Plant communities, through species richness and composition, strongly influence soil microorganisms and the ecosystem processes they drive. To test the effects of other plant community attributes, such as the identity of dominant plant species, evenness, and spatial arrangement, we set up a model mesocosm experiment that manipulated these three attributes in a full factorial design, using three grassland plant species (Anthoxanthum odoratum, Plantago lanceolata, and Lotus corniculatus). The impact of the three community attributes on the soil microbial community structure and functioning was evaluated after two growing seasons by ester-linked phospholipid fatty-acids analysis, substrate-induced respiration, basal respiration, and nitrogen mineralization and nitrification rates. Our results suggested that the dominant species identity had the most prevalent influence of the three community attributes, with significant effects on most of the measured aspects of microbial biomass, composition and functioning. Evenness had no effects on microbial community structure, but independently influenced basal respiration. Its effects on nitrogen cycling depended on the identity of the dominant plant species, indicating that interactions among species and their effects on functioning can vary with their relative abundance. Systems with an aggregated spatial arrangement had a different microbial community composition and a higher microbial biomass compared to those with a random spatial arrangement, but rarely differed in their functioning. Overall, it appears that dominant species identity was the main driver of soil microorganisms and functioning in these model grassland communities, but that other plant community attributes such as evenness and spatial arrangement can also be important.