Effect of fire and soil texture on soil carbon in a sub-humid savanna (Matopos, Zimbabwe)

We investigated the effects of changing fire regime on the stocks and isotopic composition of soil organic carbon (SOC) in a tropical savanna ecosystem at Matopos, Zimbabwe. Vegetation plots from both sandy and clay-rich soil types at this location have been subjected to fire frequencies ranging from annual burn to complete protection for the last 50 years. Gross variations in 0-5 cm SOC stocks and the δ13C value of SOC were predominantly related to soil texture, with carbon densities at the sandy sites being consistently 35-50% lower than those at comparable clay sites. Average 0-5 cm carbon densities for all the burnt plots were approximately 100 mg/cm2 and 50 mg/cm2, at the clay site and the sandy site, respectively. In both cases, lower fire frequencies had resulted in a ~10% increase; while higher fire frequencies had resulted in a ~10% decrease from these average values. Plots from which fire had been excluded experienced a 40% to 50% increase in carbon stocks in the 0-5 cm interval, compared with the average for the burned plots. There was a linear relationship between carbon density and δ13C value at both sandy and clay sites. This is controlled by the rate of delivery of C3- and C4-derived carbon to the SOC pool, by the differences in residence time for C3- and C4-derived carbon in the SOC pool (in turn controlled largely by fire frequency), and by soil texture. The distribution of carbon and 13C between size fractions is also controlled by soil texture and fire frequency. Increasing fire frequency results in a relative increase in fine particulate SOC and an increase in the δ13C value of SOC in all size fractions. Soil texture, on the other hand, controls the magnitude of the increases in both the abundance and the δ13C value of SOC in all size fractions. (C) 2000 Elsevier Science B.V.