Effect of soil carbon amendments in reversing the legacy effect of plant invasion.
Invasive plants are key drivers of global environmental changes leading to the disruption of ecosystems. Many invasive species engage in novel niche construction through plant-soil feedbacks that are driven by plant secondary compounds. These compounds can persist in the soil even after removing the invader, thus creating a legacy effect that inhibits the return of native flora and fauna. The formulation of active intervention strategies that can reverse niche construction is therefore critical for the restoration of these invaded ecosystems. We conducted this study in an old-field in Massachusetts, USA, that has been invaded by Japanese knotweed (Polygonum cuspidatum) for >20 years. We chose knotweed as a model system as it alters soil chemistry and microbial community through the input of polyphenols such as tannins that creates a legacy effect. Following the removal of knotweed biomass, we investigated the effect of two soil carbon (C) amendments (biochar and activated carbon) on the growth and establishment of newly seeded native and prairie species. We measured the percent plant cover and above-ground biomass to assess the establishment of the native and prairie species. We also measured soil and microbial characteristics including nutrient availability, extracellular enzyme activities and fungal biomass to elucidate the effect of C amendments in reversing the legacy effect. Eventhough the native species did not respond positively to C amendments, the biomass of the prairie species was 80% higher in activated carbon and biochar amended plots than the non-amended control plots. The nitrate content of the C amended plots was five times higher than the non-amended plots indicating an increased N mineralisation in the C amended plots. This could be potentially due to the amelioration of phenolic compounds by activated carbon and biocharthrough sorption. The phenol peroxidase activity also increased in the activated carbon and biochar amended plots potentially due to the less inhibition by phenolic compounds. With the decrease in polyphenols, the fungal biomass decreased in C amended plots that may have resulted in faster nutrient cycling and increased availability of soil N. Synthesis and applications. The phenolic compounds from the litter of invasive species that persist in soil C fractions can negatively affect the germination and growth of the native or non-invasive plant species. The polyphenols such as tannins from the litter can complex nitrogenous compounds in soils making the N unavailable to the native or non-invasive species. Our results revealed the potential of soil C amendments in reversing niche construction and legacy effects of polyphenol-rich invasive species and indicated that biochar could be a more economically feasible alternative to activated carbon in restoring invaded ecosystems. These results also emphasise thatunderstanding the mechanisms through which invasive species create a legacy effect is pivotal in formulating suitable knowledge-based practices for restoring invaded ecosystems.