Invasion of Solidago gigantea in contrasting experimental plant communities: effects on soil microbes, nutrients and plant-soil feedbacks

1. Plant-soil feedbacks can influence the success of non-native plant invasions. We investigated if these feedbacks and the underlying invasion effects on soil microbes and nutrients depend on the species composition of the invaded vegetation, and whether these effects are related to differences in the invasibility of native plant communities. 2. We carried out a mesocosm experiment simulating the invasion of Solidago gigantea into three wetland plant communities (Molinion, Magnocaricion and Filipendulion), each composed of five plant species but differing in productivity. To study plant-soil feedbacks, we used different soil inoculum types from invaded and non-invaded field sites of the corresponding communities and a reference site. Invasion success was assessed by measuring the biomass of S. gigantea after three growing seasons and by analysing soil properties several times during the experiment. 3. Invasion success varied significantly among communities and soil inoculum types. Solidago gigantea produced more biomass in the Molinion than in the two more productive communities. In all three communities, it exhibited a negative feedback upon itself, producing 31-46% less biomass when the substrate was inoculated with soil from a stand invaded with S. gigantea. 4. The presence of S. gigantea did not influence total biomass in any community nor Nand P availability in soil. However, it led to a decrease in soil bacterial and an increase in soil fungal biomasses. These changes were similar in the three communities and unrelated to the biomass of S. gigantea biomass in the invaded communities. 5. Synthesis. The experimental comparison between effects of an invasive plant species on soil properties in different native communities showed similar effects despite pronounced differences in the ability of the invasive species to grow in the different communities. In this system, plant-soil interactions may thus affect invasion, but not explain differences in the invasibility of different communities. The invasive species increased soil fungal biomass, particularly in its own soil, compared to native species and experienced a negative feedback, suggesting that the course of its invasion might be affected by species-specific soil pathogens.