Examining the soil legacy effects of spotted knapweed (Centaurea stoebe)

Spotted knapweed (Centaurea stoebe) is the most aggressive invasive forb in North American grasslands. Since its arrival, it has spread from the west coast of North America and reached far east. It has been able to accomplish this via the production of many small seeds per plant, and by altering soil conditions making it difficult for native plants to grow in. Control efforts have been extensive.Broadcast chemical controls have been applied, as well as biological and physical controls; however, despite these interventions, spotted knapweed continues to have negative effects on ecosystems and their functions. Spotted knapweed may have a negative legacy effect in the soils they inhabit, which perpetuates even after removal of this plant. To test for potential soil legacy effects, a greenhouse experiment was devised in which C. stoebe and rough fescue (Festuca campestris) were grown in different soil types. Activated carbon and pulp mill fly ash were used as soil amendments in each soil type in an attempt to return soils to a pristine state, and we found that F. campestris grew best in unamended invaded soils during a 90-day growing period. Pre-growing conditions of this soil displayed lower levels of both carbon and nitrogen compared to other soil types, indicating that F. campestris grew best in less hospitable conditions. As this was an unexpected result, a field experiment was designed in which different concentrations of ash were applied to transplanted rough fescue plugs; however, plug viability tapered off quickly after transplanting. Conclusions drawn from this study indicate ash as a potential soil amendment for knapweed-affected soils with directions for use in future research. Further investigation into the use of ash as a broadscale solution to negative soil legacy effects is warranted. Ash, an industrial waste product, could be potentially useful in areas heavily invaded by spotted knapweed in order to deter the spread of these noxious weeds.