Mixed-species afforestation can significantly enhance forest productivity and soil carbon stocks. However, the effects of tree species richness and composition on soil plant-derived carbon and soil organic carbon (SOC) stability are poorly understood. In this study, a manipulative experiment designed with a gradient of tree species diversity in southern subtropical China was conducted to investigate the effects of tree species richness and composition on the preservation and degradation of leaf- and root-derived carbon components in soils and quantify their contributions to SOC stability by using cutin-leaf derived and suberin-root derived biomarkers. The results showed that tree species richness significantly increased the suberin content, but had no effect on cutin content. By comparison, tree species composition significantly influenced the cutin content, but had no effect on the suberin content. Soil pH and water content mainly affected the cutin content, while soil total nitrogen and fungal/bacterial abundance ratio significantly affected the suberin content. The degradation parameters ω-C₁₆/∑C₁₆ and ω-C₁₈/∑C₁₈ for cutin and suberin were independent of tree species richness and composition, but were significantly influenced by soil pH, ammonium-nitrogen, and the ratio of carbon to nitrogen. Structural equation modeling indicated a direct and positive effect of cutin components on the stability of SOC indicated by the ratio of easily oxidized carbon to SOC(EOC/SOC)and a direct and negative effect of suberin components on EOC/SOC. Tree species richness increased root-derived carbon in soil by enhancing fine root biomass and fungal/bacterial abundance ratio, thus promoting the SOC stability. These results provide a theoretical basis for enhancing soil carbon sequestration through an appropriate silvicultural selection of tree species in subtropical plantations.