Nutrient availability and invader density regulate the diversity–invasibility relationship mediated by soil microbes

Elton's biotic resistance hypothesis, which proposes that diverse plant communities are more resistant to biological invasions, has received considerable experimental support. However, the roles of soil microbes in the diversity–invasibility relationship and how they are influenced by nutrient availability and invader density remain poorly understood in temperate forest ecosystems. We conditioned soils for nearly 1 year using nine individually grown native woody species. We then mixed the soils to create a synthetic diversity gradient of one, three or six species. The invasive plant Phytolacca americana was then grown in pots inoculated with these differently conditioned soils. Each soil treatment was crossed with two nutrient availabilities (fertilised vs. non‐fertilised) and three invader densities (one, two and four plants) to test invasion success. We found that soil treatments did not significantly affect P. americana biomass under low nutrient availability, whereas biomass increased with increasing soil inoculum diversity under high nutrient availability. Soil treatments had no significant effect at invader densities of two and four, whereas P. americana biomass increased with increasing diversity levels at a density of one. Furthermore, among all native species, Quercus acutissima reduced P. americana biomass at an invader density of four, although this suppressive effect disappeared with nutrient addition. Analysis of the soil microbial community suggests that these patterns may reflect diversity‐driven pathogen dilution and a reduction in the relative abundance of saprotrophic fungi associated with Q. acutissima . Overall, our results indicate that the diversity–invasibility relationship is modulated by soil microbes, and this modulation depends on nutrient availability and invader density. By integrating the interactive effects of plant diversity, nutrient conditions and invasion intensity, this study provides a potential microbial pathway to explain context‐dependent shifts in biotic resistance. Furthermore, our findings also highlight the important role that some specific native species can play in enhancing community resistance to invasion in particular ecological contexts. Read the free Plain Language Summary for this article on the Journal blog.