Polyphenols represent a diverse and the most abundant class of plant secondary compounds, and consists of chemically inert lignins and biologically reactive phenolic and flavonoid compounds. Traditionally, polyphenolic compounds have been implicated in many toxic plant-to-plant interactions that result in plant invasions.  Apart from the direct toxicity, polyphenols also play an important role in many ecosystem processes including soil nutrient cycling and organic matter decomposition. However, the effect of these compounds on such ecosystem processes in the invaded habitats is less understood. Japanese knotweed (Polygonum cuspidatum) is a secondary metabolite-rich 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 five knotweed invaded and adjacent non-invaded sites across eastern United States.

Physicochemical composition of plant litter and topsoil was assessed using Fourier Transform Infrared Spectrometry (FTIR). We used liquid chromatography coupled with tandem mass spectrometry analysis (HPLC-QToF) to characterize polyphenolic compounds in knotweed tissues and topsoils. We followed the degradation pattern of these compounds in the knotweed litter using field decomposition experiment and lab incubation assays.  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.

FTIR analysis demonstrated a higher concentration of recalcitrant compounds (cuticle and polymeric-phenols) in knotweed litter and in invaded soils. Knotweed litter had a higher concentration of both flavonoid aglycons and glycosides, which accounted for 1% of leaf litter by dry-weight. However, all flavonoid compounds degraded rapidly both under field and lab incubation studies. Invaded soils had a high concentration of ester-linked phenolic acids, however, none of the flavonoids were present in invaded soils in detectable quantities. Knotweed tannins were primarily composed of proanthocyanidins (PC), and accounted for 10-15% of the leaf dry weight. More than 50% of the PC was fiber bound, resulting in a reduced rate of litter decomposition, and an increase in fungal-to-bacterial ratios in invaded sites. The high polyphenol content resulted in a seasonal variation of microbial enzyme activities in the soil and in the litter, between the invaded and the adjacent non-invaded sites. The influence of knotweed polyphenols on soil nutrient cycling was also seasonal. Our overall results suggest that, the presence of polyphenol compounds provides a less conducive microclimate for the microbial decomposer community, thus resulting in the accumulation of knotweed detritus, which in turn could smother the emergence of native species.