Background/Question/Methods Old World bluestems (OWB, Bothriochloa bladhii; B. ischaemum) are perennial, warm-season grasses native to Europe and Asia introduced to the U.S. in the 1920s and planted throughout the Great Plains. It has been estimated that >1 million ha have been planted to OWB in Texas and Oklahoma, alone. Until recently, studies on OWB have focused primarily on forage potential, rather than invasibility. Recent evidence suggests these species are highly invasive and reduce native biodiversity. To assess the competitive ability of this invasive grass, a greenhouse study was conducted in which native grasses (Andropogon gerardii; Schizachyrium scoparium) were grown in monoculture or in combination with OWB.

Results/Conclusions

When the native grasses were grown with OWB, they failed to grow appreciably, comprising <10% of the community biomass after 14 weeks growth. Additionally, arbuscular mycorrhizal root colonization (%AM) of the native grasses was reduced when grown with OWB, as compared to colonization in monoculture (14% vs. 59%). To determine if OWB inhibited growth of the native grasses through alterations in soil function or soil biota, we conducted a second greenhouse study. This study examined soil from 4 prairies in OK and KS characterized by undisturbed native prairie with adjacent areas intensively invaded by OWB. We collected soil from 1) beneath native grasses (control); 2) beneath OWB grass and 3) interstitial spaces (bare soil between individual OWB bunches). To determine if alterations in soil properties were predominantly due to alterations in soil function (nutrient cycling or increases in organic compounds), we grew A. gerardii, S. scoparium, or OWB seedlings in nonsterile soil. To examine alterations in soil biota, soil was sterilized and inoculated with nonsterile soil collected beneath native grasses, thereby returning native soil microorganisms. All species (OWB and native) produced the greatest biomass in control soils. Native grasses failed to grow or become colonized by AM fungi when planted into nonsterile interstitial soils. Addition of native microbes alleviated adverse effects of interstitial soil, but these plants were substantially smaller than controls. OWB growth in interstitial soils was reduced by 60 – 80%, compared to controls. While all grass species grew in OWB soils, biomass and %AM were reduced, compared to controls. Addition of native soil microbes improved plant growth and %AM in most plant-soil combinations. Our results indicate that OWB alters both soil function and biota, and successful restoration of invaded areas may require soil remediation.