Preprints
https://doi.org/10.5194/soil-2020-69
https://doi.org/10.5194/soil-2020-69

  26 Oct 2020

26 Oct 2020

Review status: a revised version of this preprint was accepted for the journal SOIL and is expected to appear here in due course.

Continental-scale controls on soil organic carbon across sub-Saharan Africa

Sophie F. von Fromm1,2, Alison M. Hoyt1,3, Gifty E. Acquah4, Ermias Aynekulu5, Asmeret Asefaw Berhe6, Stephan M. Haefele4, Markus Lange1, Steve P. McGrath4, Keith D. Shepherd5, Andrew M. Sila5, Johan Six2, Erick K. Towett5, Susan E. Trumbore1, Tor-G. Vågen5, Elvis Weullow5, Leigh A. Winowiecki5, and Sebastian Doetterl2 Sophie F. von Fromm et al.
  • 1Department of Biogeochemical Processes, Max-Planck Institute for Biogeochemistry, Jena, Germany
  • 2Department of Environmental Systems Science, ETH Zurich, Zurich, Switzerland
  • 3Climate and Ecosystem Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA
  • 4Department of Sustainable Agriculture Sciences, Rothamsted Research, Harpenden, UK
  • 5World Agroforestry Centre (ICRAF), Nairobi, Kenya
  • 6Department of Live and Environmental Sciences, University of California Merced, Merced, CA, USA

Abstract. Earlier studies have demonstrated that soil texture and geochemistry strongly affect soil organic carbon (SOC) content. However, those findings primarily rely on data from temperate regions with soil mineralogy, weathering status and climatic conditions that generally differ from tropical and sub-tropical regions.

We investigated soil properties and climate variables influencing SOC concentrations across sub-Saharan Africa. A total of 1,601 samples were analyzed, collected from two depths (0–20 cm and 20–50 cm) at 45 sentinel sites from 17 countries as part of the Africa Soil Information Service (AfSIS) project. The dataset spans climatic conditions from arid to humid and includes soils with a wide range of pHH20 values, weathering status, soil texture, exchangeable cations, extractable metals and a variety of important land cover types.

The most important SOC predictors were identified by linear mixed effects models, regression trees and random forest models. Our results indicate that SOC is primarily controlled by aridity index (PET/MAP), exchangeable calcium (Caex) and oxalate-extractable aluminum (Alox); this was found across both depth intervals. Oxalate-extractable iron (Feox) emerged as the most important predictor for both depth intervals in the regression tree and random forest analyses. However, its influence on SOC concentrations was strong only below Feox concentrations of 0.25 wt %. This suggests that Feox can act as a pedogenic threshold – even on a continental scale. Across model-ling approaches, clay and fine silt content (< 8 µm) and land cover were not significant SOC pre-dictors, in contrast to common assumptions.

Our findings indicate that the key controlling factors of SOC across sub-Saharan Africa are similar to what has been reported for temperate regions – except for soil texture and vegetation cover. However, the strength and importance of the controlling factors vary across the environmental gradient we studied.

Sophie F. von Fromm et al.

 
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Status: closed
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Sophie F. von Fromm et al.

Sophie F. von Fromm et al.

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Short summary
(Sub)tropical soils are still understudied – especially on the African continent. We investigated soil properties and climate variables that influence soil organic carbon (SOC) concentrations across sub-Saharan Africa. Our findings indicate that the key SOC-controlling factors are similar to those for temperate regions – except for soil texture and vegetation cover. However, strength and importance of the controlling factors vary across the environmental gradient we studied.