Negative conspecific plant-soil feedback on alien plants co-growing with natives is partly mitigated by another alien

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Abstract Background and aims Naturalized alien and native plants can impact each other directly when they grow next to each other, but also indirectly through their soil legacies. These alien-native interactions can also be modified by the presence of a third alien or native species. However, it is unknown how the performance of co-growing alien and native species is affected by their soil legacies and by the presence of an additional species. Methods In our two-phase plant-soil-feedback experiment, soils were first conditioned by eight herbaceous species, four of which are naturalized and four of which are native to Germany. We then grew all 16 pairwise alien-native species combinations on soil conditioned by the respective alien or native species, on a mixture of soils conditioned by both species or on control soil. Each pair of test plants was grown on these soils without or with an additional alien or native species. Results Soil conditioning, and particularly conspecific soil conditioning, reduced growth of the alien and native test plants. The addition of another species also reduced growth of the test plants. However, the negative conspecific soil-legacy effect on alien test plants was reduced when the additional species was also alien. Conclusion The negative conspecific plant-soil feedback for alien and native plants in our study could promote their coexistence. However, as partial alleviation of negative conspecific effects on alien plants occurred with an additional alien species, the chances of coexistence of alien and native species might decrease when there are multiple alien species present.