|Beschreibung:||Radiocarbon analyses of physical and chemical soil organic matter fractions as well as of individual compounds provided information on the origin, transformation and stabilisation of organic carbon in agricultural soils. A contamination of the soil at the study site at Halle/Saale (Germany) with fossil, lignite-derived carbon was identified by high apparent 14C ages of the organic matter in the plough horizon of about 5000 years BP and a decrease in 14C age with increasing soil depth. The fossil contamination extended to all physical and chemical soil fractions, even to more specific lipid compound classes, resulting in a drastic overestimation of organic matter stability. By combining 14C-based estimates of fossil carbon in soil fractions from the Halle site with results of natural 13C-labelling it was possible to calculate turnover times of ‘natural’ organic carbon in particles-size fractions. These ranged from 40 to 170 years, similar to values obtained at ‘clean’ sites. The data indicate the importance of the silt and clay fraction, which stored more than 50 % of the total soil organic carbon and yielded slow turnover times of 160-170 years. The destruction of megaagregates by tillage and a release of up to 40 % of the total organic carbon was shown by carbon contents and 14C results of water-stable aggregates from a non-cultivated grassland trial and trials with conventional ploughing. The relatively new technique of compound-specific radiocarbon analysis was tested and applied to microbial phospholipid fatty acids (PLFAs). 14C concentrations of individual PLFAs - cell membrane components, used as proxies for living microbial biomass in soils - reflect different organic carbon sources assimilated by different soil microbes in the surface and in the subsoil at the rural Rotthalmünster site.