Soil sterilization alters interactions between the native grass Bouteloua gracilis and invasive Bromus tectorum

The invasive grass Bromus tectorum negatively impacts grass and shrublands throughout the western U.S., particularly in arid and semiarid regions. We asked whether soil microbes associated with a native grass (Bouteloua gracilis) affect growth of Bromus and competition between Bromus and Bouteloua. We also examined whether plant responses varied between soils from 15 sites in the Northern Great Plains. Bromus and Bouteloua were grown in media with sterilized or unsterilized soil, alone and together. Soil sterilization reduced biomass of Bouteloua and Bromus grown alone by an estimated 50% and 48%, respectively. Additionally, results provided evidence that sterilization increased the effect of competition on Bromus, and may have reduced the effect of competition on Bouteloua. Bouteloua likely had a stronger negative effect on Bromus in sterilized soils because sterilization reduced Bromus biomass by a greater absolute amount. Response to sterilization varied appreciably by site for Bromus, but not Bouteloua. Our results support the hypothesis that invasive species such as Bromus often have positive responses to soil biota in the invaded range. Soil microbes are one factor that may be important in determining dynamics of plant invasions, and plant responses to new sites and competition with natives.