Reduction in litter allelochemicals promotes positive biotic interactions among alien species

A key focus of invasion ecology is elucidating why multiple alien species accumulate within ecosystems. The invasional meltdown hypothesis proposes that one alien invader promotes further invasions through soil legacy effects. However, whether microbial or litter legacies drive these effects, both of which are shaped by environmental heterogeneity (e.g., alien density or nutrient gradients), remains largely unexplored. We conducted a multifactorial experiment using an invasive plant species ( Phytolacca americana ) to assess how species origin affects alien plant invasion, incorporating two interacting species origins (native vs. alien), two litter treatments (with vs. without), two soil microbial conditions (inoculated vs. sterile), three soil phosphorus levels, and three planting density levels. Alien litter had lower total phenolic levels and higher C:N ratios than native litter. Phytolacca americana grew significantly better under alien litter. Path analysis revealed that this growth enhancement was driven primarily by reduced allelochemicals rather than nutrient differences, with plant density modulating this effect. Although soil microbial communities from different origins differed in diversity and functional structure, their inoculation produced no significant differential effects on P. americana growth. These results reveal a litter‐mediated mechanism that could potentially facilitate invasional meltdown by promoting positive interactions among alien plants. Our findings provide new insights into invasion mechanisms and highlight the ecological interplay between alien species. Read the free Plain Language Summary for this article on the Journal blog.