Plant invasion and the soil microbial community: Interactions and implications for native plant performance and ecosystem function

Plant invasions present a serious threat to native ecosystem structure and function. Little is known about the role rhizosphere soil microbial communities play in facilitating or resisting the spread of invasive species into native plant communities. I examined if two invasive annual plants, Centaurea solstitialis (yellow starthistle) and Aegilops triuncialis (barb goatgrass), change the soil microbial community composition as they invade and whether invasion-induced changes in the soil community affect native plant performance or soil aggregation, an ecosystem function strongly influenced by microbial processes. The rhizosphere microbial communities of starthistle and goatgrass are different from those of five native species that may be competitively impacted by these invasive species in the field (Lotus wrangelianus, Hemizonia congesta, Holocarpha virgata, Plantago erecta, and Lasthenia californica). Rhizosphere microbial communities in newly invaded areas are more similar to the original native soil communities than are microbial communities in areas that have been invaded for several years. A greenhouse experiment found that L. californica performance was reduced when grown in goatgrass-invaded soil compared to native soil; whereas P. erecta and goatgrass performance were unaffected when grown in goatgrass-invaded versus native soil. Changing the soil microbial community in invaded areas may be a mechanism of increased plant invasion in some cases. A field study examined the effects of invaded soil microbial communities on soil aggregation. Root biomass, two fungal-derived carbon concentrations (glomalin-related soil protein (GRSP) and ergosterol), and aggregate mean weight diameter were measured in soils dominated by goatgrass, starthistle, or the native plants L. californica, P. erecta, and H. congesta. Root biomass tended to increase with invasion. GRSP concentrations varied with invasion and were either negatively or non-correlated with soil aggregation. There were no differences in ergosterol concentrations. Soil aggregation increased with goatgrass invasion, but did not increase with starthistle invasion. Abiotic factors may play a large role in aggregation of these highly-aggregated serpentine soils. Plant invasion-induced changes in the soil microbial community affect native plant performance and ecosystem function; these changes may promote future plant invasions and continued ecosystem change.