Knapweed species are considered to be among the most devastating invasive plants in the western regions of the United States, both environmentally and economically. Frequent disturbances to the native vegetation from training activities make military lands especially susceptible to such invasions and, as a result, they may become unable to sustain their use as training ranges into the future. The ability of knapweed species to produce and release allelopathic chemicals, which are potentially toxic to native species, is a supposed mechanism that contributes to their successful spread. By determining the factors regulating the phytotoxic activity of these allelochemicals in soil, it will be possible to implement measures for limiting those factors and thus control the spread of knapweed species.

The objective of this SERDP Exploratory Development (SEED) project was to determine the constituents (soil organic matter, clays, oxides, etc.) and conditions (pH and organic matter content) that provide maximum sorption of the allelochemicals (-)-catechin and 7,8-benzoflavone in soil and to use bioassays to determine if the optimum soil sorption conditions can diminish the phytotoxicity of these allelochemicals to native species.

Hydroponic screening system after seedling transplant.

Sorption batch experiments were conducted under environmentally controlled conditions. Experiments were carried out in flat-bottomed, water-jacketed (400 milliliter) glass vessels. Both sorption isotherm (amount sorbed versus equilibrium concentration of sorbate) and sorption edge (amount sorbed versus pH) experiments were conducted. The sorption experiments consisted of adding a known amount of either (-)-catechin or 7,8-benzoflavone to the reaction vessel containing a known quantity of the sorbent (soil constituent solid phase). Stock solutions of the allelochemicals were made by initially dissolving them in methanol and then bringing the dissolved material to volume with deionized water. The sorbents (soil solid constituents) consisted of the following minerals: an iron oxide (ferrihydrite), a manganese oxide (birnessite), silicate clays (montmorillinite, illite, and kaolinite), and calcite. Additionally, the sorption of (-)-catechin and benzoflavone onto well characterized soils with similar mineralogy but varying organic matter contents was examined. Once optimum sorption conditions were determined, they were evaluated for their ability to mitigate the phytotoxicity of these allelochemicals on the germination and seedling growth of native flora in the western United States.

This research has the potential to provide an environmentally sound means for controlling invasive plants, minimizing their disturbance of natural landscapes, and increasing the vegetative cover of native plant species. In particular, it targets the spread of Russian (Acroptilon repens) and spotted (Centaurea maculosa) knapweed, two invasive plants that are impacting native plant ecosystems in the western United States at tremendous environmental and economic costs. Understanding the conditions that result in optimum sorption or minimal bioavailability of these allelochemicals has provided a means to arrest the spread of Russian and spotted knapweed by eliminating the competitive edge provided by the release of phytotoxic chemicals. (SEED Project Completed - 2008)