Questions Understory vegetation is an indispensable part of the forest ecosystem. Yet, the impacts of tree species mixing on understory biomass, diversity, soil carbon, and nitrogen content and their associations remain understudied in coastal shelterbelt forests.Location Temperate forest plantations in eastern China.Methods Based on the 36 field sample plots of Japanese black pine (Pinus thunbergii)-black locust (Robinia pseudoacacia) mixtures and their respective Japanese black pine and black locust monocultures, this study analyzed the effects of tree mixture on the understory biomass and its diversity, as well as soil carbon and nitrogen.Results Species mixture had no significant effect on understory biomass, with herbaceous plants producing greater biomass than shrubs and seedlings. The Pielou index of shrubs was significantly higher in locust monocultures than in pine-locust mixtures and pine monocultures, whereas no significant difference was detected in herbaceous and seedling diversity across stand types. Both soil nitrogen and carbon contents were enhanced in pine-locust mixtures compared to pine monocultures, but lower than in locust monocultures. Shrub biomass was positively correlated with stand age, while shrub diversity showed associations with bulk density and stand type. Lastly, no significant differences were observed in plant functional groups between mixed and monospecific stands.Conclusions This study demonstrates that establishing two non-native P. thunbergii and R. pseudoacacia mixed-species plantations does not necessarily enhance the biomass and diversity of understory vegetation, but alleviates soil nutrient limitations of black pine monocultures which occupy a large proportion of forest areas in this region.