Invasion by Fallopia japonica alters soil food webs through secondary metabolites

Biological invasions are a major threat to biodiversity with varying degrees of impact. There is increasing evidence that allelopathy often plays an important role in explaining both invasion success and impact on native taxa (e.g. novel weapons hypothesis). The effects of these secondary metabolites on plant communities and microorganisms are well known. However, their direct and indirect effects on soil fauna are unresolved, despite the importance of the latter in ecosystem processes and, potentially, invasion mitigation. Japanese knotweed (Fallopia japonica), an east-Asian species, which has proved to be invasive in Europe, containing allelopathic secondary compounds inhibiting native plants and microbial communities. The focal point of this study was the allelopathic effects of knotweed on soil mesofauna (Nematoda, Collembola and Acari). During a one-month laboratory experiment we added knotweed rhizome extract (KRE) at different concentrations to soils collected in an invasion-prone area. He experiment consisted of including or excluding secondary metabolites through the use of activated carbon filtration of KRE. This enabled us to separate effects caused by nutrient addition (i.e. trophic effects) and combined (trophic and allelopathic) effects. Relative effects of nutrient and secondary metabolites addition on abiotic and biotic soil variables were then quantified. We highlighted frequently contrasting trophic and allelopathic effects influenced in some cases by KRE concentration. Microbial assemblages, through fungal/microbial biomass ratio, did not show any congruent response to KRE secondary compounds but was more negatively impacted by nutrient addition. The use of a trophic-based path analysis led us to show that changes within the soil biota had repercussions on secondary consumers (e.g. bacterivorous nematodes and Collembola). Abundance within taxa at higher trophic levels such as predatory Acari (but not predatory nematodes) was also affected although to a lesser extent, likely in part due to the limited considered timeframe. Overall, we showed that, in controlled conditions, invasive allelopathic plants such as knotweeds can have effects on soil fauna at different trophic levels through addition of both nutrients and secondary metabolites to the soil. Considering the limited knowledge of allelopathic effects on the soil fauna and soil functions, this study provides new information on above- and belowground interactions.