In the karst region of southwestern China, mixed evergreen and deciduous broad-leaved forest is typical vegetation, differing remarkably from the evergreen broad-leaved forest developed in non-karst regions in the same latitudinal zone. Litter input from deciduous trees influences the characteristics of the litter layer, which could affect decomposition dynamics. We evaluated the role of deciduous trees in regulating the decomposition of mixed leaf litter in a karst forest. Four species richness treatments (one, two, four and six species) were designed to evaluate the decomposition rates of litter mixtures for two years. The ratio of deciduous to evergreen species numbers was 1:1 in all mixtures. Litter mass loss was positively correlated with initial nitrogen concentrations and negatively correlated with carbon:nitrogen ratios, suggesting that mass loss was controlled by nitrogen concentration of the litter mixture. Litter mixing accelerated the mass loss by 5.0% and enhanced nutrient release by 4.8%–26.2% for different elements. Synergistic effects in the four-species mixture were usually strongest and increased over time. Although individual litter species within the mixtures showed idiosyncratic responses to litter species richness, mixing effects enhanced decomposition of evergreen litter species more than deciduous species. This study suggests that species composition was more important than species richness in driving non-additive effects on decomposition in this forest. Deciduous trees in karst ecosystems significantly contribute to nutrient cycling, through enhancing the decomposition of evergreen leaf litter.