"Soils play a critical role in the coupled carbon-cycle climate system. However, our scientific understanding of the role of soil biological-physicochemical interactions and of vertical transport for biogeochemical cycles is still limited. Moreover the representation of soil processes in current biosphere models operating at global scale is crude compared to vegetation processes like photosynthesis. Hence, the general aim of this project is to improve our understanding of the key interactions between the biological and the physicochemical components of the soil system that are often not explicitly considered in current experimental and modeling approaches. However, these interactions are likely to influence the biogeochemical cycles for a large part of the terrestrial biosphere and thus have the potential to significantly impact the Earth System as a whole. This will be achieved through an approach that integrates new soil mesocosm experiments, field data from ongoing European projects and soil process modeling. In mesocosm tracer experiments the fate of new and autochthonous soil organic matter will be followed under varying temperature and moisture regimes, explicitly investigating the role of microbiota. This project will test the hypothesis that transfer coefficients between soil organic matter pools, respiration and microbial biomass formation are constant as implemented in current soil organic matter models. Novel soil model structures will be developed that may explicitly account for the role of microbes and transport for soil organic matter dynamics. This will be supported by multiple-constraint model identification techniques, which allows testing and achieving model consistency with several observation types and theory. The soil modules will be incorporated into global terrestrial biosphere models which are coupled and uncoupled to the atmosphere allowing specific model experiments for investigating feedback mechanisms between soil, climate, and vegetation."