Soil fungi alter interactions between the invader Centaurea maculosa and North American natives

Soil microbes may affect the way exotic invasive plants interact with native neighbors. We investigated the effects of soil fungi on interactions between the invasive weed Centaurea maculosa (spotted knapweed) and six species native to the intermountain prairies of the northwestern United States. We also compared the effect of C. maculosa on the composition of the soil microbial community to that of the native species. In the field, fungicide (Benomyl) reduced AM mycorrhizal colonization of C. maculosa roots by >80%. Fungicide did not significantly reduce non-AM fungi. When grown alone, the biomass of C. maculosa was not affected by the fungicide application. However, depending on the combination of native competitor and fungicide, C. maculosa biomass varied from 10-fold decreases to 1.9-fold increases. In untreated soils, C. maculosa grew larger in the presence of Festuca idahoensis or Koeleria cristata than when alone. When fungicide was applied these positive effects of Festuca and Koeleria on C. maculosa did not occur. A third native grass, Pseudoroegneria spicata, had much stronger competitive effects on C. maculosa than Festuca or Koeleria, and fungicide reduced the competitive effects of Pseudoroegneria. Fungicide increased Centaurea biomass when competing with the forb Gallardia aristata. However, fungicide did not affect the way two other forbs; Achillea millefolium and Linum lewisii, interacted with C. maculosa. Rhizosphere microbial communities in the root zones of the three native bunchgrass species differed from that of C. maculosa. However, despite the strong effects of soil fungi in field interactions and differences in microbial community composition, soil biota from different plant rhizospheres did not affect the growth of C. maculosa in the absence of native competitors in greenhouse experiments. Our results suggest that successful invasions by exotic plant species can be affected by complex and often beneficial effects of local soil microbial communities. These effects were not manifest as simple direct effects, but become apparent only when native plants, invasive plants, and soil microbial communities were interacting at the same time.