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-k-_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 gm) 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 centennia• timesca,es. OREC abundance and carbon-isotope data suggest that OREC in coarse particles is progressively degraded into fmer particle sizes, with a concomitant increase in resistance to oxidafive degradation of OREC in the fmer 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.

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Tree crowns in semi-arid areas have often been found to create micro-habitats of improved soil physical and nutrient status and reduced evapotranspiration. These encourage the growth of mesic, palatable and high yielding perennial grasses in otherwise arid environments. A two-year study, in areas with a long history of light grazing, at Matopos Research Station sought to establish whether the soil nutrient status and micro-climate under tree crowns are different from those in open areas, and whether the differences are affected by soil type and tree species. The study trees were Terminalia sericea and Acacia karroo. There were consistent patterns in soil nutrient status between understorey and open areas, although these patterns were largely not significant. There was also a consistent pattern in soil volumetric water content of open areas > T. sericea > A. karroo understorey areas. Tree crowns reduced solar radiation reaching understorey areas by an average of 88% and rainfall by an average of 15%. The findings on soil nutrient status are consistent with results from disturbed systems, and challenge the dogma, at least for soil nutrient status, that conservative stocking rates are beneficial.

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