Effect of plant root symbionts on performance of native woody species in competition with an invasive grass in multispecies microcosms.

The majority of terrestrial plants form mutualistic associations with arbuscular mycorrhizal fungi (AMF) and rhizobia (i.e., nitrogen-fixing bacteria). Understanding these associations has important implications for ecological theory and for restoration practice. Here, we tested whether the presence of AMF and rhizobia influences the performance of native woody plants invaded by a non-native grass in experimental microcosms. We planted eight plant species (i.e., , , , , and ) in microcosms of field-conditioned soil with and without addition of AMF and rhizobia in a fully factorial experimental design. After seedling establishment, we seeded half the microcosms with an invasive grass . We measured shoot and root biomass of native plants and , and on roots, the percentage colonization by AMF, number of rhizobia-forming nodules and number of proteaceous root clusters. We found no effect of plant root symbionts or addition on performance of myrtaceous, and as predicted, proteaceous species as they rely little or not at all on AMF and rhizobia. Soil treatments with AMF and rhizobia had a strong positive effect (i.e., larger biomass) on native legumes (.  and ). However, the beneficial effect of root symbionts on legumes became negative (i.e., lower biomass and less nodules) if was present, especially for one legume, i.e., , suggesting a disruptive effect of the invader on the mutualism. We also found a stimulating effect of on root nodule production in .  and AMF colonization in which could be indicative of legumes' increased resource acquisition requirement, i.e., for nitrogen and phosphorus, respectively, in response to the addition. We have demonstrated the importance of measuring belowground effects because the aboveground effects gave limited indication of the effects occurring belowground.