The diversity changes in the rhizosphere microbial communities of four grass species submitted to a stress (drought) and/or a disturbance (defoliation) were examined in a full factorial common garden experiment. The diversity of bacterial and fungal communities was determined using Single Strand Conformation Polymorphism (SSCP) and multivariate analyses of variance were performed to test for the effect of experimental factors on microbial beta diversity, i.e. the variation of microbial community composition among pots. Plant species identity contributed the most to microbial beta diversity with 22% of the total variance explained and this effect was stronger for fungi than for bacteria. The effect of defoliation was significant for fungal communities, and more pronounced for plant species that exhibited an increased root biomass following defoliation. Drought had a higher effect on bacterial beta diversity, and this was particularly noticeable for plant species that were the least able to tolerate water shortage. Our results showed that both plant species identity and characteristics of plant growth response captured the plant-mediated effects on microbial diversity. They illustrate the biotic cascade between plants responding to a changing environment and the dynamics of rhizosphere microbial communities.