Soil Organic Carbon (SOC) fractions play a critical role in the stabilization of SOC. It is essential to clarify the variations in SOC fractions along forest succession for predicting soil carbon (C) source/sink capacity as forest develops. In the present study, we collected and analyzed soil samples in Total Organic Carbon (TOC) content and the concentration of its Readily Oxidizable Organic Carbon (ROC) and Non-Readily Oxidizable Organic Carbon (NROC) fractions in a Pine Forest (PF), a pine and broadleaf Mixed Forest (MF), and an old-growth monsoon evergreen Broadleaf Forest (BF) in the subtropical China. The three forests represent different successional stages of forest in southern China, with the PF being at the early, the MF at the middle and the BF at the climax stages, respectively. To clarify the reasons for why SOC fractions changed with forest succession, litterfalls in these forests were also collected to assay C functions by means of ¹³C Nuclear Magnetic Resonance (NMR) analysis. Results showed that although there was no significant difference in the content of ROC among the three forests, the ratio of ROC to TOC in the PF was significantly higher than that in both of the BF and MF. This was likely due to the higher content of O-alkyl C in fresh litter in the PF than in the other two forests. Unlike ROC, however, NROC content was the highest in the BF, followed by the MF and then PF, which could be attributed to the lower recalcitrance index in the partly-decomposed and decomposed litterfall layers in the PF than in the MF and PF. Finally, TOC concentration was significantly the highest in the climax BF, the lowest in the youngest PF, and in between in the MF. Our results suggest that the accumulation of NROC, which is the recalcitrant fraction of SOC, may be the major reason for why forests at the middle and late stages (i.e., the MF and BF, respectively) maintain higher TOC content and Organic Carbon (OC) stabilization in soils.