Natural 13C labeling via a vegetation change from plants using the C-3 photosynthetic pathway to C-4 plants (or vice versa) is a method widely used for the quantification of carbon dynamics in soils. The large isotopic difference makes it possible to trace the 13C label of the new vegetation in soil organic matter and to quantify carbon turnover times. This method relies on the assumption that organic carbon in soils is derived almost exclusively from plants growing in situ. However, anthropogenic carbon sources, such as carbon derived from fossil fuels, may contribute significantly to soil organic carbon (SOC), particularly in highly industrialized areas. Since lignite - the main contaminant in a German soil investigated in this study - has a 13C label comparable to that of the former C-3 vegetation, turnover times of 'natural', plant litter derived SOC, calculated by natural 13C labeling, are much too high. As a consequence we determined the turnover time of plant derived organic carbon in physical soil fractions by combining natural 13C labeling with AMS 14C analysis. We analyzed particle-size fractions to identify organo-mineral interactions as an important mechanism for carbon stabilization in soils. 13C based turnover time estimates for carbon in six size fractions from an agricultural surface soil range between 140 and 720 years. The data correspond to 14C concentrations of these fractions, which decrease from coarse sand (84 pMC) to fine silt (24 pMC) and again increase to clay (70 pMC), suggesting a lignite accumulation in the intermediate fractions (20-200 µm). The admixture of fossil carbon was estimated by mass balance calculation from the 14C concentrations of each fraction and of recent plant material, entering the soil since the establishment of the site. A simple decomposition model was used to calculate the 14C concentration of the plant material. The results were used to correct the underestimated portion of carbon from the new, C-4 vegetation in the litter derived SOC by mass balance calculation. Corrected turnover times range from 43 to 170 years, increasing from coarse sand to clay and are comparable to own data for a non-contaminated soil. The silt and clay fraction yield the slowest turnover times and store more than 50 % of the total SOC and thus are most important for carbon sequestration in agricultural surface soils. The dual carbon isotopic approach thus can be used to quantify carbon turnover in frequently occurring fossil fuel contaminated soils.
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