The diversity and functional type of plants can affect the microbial biomass in the soil, its respiratory activity and the diversity of its bacterial population. We have studied these effects in microcosms of reconstituted limestone grassland containing (i) a 12-species mixture of graminoids and forbs, (ii) a monoculture of the sedge Carex flacca, (iii) a monoculture of the grass Festuca ovina, and (iv) similar soil without plants. Microbial biomass was significantly greater in soil under monocultures of F. ovina than in the other microcosms. Basal respiration was largest in the F. ovina and mixed-species treatments where values were more than double those in the C. flacca and bare soil microcosms. The basal respiration was strongly linearly related to plant productivity (r = 0.89). Analysis of the active bacterial population by denaturing gradient gel electrophoresis of 16S rRNA revealed its diversity to be significantly greater in the C. flacca and bare soil treatments than in the F. ovina or mixed-species microcosms. This suggests that the functional type of plants has a strong influence on the composition of the bacterial community. We hypothesize that the discriminating functional attribute leading to a reduction of bacterial diversity in these microcosms was the presence in the F. ovina and mixed-plant communities of an active arbuscular–mycorrhizal mycelium that is absent from bare soil and monocultures of C. flacca.