Does plant invasion result in convergence of soil chemistries across ecosystems: A case study with Japanese knotweed invasion in eastern United States

Background/Question/Methods

After introduction, the invasion potential of a plant species is primarily governed by the nature of its interaction with the surrounding biotic and abiotic environments. These feedbacks are often dictated by the chemistry and composition of metabolites produced by the invading species. The ensuing changes in ecosystem process due to these interactions are well documented for many species. However, the mechanisms that drive these altered ecosystem processes are less understood, and this can hamper restoration of invaded sites even after the removal of the invasive species. Polyphenols are the most abundant class of plant secondary metabolites, and comprise a diverse array of chemically inert lignins and biologically reactive phenolic and flavonoid compounds. Apart from the direct toxicity, polyphenols play an important role in many ecosystem processes including soil nutrient cycling and organic matter decomposition. Japanese knotweed (Polygonum cuspidatum) is a noxious weed that invades diverse ecosystems across Europe and North America. We investigated the soil polyphenol profiles and the associated changes in soil processes in knotweed invaded and adjacent non-invaded sites in various forested and old-field ecosystems along eastern United States. 

Physicochemical composition and phenolic chemistry of plant litter and topsoil was assessed using Fourier Transform Infrared Spectrometry (FTIR) and liquid chromatography coupled with tandem mass spectrometry analysis (HPLC-DAD/QToF). We measured the activity of various carbon and nitrogen mineralizing enzymes in soil and decomposing litter to assess the alterations in microbial functional activity induced by knotweed litter quality. 

Results/Conclusions

Irrespective of the initial species composition, based on the quantitative analysis of   different phenolic compounds, there was a convergence of phenolic chemistries in soils invaded by knotweed, resulting in unique phenolic signatures across ecosystems. More than 60% of the proanthocyanidins was fiber bound, resulting in a reduced rate of litter decomposition, and an increase in fungal-to-bacterial ratio. The high polyphenol content resulted in seasonal variation in soil and litter enzyme activities. The regional convergence of soil polyphenol profiles and the associated soil processes identifies the plant secondary chemistry as the driver of ecosystem processes during knotweed invasion.