Estimating turnover of soil organic carbon fractions based on radiocarbon measurements

In this paper, we examine 3 different models used to estimate turnover of soil organic carbon (SOC) fractions using radiocarbon measurements: one conventional carbon dating model and two bomb C-14 models. One of the bomb C-14 models uses an atmospheric C-14 record for the period 22,050 BC to AD 2003 and is solved by numerical methods, while the other assumes a constant C-14 content of the atmosphere and is solved analytically. The estimates of SOC turnover obtained by the conventional C-14 dating model differed substantially from those obtained by the bomb C-14 models, which we attribute to the simplifying assumption of the conventional C-14 model that the whole SOC fraction is of the same age. The assumptions underlying the bomb C-14 models are more applicable to SOC fractions; therefore, the calculated turnover times are considered to be more reliable. We used Monte Carlo simulations to estimate the uncertainties of the turnover times calculated with the numerically solved C-14 model, accounting not only for measurement errors but also for uncertainties introduced from assumptions of constant input and uncertainties in the C-14 content of the CO2 assimilated by plants. The resulting uncertainties depend on systematic deviations in the atmospheric C-14 record for SOC fractions with a fast turnover. Therefore, the use of the bomb C-14 models can be problematic when SOC fractions with a fast turnover are analyzed, whereas the relative uncertainty of the turnover estimates turned out to be smaller than 30% when the turnover time of the SOC fractions analyzed was longer than 30 yr, and smaller than 15% when the turnover time was longer than 100 yr.