Invasive Plant Alters for Soil, Too, Says Colorado State University Research

Note to Editors: For photographs of spotted knapweed, please visit and click on the header for this release. Please credit Colorado State University when using photos.

Invasive plant species such as spotted knapweed are not only taking a toll on native plant species, but have a detrimental effect on the biodiversity of microbes in the soil, according to a new study by Colorado State University researchers to be published in the journal "International Society for Microbial Ecology."

The interactions between plants and soil microbes are important components of ecosystem function, said Amanda Broz, a graduate student in Colorado State’s Center for Rhizosphere Biology, who conducted the research. Invasive plants such as spotted knapweed flourish in new environments that lack their natural enemies, such as pathogens that have co-evolved with the invasive species in the original native environment.

"Spotted knapweed originated in Eurasia where it is held in check by pathogens, herbivores and other plant competitors that evolved along side of it," Broz said. "When knapweed was introduced to the American West, it escaped these natural enemies, allowing it to spread and take over many of our native grasslands."

Spotted knapweed arrived on both coasts of North America in the late 1800s as a contaminant of alfalfa seed. In addition to displacing native plant species, the weed increases water runoff leading to erosion and reduces forage for livestock and wildlife.

Researchers collected soil samples from areas near Missoula, Mont., where spotted knapweed is particularly problematic, infesting more than 4.7 million acres in the state. In areas with very high-densities of spotted knapweed, there was 80 percent less DNA of fungi than areas with low-densities of spotted knapweed. Even areas with a low-density of spotted knapweed showed changes in the amount and types of soil microbes naturally found in the area.

Soil microbes can have a profound influence on molecular and biochemical processes in individual plants, plant community and ultimately the entire ecosystem. The disruption of the balance between native plants and microbial communities in the soil can have a negative effect on native plants while benefiting invasive species.

The findings provide more information about the impacts that invasive species can have on an ecosystem, Broz said.

"A better understanding of the interactions between native plants, invasive species and the native soil community will help in developing more effective strategies in managing invasive species and restoring the landscape to its natural state," Broz said.

For more about the Center for Rhizosphere Biology at Colorado State, visit