Soil aggregation and organic carbon as affected by topography and land use change in western Iran

The study was conducted to investigate the effects of slope gradient and land use change on soil structural stability, and soil organic carbon (SOC) and total nitrogen (TN) pools in aggregate-size fractions in western Iran. Three land uses in the selected site were natural forest (NF), disturbed forest (DF) and cultivated land (CL); and three classes of slope gradient (0–10%, S1; 10–30%, S2; and 30–50%, S3) were used as a basis for soil sampling. The results showed that DF and CL treatments significantly decreased soil structural stability indices in the three slope classes. The highest percentages of macro-aggregates (i.e. 2.00–4.75 mm) and meso-aggregates (0.25–2.00 mm) were found in the lowest slope class (S1) which was related to high SOC stock in this position. The highest percentage of macro-aggregates was observed in the NF soil; but the highest percentages of micro-aggregates (0.053–0.25 mm) were observed in the CL treatment. Micromorphological observations confirmed that topsoil under natural forest mainly consisted of highly-porous crumb microstructure, excremental pedo-features or passage features, which are indicators of enhanced SOC and biological activity. The lowest values of SOC and TN were observed at the steep slope class (S3) presumably coincided with accelerated soil erosion. Overall, enhanced aggregation and aggregate-associated organic carbon pools were observed in the forest soils on the steep slopes indicating the importance of land management on C sequestration in natural environments.

► The distribution of aggregate size classes was influenced by land cultivation and topography.
► Micromorphological observations confirmed macroaggregates destruction after cultivation.
► The impact of land use on SOC was significant in the steep slopes.
► Trend of TN and C/N ratio distributions among the aggregate fractions was similar to that of OC.