Successful invasion by exotic plant species can modify the abundance and composition of soil microbial communities. Eupatorium adenophora and Chromolaena odorata are exotics that have become highly invasive plants in China. Several studies have investigated mechanisms of their successful invasions including phenotypic plasticity, genetic differentiation, and allelopathy, but little is known about their effects on soil microorganisms. Moreover, whether soil microbial community changes could cause feedback effects on these plant species is also not known. We seek a belowground microbiological mechanism supporting successful invasions by these exotic plants.

In this study, two invasive (E. adenophora and C. odorata) and two native plant species (Eupatorium japonicum and Eupatorium heterophyllum) were used to compare the soil feedback (on plant growth) before and after soil sterilization and from plant-root exudates. Bacterial and fungal biomass and community composition were also examined.

We found that soil sterilization significantly increased biomass of native species and did not affect the invasive species' biomass. After root exudates from these plants had acted on the soil biota for 10 months, soil sterilization significantly decreased the growth of E. adenophora and C. odorata and continued to significantly increase the biomass of two native species. Denaturing gradient gel electrophoresis revealed that these four plant species modified fungal rather than bacterial communities in soil.

Higher abundance of Paraglomus sp. in soil with C. odorata is likely to provide C. odorata roots with more soil nutrients. Considered together, these results strongly suggest that invasive E. adenophora and C. odorata created a belowground feedback that may be a mechanism contributing to their success as invasive species.