Understanding the relationship between plant diversity and invasibility is essential in invasion ecology. Elton proposed that species-rich communities are more resistant to invasions than species-poor communities. 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 one 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 non-fertilization). Under intraspecific competition, regardless of fertilization, the biomass of the alien species was higher in soil conditioned by six native species. In contrast, under interspecific competition, the biomass increased without fertilization but remained stable with fertilization in soil conditioned by six native species. Analysis of the soil microbial community suggests that the greater diversity of arbuscular mycorrhizal fungi (AMF) in the soil conditioned by six native species might reflect the primary influence on R. typhina growth, but the interaction between AMF and R. typhina varies depending on competition mode and nutrient status. Our findings suggest that the soil microbiome is pivotal in mediating the diversity–invasibility relationship, and this influence varies with competition mode and nutrient status.