Rainfall amplifies land-use impacts on soil carbon pools across Southern Africa

Abstract

In Southern Africa, native savanna woodlands are increasingly threatened by climate change and anthropogenic land-use pressures, with largely unexplored interactions for soil organic carbon (SOC) losses. Here, we quantified long-term changes in SOC stocks following conversion of savanna woodlands to cropland and assessed the reversibility under fallow. We hypothesized that land-use-induced SOC changes depend on climate and reference soil groups (RSGs) in a pool-specific manner, and that additional anthropogenic disturbances at savanna woodland sites attenuate climate responses. To test these hypotheses, we analyzed SOC, particulate, and mineral-associated SOC across adjacent savanna woodlands, cropland, and fallow fields along a climatic gradient spanning mean annual precipitation (MAP) values of 365–1227 mm and mean annual temperature (MAT) values of 20–24°C in Zambia and Mozambique, using stratified depth intervals. Conversion to cropland (>10 years) reduced SOC stocks in the 0–20 cm layer by 20–26%, with RSG-specific losses of 6.3, 4.2, and 3.7 t C ha −1 in Lixisols, Arenosols, and Acrisols, respectively, while the subsoils were less affected. When upscaled to maize-based croplands in Southern Africa, topsoil losses amounted to 0.06 Gt C. After an average of 20 years, only approximately 0.004 Gt C (≈7% of losses) could be offset under natural fallow due to natural revegetation processes. In Arenosols and Lixisols, SOC losses were largely attributable to reductions in particulate organic matter, and became more pronounced as MAP increased. MAT had no discernible effect. Restricting the analyses to savanna woodland sites with low anthropogenic disturbance strengthened SOC-MAP relationships, with MAP explaining up to 67% of the variability in SOC loss associated with conversion to cropland.