A study of soil feedback and allelopathy in four invasive plant species

Allelopathy occurs when a plant species releases allelochemicals into the soil that impact the germination, survival, or growth of that species or neighboring species. Soil microbes can increase or decrease the effect of these chemicals, but few studies have determined whether soil microbial effects are predictable across different plant species. Plant-soil feedback occurs when a plant species changes the soil microbial community in ways that affect the growth or reproduction of that species or neighboring species. Allelopathy and plant-soil feedback are two mechanisms that might explain how invasive plants impact native plant communities and come to dominate local habitats. Both of these mechanisms have the potential for legacy effects on the soil and plant communities after the invasive has been removed. We tested whether three local invasive shrub species of the eastern deciduous forest (Lonicera maackii, Ligustrum vulgare, and Elaeagnus umbellata) have allelopathic effects on each other or any of nine native plant species. Our results were complex and involved many interactions. Overall, we found that the allelopathic impacts of the invasive species changed depending on the soil microbial community, the seedling species being tested, and the growth stage of the seedling. We also tested whether the same three invasive shrub species, and the invasive woodland grass, Microstegium vimineum, participated in plant-soil feedback with native species. We found evidence that all four invasive species affect aspects of the soil microbial community and that these soil microbial effects can change with time and competitive context. We also found evidence for legacy effects on some components of the microbial community (e.g. mycorrhizal fungi). We did not find strong evidence that these microbial effects resulted in feedback dynamics that promote the dominance of these invasive species.;Synthesis: The effects of invasive plants can interact with the abiotic and biotic environment in complex ways. Future studies should incorporate the effects of these biotic and abiotic factors in their experimental design. We also found that these interactions can change over time and in different biotic and abiotic contexts, suggesting the value of long-term studies of these interactions and the need to unravel their context dependency in order to assess their ultimate role in driving plant community structure and dynamics.