Soils represent the largest terrestrial carbon reservoir, yet major uncertainties persist regarding soil organic carbon (SOC) turnover times and sequestration potential. Radiocarbon-based approaches have often suggested very old SOC ages, implying limited soil carbon accumulation under future climate scenarios. However, these estimates are biased by the presence of ancient radiocarbon-free organic carbon (aOC), which does not participate in contemporary carbon cycling but artificially increases apparent SOC ages.

Here, we quantify the contribution of aOC at the global scale using a linear mixing model applied to radiocarbon data from 313 soil profiles (2204 soil layers) compiled from the ISRaD and LSCE databases. This approach allows the separation of aOC from the biologically active SOC pool. Results reveal a mean aOC concentration of 2.4 ± 3.2 mg g^-1, corresponding to ~11% of SOC in topsoils (0–30 cm), ~27% in subsoils (30–100 cm), and more than 50% in deep soils (>100 cm). aOC is widespread in soils and its contributions are particularly high in Andosols and Cryosols, and vary strongly with parent material and pedogenetic history.

Accounting for aOC significantly revises global SOC age estimates. Indeed, the mean corrected age of reactive SOC to 1 m depth is ~290 years, in sharp contrast with previously reported radiocarbon ages of 3000–5000 years. These corrected values are consistent with independent isotopic proxies (¹³C, ³⁶Cl) and align more closely with Earth System Model parameterizations. Accordingly, aOC must be considered when interpreting radiocarbon data from SOC and ignoring this ancient fraction leads to a systematic overestimation of SOC persistence.

Finally, incorporating aOC dynamics provides a more realistic framework for assessing soil carbon turnover with a possible impact on sequestration potential and long-term climate mitigation strategies.

Yoann Copard, Université de Rouen.

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Laboratoire METIS, Sorbonne Université, Campus Pierre et Marie Curie
4, place Jussieu 75005 Paris
Salle Ghislain de Marsily (ex salle Darcy), tour 46-56, 3^e étage.

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https://zoom.us/j/98279972023