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 isoto pic composition of soil organic carbon (SOC) in a tropical savanna ecosyste m at Matopos, Zimbabwe. Vegetation plots from both sandy and clay-rich soil types at this location have been subjected to fire frequencies ranging fro m annual burn to complete protection for the last 50 years. Gross variation s in 0-5 cm SOC stocks and the delta(13)C value of SOC were predominantly r elated to soil texture, with carbon densities at the sandy sires being cons istently 35-50% lower than those at comparable clay sites. Average 0-5 cm c arbon densities for all the burnt plots were approximately 100 mg/cm(2) and 50 mg/cm(2), at the clay site and the sandy site, respectively. In both ca ses, lower fire frequencies had resulted in a similar to 10% increase, whil e higher fire frequencies had resulted in a similar to 10% decrease from th ese average values. Plots from which fire had been excluded experienced a 4 0% to 50% increase in carbon stocks in the 0-5 cm interval, compared with t he average for the burned plots. There was a linear relationship between ca rbon density and delta(13)C value at both sandy and clay sites. This is con trolled by the rate of delivery of C-3- and C-4-derived carbon to the SOC p ool, by the differences in residence time for C-3- and C-4-derived carbon i n the SOC pool (in turn controlled largely by fire frequency), and by soil texture. The distribution of carbon and C-13 between size fractions is also controlled by soil texture and fire frequency. Increasing fire frequency r esults in a relative increase in fine particulate SOC and an increase in th e delta(13)C value of SOC in all size fractions. Soil texture, on the other hand, controls the magnitude of the increases in both the abundance and th e delta(13)C value of SOC in all size fractions. (C) 2000 Elsevier Science B.V. All rights reserved.