Forest ecosystems are critical for carbon fixation in both above- and belowground compartments. Increasing tree diversity enhances forest productivity and litter decomposition processes. Litter decomposition is carried out by soil organisms; however, in subtropical forests where soil meso- and macrofauna abundances are rather low, we expected most of the litter decomposition process to be driven by microorganisms. In addition, there is evidence that litter diversity increases litter decomposability. We studied tree diversity effects on decomposition as well as the role of soil microbes and litter decomposability in this process in a large-scale tree diversity experiment of subtropical China (BEF China). Our results show that tree species richness increased the amount of litterfall and litter species richness. Thus, tree species richness influences litter decomposability and, thereby, microbial litter decomposition. We demonstrate that soil microorganisms are responsible for a large proportion of litter decomposition in this subtropical forest. These findings highlight the key role of tree diversity and cascading effects on different ecosystem properties in driving forest carbon and nitrogen cycles. The identified mechanisms can help to improve models on biogeochemical cycles.