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There are potential effects of converting a planted-forest type from conifer to broadleaf or mixed-species on soil organic carbon (SOC) chemical composition, and the quantity and distribution of various chemical compositions of SOC remain uncertain. An afforestation experiment with different tree species (Pinus massoniana, Erythrophleurn fordii [N-fixing species], and mixed P. massoniana and E. fordii) was conducted in clear-cut sites of P. massoniana-planted forests in a subtropical region. Topsoil organic C quality and microbial diversity were assessed after eight years of afforestation. Referring to the species diversity index in the community, Pielou's evenness index was used for estimating the evenness of distribution of various organic C compositions in the total organic C of soil, litter, and fine roots. The proportions of alkyl C and carbonyl C in SOC, as well as the alkyl C/O-alkyl C (A/O-A) ratios and the distributed evenness of the four main SOC chemical compositions were higher in the monospecific E. fordii and mixed-species-planted forests than in the monospecific P. massoniana-planted forest. Positive relationships of the A/O-A ratio and the evenness of distribution of various SOC chemical compositions in the litter as well as fine root C quality were observed. Microbial biomass C was positively correlated with the labile SOC (O-alkyl C). Multivariate analysis showed that the fine roots had a closer correlation with the chemical composition of SOC than the litter. These results highlight that introducing native N-fixing broadleaf tree species and mixing N-fixing species into conifer forests probably enhances the SOC chemical stability. It also minimizes the risk of C decomposition posed by any one soil organic matter (SOM) constituent having a huge impact during climate change through increasing the proportion of recalcitrant C components and even distribution of various SOM chemical compositions.