The soil microbial community and nitrogen availability affect the growth, biochemistry and potential allelopathic effects of the invasive plant Solidago canadensis

Plant investment in secondary metabolites can be driven by abiotic factors such as nitrogen (N) availability and variation in biotic factors such as root-associated microbes. However, few studies have tested their combined effect on allelopathy. Here, we test whether and how N addition (i.e. eutrophication) and soil microbes modify allelopathic effects of the invasive plant Solidago canadensis on germination of the native plant Crepidiastrum sonchifolium.

We first grew Solidago at three N levels with a live or sterilized soil inoculum. Then we exposed seeds of Crepidiastrum to aqueous extracts made of the Solidago plants. We analysed the biomass, soil microbiome (bacteria and fungi), and flavonoid, phenolic and saponin contents of Solidago, and the effects of the aqueous extracts on germination of Crepidiastrum.

We found that Solidago produced 67% more biomass on live soil than on sterilized soil, and that N addition only resulted in more biomass on live soil. Soils that had been sterilized accumulated higher relative abundances of bacteria involved in N transformation, and tended to have higher relative abundances of pathotrophic fungi. When grown in soil that had been sterilized, the total flavonoid content of Solidago was 22% higher, and the aqueous extracts had stronger negative allelopathic effects on germination of Crepidiastrum.

The presence of natural soil microbial communities may enhance invasiveness of Solidago by promoting its growth and thereby competitive ability, but may simultaneously decrease the negative allelopathic impact on native neighbors.