We present a study of soil organic carbon (SOC) inventories and δ13C values for 625 soil cores collected from well-drained, coarse-textured soils in eight areas along a 1000 km moisture gradient from Southern Botswana, north into southern Zambia. The spatial distribution of trees and grass in the desert, savannah and woodland ecosystems along the transect control large systematic local variations in both SOC inventories and δ13C values. A stratified sampling approach was used to smooth this variability and obtain robust weighted-mean estimates for both parameters.
Weighted SOC inventories in the 0–5 cm interval of the soils range from 7 mg cm−2 in the driest area (mean annual precipitation, MAP=225 mm) to 41±12 mg cm−2 in the wettest area (MAP=910 mm). For the 0–30 cm interval, the inventories are 37.8 mg cm−2 for the driest region and 157±33 mg cm−2 for the wettest region. SOC inventories at intermediate sites increase as MAP increases to approximately 400–500 mm, but remain approximately constant thereafter. This plateau may be the result of feedbacks between MAP, fuel load and fire frequency.
Weighted δ13C values decrease linearly in both the 0–5 and 0–30 cm depth intervals as MAP increases. A value of –17.5±1.0‰ characterizes the driest areas, while a value of −25±0.7‰ characterizes the wettest area. The decrease in δ13C value with increasing MAP reflects an increasing dominance of C3 vegetation as MAP increases. SOC in the deeper soil (5–30 cm depth) is, on average, 0.4±0.3‰ enriched in 13C relative to SOC in the 0–5 cm interval.