Slocum et al. (2026) Elevated organic carbon in African Dark Earths is not exclusively attributable to pyrogenic organic matter

Identification

• Journal: The Science of The Total Environment
• Year: 2026
• Authors: Maura P. Slocum, Riley B. Barton, Sasha J. Wagner, Johannes Lehmann, Dawit Solomon, Edward Yeboah, Alain F. Plante
• DOI: 10.1016/j.scitotenv.2026.181481

Research Groups

• Department of Earth & Environmental Science, University of Pennsylvania, USA.
• Department of Earth & Environmental Sciences, Rensselaer Polytechnic Institute, USA.
• School of Integrative Plant Science, Cornell University, USA.
• International Livestock Research Institute (ILRI), Nairobi, Kenya.
• Council for Scientific and Industrial Research-Soil Research Institute (CSIR-SRI), Kumasi, Ghana.

Short Summary

This study quantifies the contribution of pyrogenic organic matter to the elevated carbon stocks in West African Dark Earths, finding that pyrogenic carbon accounts for approximately 65% of the carbon increase. The remaining enrichment consists of non-pyrogenic organic matter, suggesting that pyrogenic material promotes the stabilization and persistence of other organic carbon forms.

Objective

• To quantify the contribution of pyrogenic organic matter (pyOM) to total soil organic matter (SOM) in paired profiles of African Dark Earths (AfDE) and adjacent soils (AS).
• To examine the depth-dependent distribution of elevated SOM and pyOM (up to 100 cm).

Study Configuration

• Spatial Scale: 11 sites in West Africa, including 6 sites in northwest Liberia and 5 sites in Ghana (Ashanti and Brong-Ahafo regions).
• Temporal Scale: Soil samples were collected in 2011 from actively managed agricultural sites; laboratory analyses were conducted through 2025.

Methodology and Data

• Models and Methods used: Ramped combustion thermal analysis (25 °C to 900 °C) coupled with CO2 evolved gas analysis (CO2-EGA); peak deconvolution of thermograms using a 425 °C threshold; Benzenepolycarboxylic acid (BPCA) molecular marker analysis via HPLC-PDA.
• Data sources: 97 soil samples from paired dark earth (DE) and adjacent soil (AS) pedogenetic horizons; comparison with previously published hydropyrolysis (HyPy) data from the same sites.

Main Results

• Carbon Enrichment: Soil organic carbon (SOC) in DE surface samples was 2.2 times higher than in AS.
• Pyrogenic Contribution: The proportion of thermally stable C (pyC) was significantly higher in DE (39.7 ± 17.7%) than in AS (22.1 ± 17.9%).
• Quantitative Partitioning: PyC accounted for 65% of the total SOC increase in DE surface soils compared to adjacent soils.
• Depth Distribution: Elevated SOM and pyC persisted to depths of 100 cm in DE profiles, with pyC proportions often peaking at intermediate depths (50–100 cm).
• Methodological Comparison: Ramped combustion (EGA) captured a wider range of the pyC continuum (28.9% of SOC) compared to HyPy (13.0%) and BPCA (4.8%).
• Non-Pyrogenic Stabilization: Non-pyC concentrations in DE (22.9 ± 7.5 mg/g) were significantly higher than total SOC in AS (16.2 ± 8.1 mg/g), indicating that pyC facilitates the accumulation of non-pyrogenic organic matter.

Contributions

• Challenges the exclusive attribution of Dark Earth fertility and carbon storage to pyrogenic matter alone.
• Provides evidence for the "synergistic effect" where pyrogenic carbon enhances the stabilization of non-pyrogenic organic matter in tropical soils.
• Offers a scientific rationale for using biochar as a soil amendment to not only sequester stable carbon but also to increase the persistence of existing soil organic matter.

Funding

• Holman Africa Research and Engagement Fund (Penn Global Research and Engagement Grant Program at the University of Pennsylvania).

Citation

@article{Slocum2026Elevated,
  author = {Slocum, Maura P. and Barton, Riley B. and Wagner, Sasha J. and Lehmann, Johannes and Solomon, Dawit and Yeboah, Edward and Plante, Alain F.},
  title = {Elevated organic carbon in African Dark Earths is not exclusively attributable to pyrogenic organic matter},
  journal = {The Science of The Total Environment},
  year = {2026},
  doi = {10.1016/j.scitotenv.2026.181481},
  url = {https://doi.org/10.1016/j.scitotenv.2026.181481}
}

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