Increasing plant diversity is known to be important for ecosystem functioning and an increase in soil organic carbon storage. The link between aboveground plant diversity and belowground diversity is currently under discussion. Furthermore, the mechanisms by which aboveground diversity impacts storage as well as the sustainability of storage are still not well understood. Addressing these issues the thesis contributes to a better understanding of the impact of plant diversity on belowground ecosystem functioning. For investigations the field site of The Jena Experiment was used. The species pool comprises 60 species common to the Central European Molinio-Arrhenatheretum grasslands. Between 2002 and 2007 organic carbon and nitrogen were stored in the top and sub soil. In the top soil the main drivers for storage were soil texture and plant diversity. While the root input was not important for storage in the top soil, it was found to significantly affect storage in the sub soil, where the system seemed to be input-limited. Due to the increased substrate use efficiency of microorganisms in the sub soil, the major part of the input was transformed and stored. Microbial transformation of input and sustainable sequestration of carbon was confirmed by the results of density fractionations. Soil microbial community was characterised by phospholipid fatty acids (PLFA). Although the abiotic factors were found to have an impact on microbial abundance, increased plant diversity lead to increased amounts of soil microbial biomass. For soil microbial biomass not only the amount of plant biomass input but also the quality and heterogeneity of input was important. The same result was found for the microbial composition and indicates that increasing diversity increases niche complementarity of soil microorganisms. The results newly establish the strong impact of plant diversity on the soil organic carbon and nitrogen storage beyond abiotic controls. Soil microorganisms were found to play the central role for transformation of organic input and were the main drivers for a sustainable storage at higher diversity levels. As specific interactions between the aboveground and belowground compartments are vital for the ecosystem functioning, this should strengthen our efforts to reduce species extinction.