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Community composition strongly affected the soil C and N storages. However, the influences of community composition on native grassland remain poorly understood. The purpose of this study is to investigate the ability of plant communities including how legumes affect the soil C and N storages in the semi-arid grassland. Experimental grassland communities were separated by whether or not containing legumes. We measured soil C and N storages and determined above-ground and below-ground biomass, litter biomass, plant species richness, and species diversity to understand the mechanisms underlying the changes of soil C and N storages and to determine the relationship of species diversity and productivity. The results showed that legumes increased above-ground and below-ground biomass and C and N storages. Soil C and N storages were significantly and positively related to above-ground and below-ground biomass, litter biomass, plant species richness, and diversity in the presence of legumes. The presence of legumes increased soil C and N simultaneously but not synchronously, which resulting in a higher C:N ratio. This study indicated that legumes increased soil C and N storages possibly through increasing biomass and soil C and N inputs. The increases are mediated by plant diversity and plant functional complementarity. We suggest that the combination of legumes-grass species may greatly enhance ecosystem services such as soil C and N storages, productivity, and diversity in semi-arid grassland. Copyright (c) 2016 John Wiley & Sons, Ltd.