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1 Rhizosphere populations of free-living bacteria and fungi associated with particular co-occurring plants were characterized to examine patterns of microbial community structure in natural plant communities. Soil was collected from three co-occurring perennial plant pairs in an undisturbed field at Smoot Hill Biological Reserve, Pullman, Washington, USA, and from three different perennial plant pairs in an old-field community in Durham, North Carolina, USA. 2 Bacteria and fungi isolated from these rhizosphere soils were tested for their ability to utilize various carbon substrates, their tolerance of antibiotics, and their sensitivity to osmotic and metal stresses. Multivariate analysis of physiological characteristics showed that rhizosphere microbial populations associated with particular plant pairs were significantly different from one another. Plots of the first and second linear discriminative functions from a canonical analysis showed clustering of bacterial and fungal physiological characteristics according to plant pair. 3 Replicates of particular plant pairs were planted into both greenhouse and field experiments to test whether bacterial community structure was influenced by different plant pairs. Repeated profile analyses showed that initially similar rhizosphere bacterial communities differentiated in both the greenhouse and field experiments and that these differences were associated with plant pairs. 4 The data suggest structure in rhizosphere populations of free-living bacteria and fungi can be influenced by plants. They provide a first step in understanding the link between the structure and function of natural plant and microbial communities. Keywords: multivariate analysis of variance, perennial grasses, soil bacteria and fungi, substrate utilization assay

1 A glasshouse study was conducted to determine the effects of three variables (sand burial, arbuscular mycorrhizal (AM) fungi and endoparasitic root nematodes) and their interactions on the growth of Ammophila breviligulata Fern. seedlings. We tested the hypotheses that burial allows A. breviligulata to escape from plant-parasitic nematodes and that inoculation with AM fungi is another factor capable of ameliorating the detrimental effects of nematodes to the plant. 2 Seedlings were grown in [Gamma]-irradiated, sterile sand and treated in a factorial manner with 5 cm of sand burial, rhizosphere inoculation with AM fungi and rhizosphere inoculation with plant-parasitic nematodes. Both AM fungi and nematodes originated from field populations of A. breviligulata. 3 Burial stimulated the foliar growth rate of plants in all combinations except when plants had been infected with nematodes prior to burial. The root dry weight was significantly reduced when nematode infected plants were buried. AM fungi improved the foliar growth rate and root dry weight of buried plants infected with nematodes. We therefore rejected the hypothesis that burial allows escape from plant-parasitic nematodes. 4 AM fungi are capable of reducing plant susceptibility to plant-parasitic nematodes and must be considered when assessing the influence of plant-parasites on the vigour of natural A. breviligulata populations. Keywords: arbuscular mycorrhizal fungi, burial in sand, plant vigour, plant-parasitic nematodes, sand dunes