Abstract
We understand little about the factors that determine and maintain local species
diversity of arbuscular mycorrhizal fungi (AMF), the reasons why a single plant has
multiple AMF partners, and how that diversity influences host plant performance. The
extent to which co-occurring AMF species occupy different niche space, based on their
ability to tolerate different soil conditions or differentially promote host plant growth in
those differing conditions, offers possible explanations for the maintenance of diversity.
AMF community composition was examined in relation to soil variability in a
naturally metalliferous serpentine grassland and along a Cu, Cd, Pb, and Zn soil
contamination gradient. Both field surveys demonstrated that AMF community
composition is strongly influenced by soil factors and provide evidence that local
diversity of AMF communities is at least partially maintained by environmental niche
partitioning among fungal species.
Because there is some evidence that AMF species can be non-additive in their
effects on plant growth, the appropriate measure of AMF function may be how much
plant growth is affected when that particular AMF species is deleted from the
community. Greenhouse experiments using this deletion approach, and the traditional
approach of evaluating host plant growth with a single AMF species, were performed.
The experiments involved two grass species: Andropogon gerardii and Sorhastrum
nutans and a subset of their natural AMF community grown in soils differing in nitrogen,
phosphorus, and nickel, which is naturally high in the plants’ native serpentine soils. This
deletion method revealed that functional redundancy, with regards to host plant growth
promotion, was the most common consequence of multiple species infecting one root.
Functional complementarity and functional synergy, which may help explain why plants
support multiple partners, were also demonstrated. Each of these interactions was found
to be soil context dependent for most fungal species. These results demonstrate that the
composition of the AMF community colonizing a host plant is important for plant
performance and the consequences of colonization change with soil condition. They also
suggest an explanation for why any one plant species supports several species of these
fungi.
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