A new model is presented that uses bomb carbon (14C enrichment) to calculate the turnover times of active soil organic carbon (SOC). The new approach overcomes major deficiencies of older models, particularly in respect to the 14C content of carbon input to SOC and of carbon output from SOC. Calculated turnover times of active carbon for ten European forest sites ranged from 4.0 to 15.1 years with a mean of 8.2 years. The model was successfully validated in soil incubation experiments, where the measured 14C content of (microbially) respired CO2 was on average 9.6% (ΔpM) higher than that of bulk SOC, but agreed considerably well with the modeled 14C content. Furthermore, we used the model's sensitivity tests to show that over long time periods (decades), soils can only sequester small amounts of carbon.