Soil microbiota and native neighbor identity affect invasive plant competitiveness

Invasive plants often compete with diverse native species, thereby altering native ecosystem structure and function. Soil microbiota can further influence invasion outcomes through their effects on plant performance, yet it remains unclear how soil microbiota and native neighbor identity jointly shape these competitive dynamics. Here, we used natural and sterilized soil to separate biotic from abiotic effects and examined competition between the invasive Alternanthera philoxeroides (Amaranthaceae) and two native species, Bidens bipinnata (Asteraceae) and Setaria viridis (Poaceae), in a common garden experiment. We assessed plant performance and their interaction relationship under different soil treatment. Soil microbiota removal reduced the invader’s total biomass when competing with S. viridis, and its biomass was lower with S. viridis than when grown alone or with B. bipinnata in sterilized soil. Competition with either native species increased the invader’s root-shoot ratio regardless of soil treatment, while sterilization enhanced its specific leaf area under competition. Moreover, both soil microbiota and neighbor identity influenced interaction relationship and competition intensity of the invader. Natural soil weakened the competition interaction and reduced the competition intensity between invasive plant and S. viridis, but not with B. bipinnata. For the native species, S. viridis showed increased shoot biomass, total biomass, and root-shoot ratio when competing with invader, while B. bipinnata showed no significant response. Overall, our findings demonstrate that soil microbiota and neighbor identity jointly regulate the competitive ability of invasive plants, underscoring their importance in shaping invasion outcomes.