The response of soil carbon dynamics to climate and land-use change will affect both the future climate and the quality of ecosystems. Deep soil carbon (> 20cm) is the primary component of the soil carbon pool, but the dynamics of deep soil carbon remains poorly understood. Therefore, radiocarbon activity (ΔD14 C), which is a function of the age of carbon, may help to understand rates of soil carbon biodegradation and stabilization. We analyzed the published 14 C contents in 122 profiles of mineral soil that were well distributed in most of the large world biomes, except for the boreal zone. With a multivariate extension of a linear mixed effects model whose inference was based on the parallel combination of two algorithms, the Expectation-Maximization (EM) and the Metropolis-Hasting algorithms, we expressed soil Δ14 C profiles as a four-parameter function of depth. The four-parameter model produced insightful predictions of soil Δ14 C as dependent on depth, soil type, climate, vegetation, land-use and date of sampling (R2 = 0.68). Further analysis with the model showed that the age of topsoil carbon was primarily affected by climate and cultivation. By contrast, the age of deep soil carbon was affected more by soil taxa than by climate and thus illustrated the strong dependence of soil carbon dynamics on other pedologic traits such as clay content and mineralogy.