Plants produce above- and below-ground biomass. However, our understanding of both production and decomposition of below-ground biomass is poor, largely because of the difficulties of accessing roots. Below-ground organic matter decomposition studies are scant and especially rare in the tropics. In this study, we used a litter bag experiment to quantify the mass loss and nutrient dynamics of decomposing twigs and small roots from an arbuscular mycorrhizal fungal associated tree, Parashorea chinensis Wang Hsie, in a tropical rain forest in Southwest China. Overall, twig litter decomposed 1.9 times faster than small roots (decay rate (k) twig = 0.255, root = 0.134). The difference in decomposition rates can be explained by a difference in phosphorus (P) concentration, availability, and use by decomposers or carbon quality. Twigs and small roots showed an increase in nitrogen concentration, with final concentrations still higher than initial levels. This suggests nitrogen transfer from the surrounding environment into decomposing twigs and small roots. Both carbon and nitrogen dynamics were significantly predicted by mass loss and showed a negative and positive relationship, respectively. Our study results imply that small roots carbon and nitrogen increase the resident time in the soil. Therefore, a better understanding of the carbon cycle requires a better understanding of the mechanisms governing below-ground biomass decomposition.