This chapter presents an overview of carbon turnover behavior of soil organic matter (SOM) types and fractions, of carbon (C) in isothermic and nonisothermic soils, in relation to the lignine/nitrogen ratio, and concerning the C pools in different climates and environments. It also presents results of radioactive and stable isotope tracer work. In order to make biomass decomposition and C turnover in specific systems visible and measurable, labeling procedures have been applied with preference. Due to its exceptional importance for fertility and potential production, the carbon turnover in soils of different climates and environments has been assessed. For an understanding of SOM-C, with 1,550 Pg the largest accessible organic carbon pool, residence time and replacement rates after alternation of vegetation with different photosynthesis mechanisms is essential. The clear results on percentage decomposition or rate of remainder by the use of uniformly labeled plant substance suggest continuation, especially with typical species of savanna and agroforestry environments, where knowledge gaps exist.