Investigating the drivers of negative diversity-invasibility relationship: The role of nutrient availability, allelopathy, soil biota, and soil legacy effects

• Nutrient addition, allelopathy, soil biota, and soil legacy effects impacted plant growth. • Allelopathy and soil microbes were key factors in invasion resistance. • A high diversity of native plants suppressed the growth of invasive species, possibly by promoting a richer community of soil microbes. • Addition of activated carbon increased biomass of the low-diversity plant communities, indicating strong allelopathic interactions in such communities. • Invasive species exhibited similar growth in soils conditioned by low- and high-diversity native communities. The successful establishment of invasive plant species depends on their invasiveness traits and the invasibility of the recipient native plant communities. Elton's diversity-invasibility hypothesis proposes that native plant communities with higher species diversity should be less susceptible to invasion by exotic plant species than those with lower species diversity. However, the exact ecological mechanisms driving this negative diversity-invasibility relationship and their relative importance within a single invaded ecosystem remain unclear. In this study, we tested whether soil nutrient availability, allelopathy, soil biota, and soil legacy effects can all influence the diversity-invasibility relationship within the same ecological system comprising invasive and co-occurring native plant species. We conducted four multi-species greenhouse experiments using 14 invasive and 20 native plant species that co-occur in grasslands in China. The results show that nutrient addition, allelopathy, soil microbiota, and soil legacy effects all independently impacted the biomass of the invasive plant species and native plant communities. However, when considering the role of native plant community diversity in resisting invasion, only allelopathy and soil microbiota emerged as potentially important factors. In the context of our study system, our findings suggest that allelopathic interactions and soil microbial communities play a more significant role than nutrient availability or soil legacy effects in shaping the invasion resistance of diverse native plant communities. These findings highlight the complexity of interactions between native and invasive species and emphasize the importance of considering multiple mechanisms when investigating the factors that contribute to the invasion resistance of diverse native plant communities.