Current plant diversity but not its soil legacy influences exotic plant invasion

Current plant diversity can influence exotic plant invasion, but it is unclear whether there is a legacy effect of plant diversity on exotic plant invasion. As plant diversity can affect soil microbial communities and physio-chemical properties, which may cascade to impact subsequent exotic plant growth, we hypothesize that the soil legacy effect of plant diversity can influence exotic plant invasion. We conducted a plant-soil feedback experiment. In the conditioning phase, we trained soils by monocultures of 12 plant species from three functional groups (4 grasses, 3 legumes and 5 forbs) and mixtures of 8 randomly selected species with all three functional groups from this 12-species pool. In the test phase, we grew the invasive plant Bidens pilosa with a co-occurring native grass (Arthraxon hispidus), with a co-occurring native forb (Pterocypsela indica) or with both in each type of the conditioned soils. The performance of B. pilosa relative to its native competitors varied depending on the functional type of both conditioning plant species in the conditioning phase and competing plant species in the test phase. Diversity of the conditioning plants did not influence the growth difference between B. pilosa and its native competitors. However, increasing diversity of the competing plant species reduced the performance of B. pilosa relative to its native competitors. Our results suggest that current plant diversity can reduce exotic plant invasion through increasing growth inequality between invasive and native plants, but the soil legacy effect of plant diversity may have little impact on exotic plant invasion.