Soil texture prevails over vegetation change in determining soil organic carbon storage in an African savanna

Tropical savannas play a significant role in the global carbon cycle, yet they are increasingly being targeted for carbon sequestration through afforestation, assuming that tree planting will substantially boost carbon stocks in both biomass and soils. However, the response of soil organic carbon (SOC) to afforestation in these ecosystems remains highly uncertain, and the mechanisms driving this variability are not well understood. The dynamic boundaries between savannas and forests provide an ideal setting to study afforestation impacts on soil carbon storage and stability. Here, we sampled surface soils (0–15 cm) across six savanna–forest boundaries in Hluhluwe-iMfolozi Park, South Africa, and analysed patterns of SOC stocks, their particulate and mineral-associated organic carbon (POC and MAOC) and pyrogenic carbon (PyC) pools. Savanna and forest soils had comparable stocks of SOC, POC, MAOC and PyC, as well as similar proportions of POC and MAOC relative to SOC, except for the PyC proportion, which was significantly higher in savannas than in forests (p = 0.012). However, savanna-to-forest transitions induced a substantial turnover in carbon sources, with C₃-derived carbon (mostly from trees) increasing from 55% to 99% in the POC pool and from 32% to 69% in the MAOC pool, thereby nearly eliminating legacy C₄-grass-derived carbon in the POC. Additionally, although savanna-to-forest transitions had limited influence on all measured carbon pool sizes, these nonetheless varied substantially across transects. Specifically, stocks of SOC and MAOC (the dominant pool) were largely driven by differences in soil clay and silt content across the landscape. Synthesis. Contrary to the expectation that savanna-to-forest transitions would increase POC pools and subsequent SOC stocks, we found that MAOC remained the dominant contributor to SOC at this site, driven primarily by local variation in soil clay and silt content. While further studies are needed to test the generality of this pattern across tropical savannas, these findings highlight that soil texture may override the influence of vegetation change on soil carbon formation and persistence, and increasing tree cover may not guarantee soil carbon gains during savanna-to-forest transitions.