Soil biodiversity is huge and determines largely the functioning of terrestrial ecosystems both at the ‘macro’ and the ‘micro’ level. Despite the general acceptance of the large impact of land use and other human activities on species loss in terrestrial ecosystems, their effects on microbial species reduction and the consequences are largely unknown. A major reason is the scarcity of experimental approaches to assess the relevance of soil microbial diversity for the functioning of soil ecosystems. The main goal of the study described in this thesis was to obtain better understanding of the diversity, structuring and functioning of bacterial communities in soil and and rhizosphere. With that purpose, we initially applied the rather old dilution approach to manipulate the diversity of microbial communities in soil by inoculation and subsequent incubation of more or less diluted soil suspensions in pre-sterilized soils.