This study is a global meta-analysis of the effects of grassland plant species richness on aboveground and belowground carbon and nitrogen dynamics. Observations from 73 studies in grasslands totaling 1385 paired observations of plant mixtures and monocultures were compiled. Response variables included nine carbon and six nitrogen processes to plant diversity, examined the effects of experimental types and age on the responses, and predicted the carbon and nitrogen balance following different biodiversity loss scenario in grasslands. We found that carbon and nitrogen functions significantly enhanced in plant mixtures, but varied with experimental types. Most of the attributes was significantly correlated with species richness and experimental age, the relationship between species richness and carbon and nitrogen processes was interacted with experimental duration in the field experiments, except for soil respiration, fungal biomass, and soil nitrate nitrogen. Importantly, our results showed that the declines in soil carbon and nitrogen pool accelerated following plant diversity loss. Our meta-analysis revealed that the plant diversity has ubiquitous impacts on carbon and nitrogen cycles in grasslands, likely driven by complementarity effects of plant diversity on plant productivity and biomass, underlined interactive effects of plant diversity, experimental types and age, and climate on carbon and nitrogen processes, and suggested that the reduction in carbon and nitrogen stocks in grasslands will be larger following biodiversity loss in the future.