We used solid-state carbon-13 nuclear magnetic resonance (NMR) to compare chemical compositions of carbon in plant litter, forest floor and soil in a native seasonal rain forest (SR) with those in a rubber plantation (RP) in Xishuangbanna, southwest China. We also applied a sequential fumigation–incubation (SFI) procedure to characterize the chemical compositional change of soil organic carbon (SOC) during soil degradation. It was found that RP had 1.5–1.7 times greater in alkyl C_0–45ppm proportions and smaller O-alkyls C₄₅₋₁₁₀ fractions in litter and forest floor materials than SR. Degradation of labile organic carbon (LOC) in fresh litter was primarily limited to O-alkyl C and O₂-alkyl C in both forests, suggesting litters of RP were less decomposable than that of SR. SOC in RP was 2.3 percent higher in aromatics C_110–160ppm proportions and 0.03 percent in aromaticity value than those in SR. With the absence of new C input and fauna, degradation of soil LOC companied with loss of carbonyls C_160−220ppm in RP, and alkyl C and aromatics C_110–160ppm in SR. Our data indicated that rubber planting induced carbon loss and extensive humification in the soil. Sources of the decomposed soil LOC can compose of both simple and complex chemical compounds.