope gradient significantly affected the OC and TN contents associated with the silt and clay particles, whereas higher OC and TN contents were observed in lower positions and the lowest value was measured in the steep slopes. Overall, the results showed that native forest land improves soil organic carbon storage and can reduce the carbon emission and soil erosion especially in the mountain regions with high rainfall in western Iran.