EGU General Assembly 2021
© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Soil carbon persistence linked to mineralogy across sub-Saharan Africa

Sophie F. von Fromm1,2, Alison M. Hoyt1, Benjamin M. Butler3, Asmeret Asefaw Berhe4, Sebastian Doetterl2, Stephan M. Haefele5, Steve P. McGrath5, Keith D. Shepherd6, Johan Six2, Erick K. Towett6, Leigh A. Winowiecki6, and Susan E. Trumbore1
Sophie F. von Fromm et al.
  • 1Department of Biogeochemical Processes, Max-Planck-Institute for Biogeochemistry, Jena, Germany (
  • 2Department of Environmental System Science, ETH Zurich, Zurich, Switzerland
  • 3The James Hutton Institute, Aberdeen, Scotland
  • 4Department of Live and Environmental Sciences, University of California Merced, Merced, CA, USA
  • 5Department of Sustainable Agriculture Sciences, Rothamsted Research, Harpenden, England
  • 6International Centre for Research in Agroforestry, Nairobi, Kenya

Recent compilations of global soil radiocarbon data suggest that current Earth System Models underestimate the mean age of soil carbon (C). The discrepancy between data-derived estimates and model calculations might be due to an inadequate representation of processes that control C persistence in soils – especially in understudied regions.

Here, we investigate the relationships between soil mineralogy, soil properties, climate and radiocarbon (Δ14C) in soils sampled as part of a comprehensive soil survey (AfSIS) for sub-Saharan Africa. A total of 510 samples were analyzed, comprised of soils collected from two depths (0–20 cm and 20–50 cm) at 30 sites in 14 countries. To determine soil mineralogy, we analyzed X-ray powder diffraction (XRPD) data, which provides a precise and detailed mineralogical signature of each soil sample. The studied soil profiles vary greatly in their mineralogy, reflecting a diverse range of parent materials and soil forming factors.

The median soil C age is 182 years in the topsoils and 563 years in the subsoils, corresponding to a total Δ14C value range of -432 to 95 ‰. In general, Δ14C values decrease (older mean C ages) with increasing clay particle size fractions. This corresponds to an increase in short range-order minerals expressed as oxalate-extractable aluminum and iron (Alox and Feox). Separately, mineralogically defined variables – derived from the XRPD data using principal component analysis – are found to correlate strongly with a range of soil properties (pH, weathering status, exchangeable calcium, Alox and Feox, and soil texture) and climatic variables (aridity index and mean annual temperature). This provides a holistic assessment of the processes that have formed each soil along with the properties that it currently exhibits. Our analyses with random forests show that these XRPD-derived mineralogical variables alone can explain up to 30% of the variation in Δ14C across sub-Saharan Africa. They also allow the identification of specific minerals that contribute to this variation and how they are linked to the C mean age of the soil. In conclusion, our results suggest that soil mineral data can help to better understand C persistence in subtropical and tropical soils.

How to cite: von Fromm, S. F., Hoyt, A. M., Butler, B. M., Berhe, A. A., Doetterl, S., Haefele, S. M., McGrath, S. P., Shepherd, K. D., Six, J., Towett, E. K., Winowiecki, L. A., and Trumbore, S. E.: Soil carbon persistence linked to mineralogy across sub-Saharan Africa, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10703,, 2021.


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