Environmental factors controlling soil organic carbon storage in loess soils of a subhumid region, northern Iran

•Effects of land use type and topography on SOC were significant.•Subsurface soil layer (30-100 cm) was responsible for 45% of SOC storage.•46.7% of SOC storage occurred in one-third of the studied area under forest.•The highest amounts of SOC loss was observed in E and W aspects of deforested/cultivated areas.

Soil organic carbon (SOC) storage is a basic measure used to study soil productivity, hydrology and the balance among greenhouse gases. Variation of SOC is controlled by environmental factors such as land use and topography. Toshan watershed located in northern Iran was selected to study the effects of different land uses i.e. forest (FO), cropland (CP), orchard (OR) and abandoned land (AB) on different slope gradients and aspects on SOC both in surface (0-30 cm) and subsurface (30-100 cm) layers. A total of 364 soil samples plus 1638 undisturbed ones were collected from two soil layers in 182 sampling sites. Results showed that the surface 30 cm soil layer was solely responsible for 54.8% of SOC density. On average, FO with 22.84 kg m− 2 had the highest SOC density in 0-100 cm layer. Deforestation and agricultural activities have resulted in a significant 48.2% decrease of SOC density in 0-30 cm soil layer. North facing slope (N) aspect and also flat area of all land uses had the higher SOC density compared to east (E) and west facing slope (W) aspects in this subhumid region in 0-100 cm layer. Generally, in the upper 100 cm soil layer of deforested lands, gentle and moderate slopes had higher SOC density than steeper slopes. There was a positive significant correlation between SOC density and clay content. The largest amount of SOC storage was observed in the surface 30 cm layer accounting for 74,907.94 Mg (54.4% of total SOC storage), indicating the important key role of topsoil in conserving SOC. FO with one-third proportion of the total area stored the largest amount of SOC (39,325.55 Mg; 52.5%) in surface layer. In conclusion, protection of forest lands is higher important to increase SOC storage. Agricultural activities on steep E and W aspects in deforested lands must be reduced or prohibited. The subsoil has almost the same contribution to SOC storage and therefore should be carefully considered for management measures. Generally, interaction between environmental factors on storing SOC and also rate of carbon loss to the atmosphere was significant.