Arbuscular mycorrhizal fungi (AMF) are important plant symbionts, but we know little about the effects of plant taxonomic identity or functional group on the AMF community composition. To examine effects of the surrounding plant community, of host, and of the AMF pool on the AMF community in plant roots, we manipulated plant community composition in a long-term field experiment. Within four types of manipulated grassland plots, seedlings of eight grassland plant species were planted for 12 weeks, and AMF in their roots were quantified. Additionally, we characterised the AMF community of individual plots (as their AMF pool) and quantified plot abiotic conditions. The largest determinant of AMF community composition was the pool of available AMF, varying at metre scale due to changing soil conditions. The second strongest predictor was the host functional group. The differences between grasses and dicotyledonous forbs in AMF community variation and diversity were much larger than the differences among species within those groups. High cover of forbs in the surrounding plant community had a strong positive effect on AMF colonisation intensity in grass hosts. Using a manipulative field experiment enabled us to demonstrate direct causal effects of plant host and surrounding vegetation.