Key species groups that influence major ecological processes are essential components of community diversity. Many of these groups are found in the soil, including mycorrhizal fungi that may connect plants into a functional "wood-wide web." Arbuscular mycorrhizal fungi, which form associations with 90% of land plant families, are thought to have a broad host range. However, this study shows that the diversity of these fungi is strikingly low in arable sites compared to woodland. The arbuscular mycorrhizal fungi colonizing roots cannot be reliably identified below the genus level except by molecular methods. Roots from five abundant woodland plant species were examined at four sites within a broadleaved wood dominated by oak. Partial fungal small-subunit ribosomal DNA sequences were amplified, cloned, and analyzed using restriction-fragment length polymorphism (RFLP). Sequencing of selected clones determined their phylogenetic position. Comparison was made with samples of pea, maize, and wheat crops on three farms near the woodland site. Three families in the Glomales were represented in the samples by the genera Glomus, Acaulospora, and Scutellospora. These genera were confirmed morphologically. In arable sites, 92% of sequences represented Glomus mosseae or closely related taxa, while those from woodland were much more diverse. The combined woodland samples had a higher diversity of RFLP types than arable samples. This suggests that the broad host range of some arbuscular mycorrhizal fungi may be realized in nature. The change in sequence composition and low diversity in arable fields is likely not due to plant monoculture but reflects other aspects of the agronomic regime, such as ploughing, fertilization, or fungicide application. The dominant arbuscular mycorrhizal fungal type in arable fields was a putative G. mosseae not found in woodland. This species sporulates abundantly and colonizes readily from spores, which may be more important in ploughed fields than in woodland. The low taxonomic diversity of arbuscular mycorrhizal fungi in arable fields indicates their functional contribution may be less than in woodland. 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.Key species groups that influence major ecological processes are essential components of community diversity. Many of these groups are found in the soil, including mycorrhizal fungi that may connect plants into a functional "wood-wide web." Arbuscular mycorrhizal fungi, which form associations with 90% of land plant families, are thought to have a broad host range. However, this study shows that the diversity of these fungi is strikingly low in arable sites compared to woodland. The arbuscular mycorrhizal fungi colonizing roots cannot be reliably identified below the genus level except by molecular methods. Roots from five abundant woodland plant species were examined at four sites within a broadleaved wood dominated by oak. Partial fungal small-subunit ribosomal DNA sequences were amplified, cloned, and analyzed using restriction-fragment length polymorphism (RFLP). Sequencing of selected clones determined their phylogenetic position. Comparison was made with samples of pea, maize, and wheat crops on three farms near the woodland site. Three families in the Glomales were represented in the samples by the genera Glomus, Acaulospora, and Scutellospora. These genera were confirmed morphologically. In arable sites, 92% of sequences represented Glomus mosseae or closely related taxa, while those from woodland were much more diverse. The combined woodland samples had a higher diversity of RFLP types than arable samples. This suggests that the broad host range of some arbuscular mycorrhizal fungi may be realized in nature. The change in sequence composition and low diversity in arable fields is likely not due to plant monoculture but reflects other aspects of the agronomic regime, such as ploughing, fertilization, or fungicide application. The dominant arbuscular mycorrhizal fungal type in arable fields was a putative G. mosseae not found in woodland. This species sporulates abundantly and colonizes readily from spores, which may be more important in ploughed fields than in woodland. The low taxonomic diversity of arbuscular mycorrhizal fungi in arable fields indicates their functional contribution may be less than in woodland. 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.