The sequestration of carbon in soil humus is given great attention during the last years as this represents a mitigation strategy for removing radiative forcing carbon dioxide from the atmosphere (Lal et al., 2004). For the analysis of C sequestration in soil, the precise estimations of soil carbon stocks, of the resistant and readily available pools, the carbon dioxide release and primary productivity need to be combined which can vary due to heat and drought (Ciais et al., 2005). On the other hand, microbiological methods including respirometry are used for assessing soil quality in research, monitoring and routine analysis in European countries (Bloem et al., 2005). These biological indicators contributed to the understanding of soil carbon sequestration in young coastal soils in Germany where both C sequestration and soil respiratory activity were high (Dilly et al., 2005). Soil respiration can be estimated on the use of carbon dioxide evolution and oxygen uptake which is defined as respiratory quotient. The two measures may vary significantly dependent the substrate being readily available (Dilly 2001; 2003). The analysis of respired carbon dioxide indicated priming effects which occurred mainly after the addition of readily available carbon (Zyakun and Dilly, 2005). The presentation will give insights on the use of microbial indicators and mass spectrometry of carbon dioxide when dealing with humus accumulation in agricultural and forest soils. References: (i) Bloem J., Hopkins D., Benedetti A., 2005. Microbiological methods for assessing soil quality. CABI, Wallington. (ii) Ciais P. et al., 2005. Europe-wide reduction in primary productivity caused by the heat and drought in 2003. Nature 437, 529-533. (iii) Dilly O., 2001. Microbial respiratory quotient during basal metabolism and after glucose amendment in soils and litter. Soil Biology and Biochemistry 33, 117-127 (iv) Dilly O., .2003. Regulation of the respiratory quotient of soil microbiota by availability of nutrients. FEMS Microbial Ecology 43, 375-381. (v) Dilly O., Gnass A., Pfeiffer E.-M., 2005. Humus accumulation and microbial activities in Calcari-Epigleyic Fluvisols under grassland and forest diked in for 30 years. Soil Biology & Biochemistry 37, 2163-2166 Lal, R., Griffin, M., Apt, J., Lave, L., Morgan, M.G., 2004. Managing soil carbon. Science 304, 393. (vi) Zyakun A., Dilly O., 2005. Respiratory quotient and priming effect in an arable soil induced by glucose. Applied Biochemistry and Microbiology 41, 512-520