While most alien species fail to establish, some invade native communities and become widespread. Our understanding of invasion success is derived mainly from pairwise interactions between aliens and natives, while interactions among more than two species remain largely unexplored. Here, we experimentally tested whether and how a third plant species, either native or alien, affected the competitive outcomes between alien and native plants through its soil legacy. We first conditioned soil with one of ten species (six natives and four aliens) or without plants. We then grew on these 11 soils five aliens and five natives without competition, or with intra- or interspecific competition. We found that aliens were not more competitive than natives when grown on soil conditioned by other natives or on non-conditioned soil. However, aliens were more competitive than natives on soil conditioned by other aliens (that is, invasional meltdown). Soil conditioning did not change competitive outcomes by affecting the strength of competition between later plants. Instead, soil conditioned by aliens pushed competitive outcomes towards later aliens by affecting the growth of aliens less negatively than that of natives. Microbiome analysis verified this finding, as we showed that the soil-legacy effects of a species on later species were less negative when their fungal endophyte communities were less similar, and that fungal endophyte communities were less similar between two aliens than between aliens and natives. Our study reveals invasional meltdown in multispecies communities and identifies soil microorganisms as a driver of the invasion success of alien plants.

虽然大多数外来物种未能建立，但有些入侵本土社区并变得广泛。我们对入侵成功的理解主要源于外星人与当地人之间的成对相互作用，而两个以上物种之间的相互作用在很大程度上尚待探索。在这里，我们通过实验测试了第三种植物物种（本地或外来物种）是否以及如何通过其土壤遗产影响了外来和本地植物之间的竞争结果。我们首先用十个物种之一（六个本地人和四个外星人）或没有植物来调节土壤。然后，我们在这11种土壤上生长了五个外星人和五个本地人，没有竞争，也没有种内或种间竞争。我们发现，在由其他本地人适应的土壤或未经条件处理的土壤上生长时，外星人没有比本地人更具竞争力。然而，在其他外星人适应的土壤上（即入侵崩溃），外星人比本地人更具竞争力。土壤调理不会通过影响后期植物之间的竞争强度来改变竞争结果。相反，受外星人制约的土壤通过对外星人的生长的负面影响小于对本地人的增长，将竞争结果推向了后来的外星人。微生物组分析证实了这一发现，因为我们表明，当一个物种的真菌内生菌群落不太相似时，该物种对后代物种的土壤遗产影响较小，并且两个外星人之间的真菌内生菌群落比外星人和本地人之间的相似性更低。我们的研究揭示了多物种群落中的入侵融化，并确定了土壤微生物是外来植物入侵成功的驱动力。