The invasive plant Polygonum cuspidatum reduces diversity of soil microbes via litter chemistry

Background/Question/Methods

One of the most important community-level effects of introduced plants is to alter the communities of microbes in the soil.  Discovering the mechanisms for this effect could help show why some introduced plants are more invasive than others and suggest how to restore invaded communities.  We hypothesized that the introduced, invasive plant Polygonum cuspidatum (Japanese knotweed) alters communities of soil fungi by producing large quantities of litter with high concentrations of persistent secondary compounds that change soil chemistry and select for fungi that tolerate or metabolize these compounds.  We tested this hypothesis by comparing soil chemistry and fungi under and adjacent to a stand of P. cuspidatum at each of four sites in eastern North America.

Results/Conclusions

Although flavonoids and tannins comprised 15% of the dry mass of the litter of P. cuspidatum, they were undetectable in soil extracts.  In contrast, monophenolics appeared highly persistent; they made up < 5% of the total phenolics in litter but > 50% of the phenolics identified in soils and were 50% higher under than adjacent to stands.  Moreover, profiles of extractable monophenolics and composition of soil organic matter were more similar among soils under stands at different sites than between soils under and adjacent to a stand within a site.  Soils under stands showed three times higher abundance of fungi than adjacent soils, but lower diversity of fungi, and higher activity of enzymes that can break down phenolics.  These results are consistent with the hypothesis and suggest that P. cuspidatum decreases soil fungal diversity within and between sites by favoring fungi that can metabolize monophenolics that are released from litter and persist in the soil.