Stability of elemental carbon in a savanna soil

Abstract

We have investigated the stability of oxidation‐resistant elemental carbon (OREC) in a sandy savanna soil at the Matopos fire trial site, Zimbabwe. The protection of some soil plots from fire for the last 50 years at this site has enabled a comparison of OREC abundances between those plots which have been protected from fire and plots which have continued to be burnt. The total 0–5 cm OREC inventory of the soil protected from fire is estimated to be 2.0±0.5 mg cm−2; approximately half the “natural” OREC inventory at the study site of 3.8±0.5 mg cm−2 (the mean for plots burnt every 1–5 years). The associated half‐life for natural OREC loss from the 0–5 cm interval of the protected plots is calculated to be <100 years, with the half‐life for large carbonized particles (>2000 μm) in the soil being considerably <50 years. These results suggest that at least in well‐aerated tropical soil environments, charcoal and OREC can be can be significantly degraded on decadal to centennial timescales. OREC abundance and carbon‐isotope data suggest that OREC in coarse particles is progressively degraded into finer particle sizes, with a concomitant increase in resistance to oxidative degradation of OREC in the finer particle sizes due to the progressive loss of more readily degraded OREC. It remains unclear whether the OREC that is degraded is oxidized completely to CO2 and subsequently emitted from the soil, reduced to a sufficiently small particle size to be illuviated to deeper parts of the soil profile, solubilized and lost from the profile as dissolved organic carbon or transmuted into a chemical form which is susceptible to attack by the acid‐dichromate reagent. The conclusion that a significant proportion of OREC can undergo natural degradation in well‐aerated environments on decadal/centennial timescales suggests that only a fraction of the total production of OREC from biomass burning and fossil fuel combustion is likely to be sequestered in the slow‐cycling “geological” carbon reservoir.