The age of global soil carbon and implications for sequestration potential

Thursday, December 12, 2019 - 09:45
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Soil is the largest organic carbon (C) reservoir on land, containing more C than the atmosphere and vegetation combined. Soil radiocarbon measurements (∆14C) offer valuable insights into soil C stabilization and rates of C cycling in soils, but to date soil radiocarbon has not been mapped globally. Here, we generate globally-gridded datasets of soil ∆14C and corresponding soil C age down to 1-meter depth from 834 globally distributed soil radiocarbon profiles. We find that soil depth is a primary driver of ∆14C, whereas climate (e.g. mean annual temperature) is a major control on the spatial pattern of ∆14C in surface soil. Only 11±1% of global soil C has positive values of ∆14C, a sign of significant incorporation of ‘bomb’ C over the past 75 years; the remaining soil has negative ∆14C values, implying a relatively small sensitivity of this fraction to rapid change. The mass-weighted age (derived from turnover time of one-pool model) of soil C to a depth of 1 meter is 4150 ± 440 years, with surface soils considerably younger (1290 ± 100 years for 0-30cm) than deeper soils (6210 ± 720 years for 30-100cm). Soil radiocarbon simulated by state-of-the-art global land models with depth resolution is less depleted and thus younger than the global-gridded datasets. Our observations imply limits on the C sequestration potential of global soils. Even improved soil models with depth resolution may overestimate the response of soil carbon to increasing atmospheric CO2 levels.

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