We studied the effects of soil fungi on interactions between Centaurea melitensis, an exotic invasive weed in central California, and two co-occurring grasses, Nassella pulchra and Avena barbata. The fungicide benomyl reduced the abundance of arbuscular mycorrhizal (AM) fungi in plant roots but did not affect non-AM fungi. Centaurea plants grown alone were \u3e50% smaller with the resident microbial community intact than when benomyl was applied. When grown with Nassella, the effect of benomyl was reversed. Centaurea grew almost five times larger with the resident microbial community intact. Fungicide had no effect on the biomass of Centaurea grown with Avena, but biomass of Centaurea was significantly lower when grown with Avena than when grown with Nassella or alone. Photosynthetically fixed carbon may have been transferred from Nassella via soil fungi to Centaurea, constituting a form of soil fungi-mediated parasitism, but such a transfer did not occur from Avena to Centaurea. Second, Nassella may have been more inhibited by soil pathogens in the presence of Centaurea than when alone, and the inhibition of Nassella may have released Centaurea from competition. A third possibility is that Nassella has strong positive effects on the growth of soil fungi, but the positive feedback of beneficial soil fungi to Nassella is less than the positive feedback to Centaurea. Regardless of the mechanism, the difference in soil fungicide treatment effects on competition between Centaurea and Nassella vs. Centaurea and Avena has important implications for the invasion of California grasslands
