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Understanding the relationship between plant diversity and invasibility is essential in invasion ecology. Species‐rich communities are hypothesized to be more resistant to invasions than species‐poor communities. However, while soil microorganisms play a crucial role in regulating this diversity–invasibility relationship, the effects of plant competition mode and soil nutrient status on their role remain unclear. To address this, we conducted a two‐stage greenhouse experiment. Soils were first conditioned by growing nine native species separately in them for 1 year, then mixed in various configurations with soils conditioned using one, three, or six species, respectively. Next, we inoculated the mixed soil into sterilized substrate soil and planted the alien species Rhus typhina and native species Ailanthus altissima as test plants. We set up two competition modes (intraspecific and interspecific) and two nutrient levels (fertilization using slow‐release fertilizer and nonfertilization). Under intraspecific competition, regardless of fertilization, the biomass of the alien species was higher in soil conditioned by six native species. By contrast, under interspecific competition, the biomass increased without fertilization but remained stable with fertilization in soil conditioned by six native species. Analysis of soil microbes suggests that pathogens and symbiotic fungi in diverse plant communities influenced R. typhina growth, which varied with competition mode and nutrient status. Our findings suggest that the soil microbiome is pivotal in mediating the diversity–invasibility relationship, and this influence varies according to competition mode and nutrient status. [ABSTRACT FROM AUTHOR]