The effects of plant-soil feedback on invasion resistance are soil context dependent

No evidence that modification of soil microbiota by woody invader facilitates subsequent invasion by herbaceous species

Many terrestrial ecosystems are co-invaded by multiple exotic species. The "invasional meltdown" hypothesis predicts that an initial invasive species will facilitate secondary invasions. In the plant kingdom, the potential underlying mechanisms of this hypothesis may be that modification of the soil properties by the initial invaders benefits for the subsequent exotic species invasion. In this study, we analyzed the composition of soil microbial communities and soil chemical properties from sites invaded by woody Rhus typhina, as well as uninvaded sites, to assess the impact of R. typhina invasion. Furthermore, we conducted a greenhouse experiment with multiple native-invasive pairs of herbaceous species to test whether R. typhina invasion facilitates subsequent exotic herb invasion. Our results showed that R. typhina invasion significantly altered the composition of soil fungal communities, especially pathogenic, endophytic, and arbuscular mycorrhizal fungi. However, this change in microbial composition led to neither direction nor magnitude changes in negative plant-soil feedback effects on both native and invasive species. This indicates that initial R. typhina invasion does not facilitate subsequent herb invasion, which does not support the "invasional meltdown" hypothesis. Additionally, R. typhina invasion significantly decreased soil total nitrogen and organic carbon contents, which may explain the significantly lower biomass of herbaceous roots grown in invaded soils compared with uninvaded soils. Alternately, although invasive herb growth was significantly more inhibited by soil microbiota compared with native herb growth, such inhibition cannot completely eliminate the risk of exotic herb invasion because of their innate growth advantages. Therefore, microbial biocontrol agents for plant invasion management should be combined with another approach to suppress the innate growth advantages of exotic species.

A global meta-analysis of the impacts of exotic plant species invasion on plant diversity and soil properties

Plant diversity and biogeochemical cycles are rapidly changing in response to exotic plant species invasion. However, there are conflicting conclusions regarding the quantification of such changes in the soil properties and plant diversity. Moreover, the relationships between soil properties and plant diversity are unclear. Here, a global meta-analysis was conducted on the impact of exotic species invasion on soil physicochemistry, microbial activity, and plant diversity using data from 123 published reports and 332 samples. Exotic species invasion significantly enhanced the soil pH, soil microbial activity, and soil nutrient content. The impact was more substantial for grass than for shrub and tree. Exotic species invasion did not significantly affect soil texture, but significantly reduced the plant diversity, richness, and evenness by 36.97%, 64.72%, and 47.21%, respectively. Soil pH, soil organic carbon, and total nitrogen were significantly correlated with plant diversity reduction. The response ratio of plant richness and evenness gradually increased with precipitation. However, the response ratio of phosphatase, microbial biomass nitrogen, microbial biomass phosphorus, total nitrogen, and soil moisture gradually decreased with precipitation. Overall, exotic species invasion significantly increased the soil nutrient content and soil microbial activity, but significantly decreased plant diversity. These effects were influenced by exotic species types and precipitation.