Objective The aim was to understand the distribution characteristics of soil organic carbon (SOC) components in the understory of Abies georgei var. smithii forests along different altitude gradients, and the factors affecting SOC components were investigated to provide scientific basis for assessing the carbon sink function of forest soil in this region.

Method Soil samples were collected from Abies georgei var. smithii forests located at varying altitudes (3600 m to 4200 m) on the western slope of Sejila Mountain. Sampling was performed across distinct soil layers (0-10 cm, 10-20 cm, and 20-30 cm depth), and the contents of organic carbon, inert organic carbon, particulate organic carbon, heavy fraction organic carbon, light fraction organic carbon and readily oxidizable organic carbon were determined in the samples. The distribution characteristics of SOC components at different altitudes and soil depths and the effects of soil physicochemical factors were analyzed.

Result The results showed that SOC and its components showed a decreasing trend with the depth of soil layer, and the contents of organic carbon, inert organic carbon, heavy fraction organic carbon and readily oxidizable organic carbon showed an increasing and then decreasing trend with the altitude, and reached the maximum value at 3820 m above sea level, while the contents of particulate organic carbon and light fraction organic carbon gradually increased with the altitude. ② Among the contents of the SOC pool components, particulate organic carbon accounted for the largest proportion, and readily oxidizable organic carbon accounted for the smallest proportion. ③ There were significant effects of altitude and soil layer on SOC components, and there were interaction effects between altitude and soil layer on inert organic carbon, particulate organic carbon, heavy fraction organic carbon and readily oxidizable organic carbon, but not on light fraction organic carbon. ④ The results of correlation analysis showed that soil bulk density, soil moisture content, electrical conductivity, total nitrogen, total phosphorus, alkali-hydro nitrogen correlated well with organic carbon and components, and the results of RDA analysis showed that total nitrogen, total phosphorus, available phosphorus, and bulk density had a greater contribution to the explained amount of SOC and components.

Conclusion There were obvious distribution patterns of SOC and fractions under A. georgei var. smithii forest at different altitude gradients in Sejila Mountain. The physical and chemical properties of the soil were important factors leading to the variability of organic carbon and fractions.