Increasing exotic dominance generates soil microbial legacies that favour exotic over native plants

Ecosystems worldwide are undergoing a pervasive transition toward exotic‐dominated states, a shift that threatens biodiversity and disrupts key ecosystem functions. Although positive abiotic feedbacks are frequently cited as drivers of these transitions, the microbial mechanisms involved remain poorly understood. In a two‐phase greenhouse experiment, we conditioned soils along a gradient of increasing exotic plant dominance using 10 exotic and 10 native species. We then assessed plant performance on these soil microbial inocula under no, intraspecific and interspecific competition. We found that increasing exotic dominance progressively reshaped soil microbial communities in ways that favoured exotic over native growth, without altering competitive strengths. This growth asymmetry was tightly linked to arbuscular mycorrhizal fungi (AMF), whose richness increased with exotic dominance and was positively correlated with exotic performance. Complementary field surveys of the same exotic species confirmed a positive correlation between their abundance and AMF richness in natural communities. Synthesis . Our findings demonstrate that increasing dominance of exotic plants can generate microbial legacies that progressively favour exotic plants over native plants. These microbial legacies provide a mechanism linking changes in plant community structure to subsequent competitive interactions and offer a framework for understanding how biotic interactions, particularly those involving soil mutualists, can contribute to the persistence and spread of invasive plants.