Soil microbes provide multiple ecosystem functions such as nutrient cycling, decomposition and climate regulation. However, we lack a quantitative understanding of the relative importance of microbial richness and composition in controlling multifunctionality. This knowledge gap limits our capacity to understand the influence of biotic attributes in the provision of services and functions on which humans depend. We used two independent approaches (i.e. experimental and observational), and applied statistical modeling to identify the role and relative importance of bacterial richness and composition in driving multifunctionality (here defined as seven measures of respiration and enzyme activities). In the observational study we measured soil microbial communities and functions in both tree- and bare soil-dominated microsites at 22 locations across a 1200 km transect in southeastern Australia. In the experimental study we used soils from two of those locations and developed gradients of bacterial diversity and composition through inoculation of sterilized soils. Microbial richness and the relative abundance of γ-Proteobacteria, Actinobacteria and Bacteroidetes were positively related to multifunctionality in both the observational and experimental approaches; however, only Bacteroidetes was consistently selected as a key predictor of multifunctionality across all experimental approaches and statistical models used here. Moreover, our results, from two different approaches, provide evidence that microbial richness and composition are both important, yet independent, drivers of multiple ecosystem functions. Overall, our findings advance our understanding of the mechanisms underpinning relationships between microbial diversity and ecosystem functionality in terrestrial ecosystems, and further suggest that information on microbial richness and composition needs to be considered when formulating sustainable management and conservation policies, and when predicting the effects of global change on ecosystem functions. Usage notes Dataset_06_08_2017 There are five spreadsheets in this excel file. The spreadsheets "Microcosm soil A”, “Microcosm soil B” and “Environmental gradient" contain the raw data used in this paper. The spreadsheets "Microcosm metadata” and “Env. gradient metadata" contain the associated metadata, where a description of all the variables and units can be found.