Direct measure of soil carbon persistence over annual to centennial timescales

We measure the age of soil organic matter (SOM) in soil depth profiles using fallout radionuclide (FRN) chronometry. The FRN age model quantifies the well-known lag in ∆14C which is the time between biological carbon fixation and its incorpration into SOM 1–3. The FRN model also reveals sharp excursions in ∆14C at depth to extremely old ages, which we attribute to legacy petrogenic, pedogenic, or passive carbon pools because corresponding soil carbon fluxes based on legacy ∆14C do not reconcile with independent ecosystem measures. FRN ages may thus resolve a foundational uncertainty regarding ages of SOM, agreeing with mass balance of carbon pools and fluxes at both soil profile and global scales 4,5, 𝛿13C experiments at profile and global scales 6,7, compilation of global experimental soil carbon fluxes 8, and independent 35Cl bomb-pulse dating 9. We confirm that the pool of SOM relevant to climate transition is up to 10 times younger than deduced from global ∆14C turnover times. We thereby estimate that temperate and tropical forest soils store ca. 10% of Net Primary Productivity (NPP) over decadal timescales. These observations demonstrate that SOM cycling is more dynamic than deduced from 14C and may respond rapidly to both climate change as well as mitigation efforts aimed at sequestering atmospheric CO2 over annual to decadal timescales.