Key species groups that affect major ecological processes are vital components of community diversity. Many such key groups are found in the soil, including the mycorrhizal fungi that may connect plants into a functional “wood-wide web”1. Arbuscular mycorrhizal associations are formed by fungi of the order Glomales with 90% of land plant families, and many arbuscular mycorrhizal fungi are thought to have a broad host range2. Here we show that, despite this broad host range, the diversity of arbuscular mycorrhizal fungi is strikingly low in arable sites compared with a woodland.
The arbuscular mycorrhizal fungi that colonize roots cannot be reliably identified below the genus level except by molecular methods. We examined roots from five abundant woodland plant species at four sites within a broadleaved wood dominated by oak (Quercus petraea, colonized by ectomycorrhizal fungi), and sycamore (Acer pseudoplatanus, colonized by arbuscular mycorrhizal fungi), at Castle Howard, North Yorkshire, UK. Partial fungal small-subunit ribosomal RNA sequences were amplified, cloned and screened for differences in restriction pattern by restriction-fragment length polymorphism (RFLP). We sequenced selected clones to determine their phylogenetic position (Fig. 1). For comparison, we sampled pea, maize and wheat crops on three farms within a 55-km radius of the woodland site.
There are three families in the Glomales, represented in our samples by the genera Glomus, Acaulospora and Scutellospora, and these are readily distinguished as distinct sequence clusters (Fig. 1). Morphological studies confirmed that these three genera were present in these roots3. In arable sites, 92% of sequences represented Glomus mosseae or closely related taxa, whereas those from woodland were much more diverse (Fig. 1). This was true even though the arable sites were separated by up to 66 km and three host species were sampled. The combined woodland samples had a much higher diversity of RFLP types (Shannon-Weiner H = 0.144) than the combined arable samples (H = 0.398). In both wood and field, we often obtained identical sequences from different plant species, suggesting that the broad host range exhibited by many cultured arbuscular mycorrhizal fungi2 may also be realized in nature.
Given the broad host range of some arbuscular mycorrhizal fungal taxa, the change in sequence composition and low diversity of the fungi in arable fields is probably not a result of plant monoculture per se, but reflects other aspects of the agronomic regime such as ploughing, fertilization or fungicide application. In all the arable fields, regardless of host plant or location, the dominant arbuscular mycorrhizal fungal type was a putative G. mosseae not found in the woodland. This species sporulates abundantly and colonizes readily from spores, which may be more important in a field that is ploughed annually than in woodland4.
Arbuscular mycorrhizal fungi differ widely in their biological properties, and presumably have several different roles in ecosystems5. The low taxonomic diversity of arbuscular mycorrhizal fungi in arable fields indicates that their functional contribution may be less there than in woodland. It has been suggested that low ecosystem diversity may be associated with impaired function6,7 and reliability8,9. Our results show that microbes need to be considered in any assessment of the effects of agriculture on biological diversity and that intensive arable agriculture may be operating at minimum levels of diversity for at least one key functional group.
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Helgason, T., Daniell, T., Husband, R. et al. Ploughing up the wood-wide web?. Nature 394, 431 (1998). https://doi.org/10.1038/28764
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