Global stocks and capacity of mineral-associated soil organic carbon

Authors: GEORGIOU, KATERINA - Stanford University; JACKSON, ROBERT - Stanford University; VINDUSKOVA, OLGA - University Of Antwerp; ABRAMOFF, ROSE - Laboratoire Des Sciences Du Climat Et De L'Environnement (LSCE); AHLSTROM, ANDERS - Lund University; FENG, WENTING - Chinese Academy Of Agricultural Sciences; HARDEN, JENNIFER - Stanford University; PELLEGRINI, ADAM - University Of Cambridge; Polley, Herbert; SOONG, JENNIFER - Colorado State University; RILEY, WILLIAM - Lawrence Berkeley National Laboratory; TORN, MARGARET - University Of California

Submitted to: Nature Communications
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/9/2022
Publication Date: 7/1/2022
Citation: Georgiou, K., Jackson, R.B., Vindušková, O., Abramoff, R.Z., Ahlström, A., Feng, W., Harden, J., Pellegrini, A., Polley, H.W., Soong, J., Riley, W.J., Torn, M.S. 2022. Global stocks and capacity of mineral-associated soil organic carbon. Nature Communications. 13(1). Article 3797. https://doi.org/10.1038/s41467-022-31540-9.
DOI: https://doi.org/10.1038/s41467-022-31540-9

Interpretive Summary: Soil is a huge reservoir of carbon. Soil carbon storage is vital to climate regulation. Soil storage lessens carbon accumulation in gaseous forms (e.g., carbon dioxide) in the atmosphere that modify climate. Storage of soil organic carbon is associated with chemical and physical processes that bind organic matter to mineral surfaces, but the capacity of soils to store carbon remains uncertain. We used data from more than 1,200 reports spanning sites with diverse biomes, soil types, and climates to derive a global estimate of soil carbon storage capacity. We find that the maximum capacity of soils to store carbon depends on soil mineral properties. Storage capacity greatly exceeds current carbon stores for most soils. The ‘deficit’ in soil carbon relative to storage capacity is greatest in deep soils and degraded lands. Globally, soil carbon stocks are only 21-33% of capacity. This global estimate and its spatial distribution provide a basis to geographically prioritize soil management and conservation efforts to facilitate carbon storage. Our results indicate that there is enormous potential to store soil carbon and thereby limit the climate-modifying consequences of the accumulation of carbon gases in air.

Technical Abstract: Soil is the largest terrestrial reservoir of carbon and has the potential to sequester large amounts of carbon. However, the maximum capacity of soils to store carbon globally is unknown. Chemical and physical associations of organic matter with mineral surfaces are known to play a critical role in this storage, but this role remains unquantified. We conducted a comprehensive analysis of more than 1,200 observations of mineral-associated carbon from 171 sites spanning diverse biomes, soil types, and climates to produce the first global estimates of storage capacity. We find that soil mineralogical properties dictate the maximum capacity of soils to stabilize organic matter associated with minerals, and that most soils contain substantially less carbon than their mineralogical carbon capacity. We show how climate and vegetation influence the spatial variability of this undersaturation, using a data-driven, machine-learning approach. The departure from carbon saturation is largest in deeper soils and in poorly-managed and degraded lands. We leverage theory and model insights to calculate the mineralogical capacity of low- and high-activity mineral soils (42 +/- 3 and 86 +/- 5 mg carbon g-1 mineral, respectively). Using these empirical relationships and conservative extrapolations, we estimate the global potential for soil minerals to stabilize carbon at 6,006 + 342 Pg C, and that soils are currently at 21-33% of their carbon capacity. Our findings are essential for developing explicit representations of mineral-organic associations, currently lacking in Earth system models. Moreover, this global estimate and its spatial distribution provide crucial insights for prioritized soil management and conservation efforts. Our results suggest that there is a large potential for sequestering soil carbon through restoration, which can mitigate CO2 emissions with enduring co-benefits.