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

von Fromm, Sophie F.; Hoyt, Alison M.; Acquah, Gifty E.; Aynekulu, Ermias; Berhe, Asmeret Asefaw; Haefele, Stephan M.; Lange, Markus; McGrath, Steve P.; Shepherd, Keith D.; Sila, Andrew M.; Six, Johan; Towett, Erick K.; Trumbore, Susan E.; Vågen, Tor-G.; Weullow, Elvis; Winowiecki, Leigh A.; Doetterl, Sebastian

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

Preprints

https://doi.org/10.5194/soil-2020-69
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Preprints

26 Oct 2020

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

Sophie F. von Fromm^1,2, Alison M. Hoyt^1,3, 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

Received: 19 Oct 2020 – Accepted for review: 23 Oct 2020 – Discussion started: 26 Oct 2020

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 pH_H20 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 (Ca_ex) and oxalate-extractable aluminum (Al_ox); this was found across both depth intervals. Oxalate-extractable iron (Fe_ox) 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 Fe_ox concentrations of 0.25 wt %. This suggests that Fe_ox 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.