Abstract: Elton’s biotic resistance hypothesis predicts that species-rich native communities are more resistant to invasion. However, explanations for the diversity-invasibility relationship have focused mainly on resource competition, whereas the role of allelopathy in this relationship and whether its effects are mediated by soil microbes remain unclear. We conducted a greenhouse experiment using Phytolacca americana as a model invader. Litter leachates from 15 native plant species were assembled into five richness levels (1, 3, 6, 9, and 15 species) and applied to sterilized and unsterilized soils to test how diversity-driven allelopathic inputs and soil microbial context jointly affect invader germination, growth, and biomass allocation. Seed germination was insensitive to litter richness and was influenced mainly by soil microbial context, with higher germination in unsterilized soils. By contrast, in sterilized soils, increasing litter richness reduced post-germination above-ground biomass but increased below-ground biomass and the root:shoot ratio (R/S). These responses were markedly weakened in unsterilized soils. Composition-specific analyses showed that observed values of total phenolic concentration (TPC) exceeded their corresponding additive expectations, providing complementary evidence of mixture-level non- additivity. Overall, native plant diversity can influence community invasibility through allelopathic inputs, but the realized expression of this effect depends strongly on soil microbial context and is manifested mainly in invader post-germination growth and biomass allocation. These findings indicate that allelopathy and its mediation by soil microbes are important components of the diversity-invasibility relationship and suggest that assessing invasion resistance in forest communities requires consideration of above-ground diversity, litter inputs, and below-ground microbial processes.