Invasive and native grasses exert negative plant-soil feedbacks on the woody shrub Artemisia tridentata.

Displacement of diverse native plant communities by low-diversity invasive communities is a global problem. In the western United States, the displacement of sagebrush-dominated communities by cheatgrass has increased since the 1920s. Restoration outcomes are poor, potentially due to soil alteration by cheatgrass. We explored the poorly understood role of plant-soil feedbacks in the dominance of cheatgrass in a greenhouse study where uninvaded sagebrush soils were conditioned with either cheatgrass, a native bunchgrass or sagebrush. Sagebrush seedlings were grown in the soils that remained following the removal of conditioning plants. We expected cheatgrass to strongly suppress sagebrush due to a change in belowground microbial communities, conspecifics to have neutral effects and the native bunchgrass to have intermediate effects as it coevolved with sagebrush but belongs to a different functional group. We assessed the effects of conditioning on sagebrush growth, tissue nutrients, and carbon allocation. We also characterized the abundance, diversity and community composition of root microbial associates. Cheatgrass strongly suppressed sagebrush growth at high and low conditioning densities, the native bunchgrass showed suppression at high conditioning densities only and conspecific effects were neutral. Tissue nutrients, amount of root colonization by soil fungi or root microbial community composition were not associated with these plant-soil feedbacks. Although we did not identify the precise mechanism, our results provide key evidence that rapid soil alteration by cheatgrass results in negative plant-soil feedbacks on sagebrush growth. These feedbacks likely contribute to cheatgrass dominance and the poor success of sagebrush restoration.