1. Decomposition is a vital process underlying many ecosystem functions. Although a growing number of studies have tested how litter richness affects the decomposition of aboveground plant organs, knowledge remains limited about the decomposition of root mixtures. Here, we used a field experiment in a subtropical forest to investigate how species richness in root litter mixtures (air-dried fresh fine roots) affects the decomposition of root litter material. 2. Based on the concept of resource complementarity, we hypothesized that root litter would decompose faster as the richness of the root litter mixture increased. In addition, we expected the presence of detritivores to modify the effect of root richness on mass loss, because detritivores might experience bottom-up effects from specific plant species and might affect microbial decomposer communities. 3. We found that the richness level of root litter mixtures did not affect mass loss in the absence of detritivores. In the presence of detritivores, all root litter types decomposed faster. Notably, the positive effect of detritivores was stronger at low root litter richness than at high root litter richness, particularly during the early stages of decomposition (the first two sampling points) when litter mass loss was roughly double at low root litter richness compared to that at high root litter richness. The composition of the root fungal community measured at the last sampling point did not differ significantly across root richness levels, and was not affected by the presence of detritivores. 4. Synthesis. Our findings demonstrate that detritivores modify the relationship between root litter diversity and root litter decomposition in subtropical forest ecosystems. This highlights an importance of cascade effects between different trophic organisms on ecosystem functioning.