Aggregate structure and soil organic carbon pools in aggregate sizes fractions as affected by slope gradient and land use change in hilly regions, western Iran

This study was conducted to investigate the effects of slope gradient and land use changes on aggregate stability and soil organic carbon (SOC) pools in aggregate size fractions in western Iran. Three land uses in the study area 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 for soil sampling. A total of 99 soil samples were taken from the 0-10 cm surface layer in the whole studied hilly region. The results showed that forest disturbances and continuous cultivation after deforestation significantly decreased water stable aggregates percentage (WSA)، mean weight diameter (MWD) and geometric mean diameter (GMD) values in three selected slope classes. The highest percent of macro-aggregates (2. 00-4. 75mm) and meso-aggregates (0. 25–2. 00 mm) were found in the lower slope (S1) induced by higher C stocks in this position. The highest percentage of macro-aggregates was observed in the natural forest soil and that of micro-aggregates (0. 053-0. 25 mm) was observed in the cultivated and disturbed forest soils. There were significant differences of SOC among three land uses for macro- aggregates، but no significant differences were identified for meso- and micro-aggregates between disturbed forest and cultivated soils. The result also revealed that those significant differences among gradient slopes according to SOC and TN in aggregates for three land uses and the lowest was observed in the steep slopes (S3) with highest soil erosion rate. The relative distribution of total nitrogen among the aggregate fractions and C/N ratios were similar to the SOC distribution.