Elevated organic carbon in African Dark Earths is not exclusively attributable to pyrogenic organic matter

Anthropogenic Dark Earths are characterized by dark colors and high organic carbon, frequently attributed to the presence of pyrogenic organic matter. We sought to quantify the contribution of pyrogenic organic matter to total soil organic matter (SOM) in paired deep soil profiles of dark earths and adjacent soils from sites in Ghana and Liberia by examining the depth to which elevated SOM occurs in dark earths as well as the contribution of pyrogenic organic matter as a function of depth. Thermally stable C was quantified using ramped combustion thermal analysis coupled to CO2 evolved gas analysis. The proportion of thermally stable C to total soil organic C (SOC) was nearly twice as large in dark earth samples (39.7 ± 17.7%) compared to adjacent soils (22.1 ± 17.9%), suggesting that thermally stable organic matter may be contributing to the persistence of SOM in dark earths, even at depths of up to 100 cm. Proportions of thermally stable C to total SOC in surface soils were substantially greater than pyrogenic C to SOC proportions measured using benzenepolycarboxylic acid (BPCA) markers and previously published values using hydropyrolysis. While the thermal method captures a wider range of pyrogenic organic matter than hydropyrolysis and BPCA, the latter methods were well correlated with thermograms at temperatures >425 °C. Calculated non-pyC concentrations in dark earth samples determined by ramped combustion often exceeded total SOC concentrations in adjacent soil samples, suggesting that C enrichment in dark earths was not attributable exclusively to pyrogenic C. Pyrogenic C accounted for 65% of the increase in total SOC in dark earths versus adjacent surface soils. These results suggest that the presence of pyrogenic C may promote greater persistence of non-pyrogenic SOM, thus providing a compelling rationale for the use of biochar as a carbon-enriching amendment in contemporary agricultural systems.