Assessing the potential of biochar and charcoal to improve soil hydraulic properties in the humid Ethiopian Highlands: The Anjeni watershed

• Effect of charcoal and biochar amendments were studied in the Ethiopian highlands.
• Wood charcoal increased permeability of soils near saturation (tension < 100 kPa).
• Wood charcoal reduced moisture retention at similar tensions (< 100 kPa).
• Wood biochar had a similar effect as wood charcoal, but results were not significant.
• Nonwood biochar (corn stover) had positive or no effect on moisture retention.

Biochar has shown promise for restoring soil hydraulic properties. However, biochar production could be expensive in the developing world, while charcoal is widely available and cheap. The objective of this study is therefore to investigate whether some of the charcoal made in developing countries can also be beneficial for improving soil hydraulic properties, and explore whether charcoal could potentially restore the degraded African soils. Laboratory and field experiments were conducted in the Anjeni watershed in the Ethiopian highlands, to measure soil physical properties including soil moisture retention and infiltration rates. Soils were dominantly clayey with pH in the acidic range, low organic carbon content, and steady infiltration rates ranging between 2 and 36 mm/h. Incorporation of woody feedstock (Acacia, Croton, and Eucalyptus) charcoals significantly decreased moisture retention at lower tensions (10 and 30 kPa), resulting in an increase in relative hydraulic conductivity coefficients at these tensions. While wood (oak) biochar decreased moisture retention at low tensions, corn biochar increased retention, but effects were only slight and not significant. Surprisingly, available water content was not significantly affected by any of the amendments. Overall findings suggest that wood charcoal amendments can improve soil hydraulic properties of degraded soils, thereby potentially reducing runoff and erosion.