Background Forest is the most important carbon storage pool in the terrestrial ecosystem. Studying the spatial distribution pattern of soil organic carbon density (SOCD) in forest land and understanding its influencing factors contribute to enhancing soil fertility, thereby promoting the stability of forest stands and improving vegetation productivity in ecosystems. Due to the fact that the Ta'ergou small watershed in Datong county, northeastern Qinghai province is a typical representative area for implementing the Grain for Green Project in the Qilian Mountains region, this area has been selected as the research area.

Methods This study selected three typical forest types, namely Picea crassifolia forest, Betula platyphylla forest, and Picea crassifolia & Betula platyphylla forest, in the Ta'ergou watershed of Datong county, Qinghai province. Set up 5 plots in each forest type, totaling 15. Sampling was conducted on soil layers (0-20, 20-40, and 40-60 cm) at different depths under different forest types, the organic carbon content was measured to calculate the SOCD, and the grey correlation analysis method was used for research and analyze the distribution characteristics and influencing factors of soil organic carbon density in the forest land.

Results The findings indicated that the soil organic carbon density (SOCD) content in different forest land types demonstrated that the P. crassifolia & B. platyphylla was higher than the B. platyphylla (P < 0.05), and the P. crassifolia was the lowest. The organic carbon density on the vertical profile decreased with the increase of soil depth and was significantly positively correlated with the shrub biomass and canopy density in the vegetation layer factors, while significantly negatively correlated with the angular scale. Among the three forest types, the correlation between available phosphorus (AP) and soil organic carbon density is the highest in the P. crassifolia forest at 0-20 cm (correlation degree is 0.92). In the B. platyphylla forest, the factors with strong correlation with organic carbon density are 0-20 cm alkaline nitrogen (AN), 0-40 cm available phosphorus (AP), and average tree height (H); In the P. crassifolia & B. platyphylla, the correlation between canopy density (CD) and organic carbon density is the highest (correlation degree is 0.95).

Conclusions The P. crassifolia & B. platyphylla has the best comprehensive properties, which is more favorable for the storage and buildup of soil organic carbon. It has important guiding significance for the improvement of forest stand function in the region.