Soil organic C and total N pools in the Kalahari: potential impacts of climate change on C sequestration in savannas

AIMS: The impacts of climate change on carbon (C) and nitrogen (N) fluxes are not fully understood, especially in savanna ecosystems. This study aimed to assess the potential impacts of climate change in soil C and N pools in the savannas. METHODS: Soil organic carbon (SOC) and total nitrogen (TN) were quantified at four sites with consistent sandy soils along a precipitation gradient in the southern Africa savannas. Soils were sampled at 240 locations at each site at four depths: 10, 30, 70 and 120 cm. RESULTS: The driest site and the wettest site had the SOC content of 1,397 g m⁻² and 1,982 g m⁻², respectively. Mean C:N ratio was highest (26 ± 6) at the wettest site and lowest at the next-wettest (15 ± 2). Our results support the hypothesis of low nitrogen fixation in the area, particularly in the dry southern region. We found a significant spatial relationship between SOC/TN content and tree height or tree mass only in the driest. The Botswana Kalahari (BK) soil is estimated to contain about 0.99 × 10¹⁵ g C belowground of which about 92 % is SOC. Furthermore, under IPCC RCP4.5 and RCP8.5 carbon emission scenarios, total belowground C is predicted to drop to 0.92 and 0.86 × 10¹⁵ g C, respectively by the end of the century. CONCLUSIONS: Our results provide insight into soil C stocks in Kalahari savannas and suggest that savannas may be bigger stores of organic C than currently thought. The current climate change projections together with field measurements suggest that C storage in these savannas may decrease in the future as the region warms and dries into the next century.