The specificity of arbuscular mycorrhizal fungi in perennial ryegrass–white clover pasture

For nitrogen to be transferred from a legume to grass through hyphae of arbuscular mycorrhizal fungi (AMF), as reported for a number of grass/legume combinations, the two species should be linked by a common hyphal network, which gives rise to the question of host–fungus preference. To examine the relative preference of perennial ryegrass and white clover for co-existing AMF, two experiments, adopting a ‘spreader–receiver’ system, were carried out in a glasshouse. Both experiments demonstrated quantitative host preference by perennial ryegrass and white clover for AMF associated with their own rhizospheres. However, the effect seemed to be temporal, as, by Week 17, in Experiment 2, AMF not associated with the roots of the spreader may have had an opportunity to infect the receiver. Differences in the population of AMF spores around the roots of the two plant species also suggest differences in preference by the two hosts. The study confirms that white clover is more highly mycotrophic than grass. It is concluded that quantitative differences in preference by perennial ryegrass and white clover for specific AMF may reduce the ecological and agricultural significance of N transferred from clover in the field compared with laboratory estimates. ©2000 Elsevier Science B.V. All rights reserved.