Many invasive plants are allelopathic, and it has been hypothesized that allelopathy may generally offer an advantage to plants in novel ecosystems due to lack of prior selection of other competing species to specific allelochemicals (i.e., a “novel chemistry” advantage). Control measures for allelopathic invasive species are often limited, since many allelochemicals are persistent in the soil, and removal of aboveground structures can stimulate allelochemical production. Activated carbon has commonly been in assays for allelopathic effects, and is recognized for its ability to sorb allelochemicals. Biochars – charcoals used as soil amendments – differ from activated carbon in terms of feedstock (biochars are derived from organic waste materials whereas most activated carbon is made from anthracitic coals), and also physical properties (e.g., biochars generally have lower surface area / mass ratios).
Using seed germination bioassays, we assessed the capacity of a variety of biochars to sorb allelochemicals produced by a range of allelopathic invasive plants, including two tropical invasives (strawberry guava: Psidium cattleianum; Lemongrass: Cymbopogon citratus), and three temperate-boreal invasives (yellow sweet clover: Melilotus officinalis; garlic mustard: Alliaria petiolata; tree of heaven: Ailanthus altissima). Fresh leaf extracts from each species were treated with biochars in a water slurry, and the resulting filtrate tested for effects on seed germination and early seedling development of agricultural and native species.
In all cases biochar treatments reduced the allelochemical effects at some combination of extract concentrations and biochar dosages. In some cases dramatic “rescue effects” were observed, where biochars completely alleviated allelopathic effects on seed germination and early seedling development. Aqueous extracts of biochars were included as an alternative control (in addition to distilled water); these extracts commonly had small positive effects on seed germination and early seedling development, possibly as consequence of high soluble potassium levels in biochars. The chemistry of the known or presumed allelochemicals produced by the surveyed species is diverse, suggesting that biochars may very generally be of use as a soil treatment to alleviate invasive allelopathic plant impacts on plant communities. Initial results from field trials conducted at acute sites of plant invasions (on the island of Mauritius and at a mine site in the Ontario Far North region) also support this conclusion.