Soil organic carbon pools in particle-size fractions as affected by slope gradient and land use change in hilly regions, western Iran

This study was conducted to explore the effects of topography and land use changes on particulate organic carbon (POC), particulate total nitrogen (PTN), organic carbon (OC) and total nitrogen (TN) associated with different size primary particle fractions in hilly regions of western Iran. Three popular 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 employed as the basis of soil sampling. A total of 99 soil samples were taken from the 0–10 cm surface layer in the whole studied hilly region studied. The results showed that the POC in the forest land use in all slope gradients was considerably more than the deforested and cultivated lands and the highest value was observed at NF-S₁ treatment with 9.13%. The values of PTN were significantly higher in the forest land use and in the down slopes (0.5%) than in the deforested and cultivated counterparts and steep slopes (0.09%) except for the CL land use. The C:N ratios in POC fraction were around 17–18 in the forest land and around 23 in the cultivated land. In forest land, the silt-associated OC was highest among the primary particles. The enrichment factor of SOC, EC, was the highest for POC. For the primary particles, EC of both primary fractions of silt and clay showed following trend for selected land uses and slope gradients: CL> DF> NF and S₃ > S₂> S₁. Slope gradient of landscape 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 mountainous regions with high rainfall in west of Iran.