Depth-dependent driver of global soil carbon turnover times

Soil carbon fixation has the potential to offset anthropogenic carbon emissions and mitigate climate change. However, the carbon fixation capacity still remains uncertain at the global scale, and little is known about the patterns and controls of soil carbon turnover times. Here we synthesize 5188 radiocarbon measurements at the global scale, and random forest models are applied to assess the key drivers of soil carbon turnover times in different soil layers. We find that across the globe, the mean soil carbon turnover time (tau) is 4178 +/- 106 years (mean +/- standard error), but the turnover time varies significantly across different regions and land cover types, with the longest values of tau being observed in tundra and the shortest in temperate forests. Longer soil carbon turnover times are observed in the northern permafrost regions, where the mean tau value is nearly twice that of the non-permafrost regions. Furthermore, tau is generally longer in subsoil than that in topsoil across all ecosystems. Moreover, we find the key drivers of tau are depth-dependent. The most important factors affecting topsoil tau are microbes (bacteria, fungi), while soil mineral protection is the major contributor to subsoil tau. These results highlight the necessity to integrate depth-specific soil carbon turnover time and its associated drivers in carbon cycling models into future climate change scenarios.