Soil Carbon Dynamics Reshaped by Ancient Carbon Quantification

Soil is a major terrestrial carbon reservoir, and enhancing its carbon stock is a central strategy to mitigate climate change. Earth system models project a net soil carbon sink by 2100, the magnitude of which is still under debate, differing significantly between approaches. Radiocarbon-based studies often suggest a limited soil carbon accumulation capacity, but these estimates are biased by the presence of ancient, radiocarbon-free, organic carbon (aOC). This carbon no longer contributes to soil carbon dynamics and increases the average ¹⁴C age of soil carbon because it is radiocarbon-depleted. This known radiocarbon caveat can be overcome with a better understanding of the aOC (ancient radiocarbon-free OC) distribution in the world's soils. Here we apply a mixing linear equation to 313 soils worldwide from radiocarbon databases to estimate the aOC contained in soils. The aOC contained in soils has different origins, from rock-derived to old biospheric C strongly associated with mineral particles during pedogenesis. Our findings show a mean aOC content of 2.4 mg/g ±3.2 SD with an aOC contribution up to 11% of the soil organic carbon in topsoils (0-30 cm depth), reaching 25% in subsoils (30-100 cm depth) and more than half in deep soil (> 100 cm depth). We demonstrate that the aOC content is particularly high in Andosols and Cryosols. We subtracted the aOC contributions to calculate a global mean corrected age of non-aOC carbon to 1 m depth of 290 years, contrasting sharply with previously reported values of 3100 to 4830 years. This corrected estimate aligns more closely with independent isotopic proxies (¹³C and ³⁶Cl) of soil carbon dynamics. These results also reconcile empirical data with the parameterization of Earth system models.