1. Invasive plant species are key drivers of global environmental changes leading to the disruption of ecosystems they invade. Many invasive species engage in novel niche construction through plant-soil feedbacks facilitated by the input of secondary compounds, which help their further spread and survival. These compounds can persist in soil even after the removal of the invader thus creating a legacy effect that inhibits the return of native flora and fauna. Thus, formulating active intervention strategies that can reverse niche construction is critical for the restoration of these invaded ecosystems.

2. We hypothesized that the management practices that can reverse the soil carbon and nutrient cycling in invaded ecosystems can facilitate the rapid restoration of the invaded sites. We predicted that adding soil C amendments such as activated carbon and biochar can alter the microbial functional activity and nutrient cycling leading to the restoration of invaded habitats. We tested this hypothesis in an old-field in Massachusetts that has been invaded by Japanese knotweed (Polygonum cuspidatum) for >20 years.

3. After two years of treatment application, the activated carbon and biochar amended plots had 80% more biomass of the prairie species than the control plots. The C amendments also altered soil nutrient cycling and fungal biomass and enzyme activity compared to the control plots. The nitrate content of C amended plots was 5 times higher than the non-amended control plots indicating an increased nitrogen mineralization in C amended plots potentially due to the sorption of phenolic compounds by activated carbon and biochar that makes them unavailable. This was further supported by the increased phenol oxidase activity which might have been less inhibited by tannins and led to increased organic matter decomposition.

4. Synthesis and conclusions: Our results thus reveal the potential of soil C amendments in reversing niche construction and legacy effects of polyphenol-rich invasive species and indicate that biochar could be a more economically feasible alternative to activated carbon in restoring invaded ecosystems. Our results also emphasize that understanding the mechanism through which invasive species engage in niche construction is vital in formulating suitable knowledge-based restoration practices for invaded ecosystems.