There is an increasing awareness that soil microbes are important for agriculture and nature. Soils contain a large number of microbial species, but relatively little is known about the specific role of these different species on soil functioning. During my PhD research I looked at the effect of land use change on the microbial community composition and the influence on related soil processes. In particular I studied microbial decomposition of soil organic matter, an important process that influences nutrient availability for plants and soil carbon storage. The results shows that the composition of the microbial community is of great importance for processes as nitrogen retention and the decomposition efficiency of organic matter. This information ensures that we can predict the long-term consequences of land use on soil functioning more precisely, and will help in achieving an optimal and sustainable management of our soils.