JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org. Summary1 In laboratory microcosm experiments, co-occurring plant species were found to support very different rates of sporulation of arbuscular mycorrhizal (AM) fungi. These differences were not affected by the time of harvest, suggesting that they reflect host-dependent differences in fungal growth rates, rather than host-dependent timing of sporulation. 2 Spore counts in field soil and estimates from sorghum trap cultures showed that the association of AM fungi with particular host plants in the field was positively correlated with the sporulation rates observed on those hosts in the microcosm experiments. 3 The AM fungal species richness observed at the field site was high relative to estimates made in previous studies. 23 distinct species of AM fungi were found, seven of which have not been previously described. 4 The host-dependence of the relative growth rates of fungal populations may play an important role in the maintenance of fungal species diversity.

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Summary• Arbuscular mycorrhizal fungi (AMF) are known to promote plant growth when phosphorus is limiting, but the role of AMF in plant growth under nitrogen (N) limiting conditions is unclear.• Here, we manipulated N (control vs inorganic and organic forms) and AMF species (control vs four AMF species) for five old-field perennials grown individually in a glasshouse under N-limiting conditions.• We found that AMF were at best neutral and that some AMF species depressed growth for some plant species (significant plant-fungus interaction). Native plant species growth was strongly depressed by all but one AMF species; exotic plant species were less sensitive to AMF. We found no evidence of plant N preferences. Both natives and exotics were able to acquire more N with N addition, but only exotics grew more with added N.• Our results suggest that AMF do not promote plant N acquisition at low N supply, and our results are consistent with other research showing that AMF can act as a parasitic carbon drain when phosphorus availability is relatively high.

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Andropogon gerardii seed obtained from Kansas and Illinois was grown in a controlled environment in their own and each other's soils, with and without arbuscular mycorrhizal fungi (AMF). Each ecotype grew comparatively better in its own soil indicating adaptation to its soil of origin. Overall, A. gerardii benefited more from AMF in low-nutrient Kansas soil than Illinois soil. The two ecotypes, however, did not benefit equally from mycorrhizal infection. The Kansas ecotype was three times more responsive to mycorrhizal infection in the Kansas soil than was the Illinois ecotype. Our results indicate that plant adaptation to the nutrient levels of their local soils is likely to be due, at least in part, to a shift in their dependence on mycorrhizal fungi. The Illinois ecotype of A. gerardii has evolved a reduced dependence upon these fungi and greater reliance on a more highly branched root system. In contrast, the Kansas ecotype had a significantly coarser root system and invested proportionately greater carbon in the symbiotic association with AMF as measured by spore production. This study provides the first demonstration that plants can adapt to changing soil nutrient levels by shifting their dependence on AMF. This result has broad implications for our understanding of the role of these fungi in agricultural systems.