Mineralization activity of soil microorganisms is a crucial parameter in evaluating the biosphere potential to regulate CO2 level in the atmosphere. Priming effects (PE) are strong changes in the turnover of native soil organic matter (SOM) induced by the input of fresh organic matter to the soil. In the course of priming, large amounts of C, N and other nutrients can be released or immobilized in soil on a very short timescale. Hence, the intensity of carbon sequestration and turnover in soil are regulated by PE.The aim of this project is to elucidate the function of microorganisms with different growth strategies (r or K) in accumulation and mineralization of SOM. Specific goals include (i) tracing the succession and functions of the soil microbial community during decomposition of applied organic substances, (ii) determination of the extent of priming effects - extra CO2 evolution and N mineralization - in the course of the microbial succession, (iii) identification of the environmental factors and substrate properties (C/N ratio, polymerization) controlling the intensity of the PE and microbial succession, (iv) development of a process-oriented model simulating C sequestration in soil under various scenarios of priming effects.Amount of C and nutrients mobilized or immobilized from SOM by PE will be assessed by application of 14C, 13C and 15N labelled primer substances and their tracing in different SOM pools, released CO2 and mineral N. Microbial communities at different succession stages of fresh organic matter decomposition will be characterized by functional activity and by metabolic diversity (BIOLOG). Microbial community structure will be determined by molecular methods such as 13C-PLFA, DGGE and 14C-microautoradiography and will be related to functions.The project will contribute to clarification and systematization of PE mechanisms and to development of successful strategies for C sequestration at the field and regional scale.