Is mycorrhiza functioning influenced by the quantitative composition of the mycorrhizal fungal community?

The identity and diversity of arbuscular mycorrhizal (AM) fungal symbionts strongly affect the functioning of mycorrhiza, but little is still known about the functional relevance of the individual taxa abundances within AM fungal communities. We hypothesized that proportions of AM fungal taxa influence mycorrhizal benefits to the host, and that a community with spontaneously established ratios of AM fungal species is more beneficial than communities with artificially manipulated ratios. Medic (Medicago truncatula) was inoculated with synthetic AM fungal communities composed of five fungal species in different proportions, and a 'functionally optimized' community generated by previous co-cultivation of the five AM fungi. The composition of the communities was monitored along with the host plant responses to mycorrhiza in three sequential harvests. Most of the artificial AM fungal communities differed compositionally and functionally from the presumably 'functionally optimized' community, which indeed promoted plant growth to the greatest extent. The higher ability to promote plant growth was partly explained by higher intraradical fungal biomass, but functional differences between the communities were also related to the abundances of certain AM fungal species. Thus, the experiment demonstrated functional relevance of species' abundances within AM fungal communities. The observed 'functional optimization' of the AM fungal community is discussed in context with the host plant's and AM fungal species' traits.