Spotted knapweed is an invasive mycorrhizal weed prevalent in the Pacific Northwest of the United States. Little is known about the effects of spotted knapweed or its management methods on soil quality and soil structure. This study compared soils from spotted knapweed–infested areas with areas where spotted knapweed is being managed using several herbicides and mechanical treatments. We measured concentrations of glomalin, a glycoprotein produced by arbuscular mycorrhizal fungi (AMF), that is correlated with soil aggregate stability, AMF hyphal length, and percent water-stable aggregates (WSA) in soils from managed and unmanaged areas. Areas with high knapweed density (unmanaged areas) generally had lower glomalin concentrations and AMF hyphal lengths compared with areas receiving chemical and combined mechanical–chemical management treatments. Total glomalin was significantly negatively correlated with percent knapweed cover. However, WSA was high (70 to 80%) in soils from all management treatments and was not affected by knapweed cover. Our results suggest that spotted knapweed does not have negative effects on soil quality from our study site, likely because of the high aggregate stability of the soils in the area. However, Centaurea maculosa may have negative effects on soil quality in soils with lower aggregate stability.

Nomenclature: Spotted knapweed, Centaurea maculosa Lam. CENMA.