Soil carbon in tropical savannas mostly derived from grasses

Grasses contribute more than half of the soil organic carbon across tropical savannas, according to a case study in South Africa combined with a synthesis of data from tropical savannas globally. Tropical savannas have been increasingly targeted for carbon sequestration by afforestation, assuming large gains in soil organic carbon (SOC) with increasing tree cover. Because savanna SOC is also derived from grasses, this assumption may not reflect real changes in SOC under afforestation. However, the exact contribution of grasses to SOC and the changes in SOC with increasing tree cover remain poorly understood. Here we combine a case study from Kruger National Park, South Africa, with data synthesized from tropical savannas globally to show that grass-derived carbon constitutes more than half of total SOC to a soil depth of 1 m, even in soils directly under trees. The largest SOC concentrations were associated with the largest grass contributions (>70% of total SOC). Across the tropics, SOC concentration was not explained by tree cover. Both SOC gain and loss were observed following increasing tree cover, and on average SOC storage within a 1-m profile only increased by 6% (s.e. = 4%, n = 44). These results underscore the substantial contribution of grasses to SOC and the considerable uncertainty in SOC responses to increasing tree cover across tropical savannas.

Automated summary

A case study in South Africa combined with a synthesis of global data on tropical savannas shows that grasses contribute more than half of the soil organic carbon (SOC) across these regions. The assumption that increasing tree cover leads to significant gains in SOC may not reflect the actual changes, as SOC in savannas is also derived from grasses. The study highlights the substantial contribution of grasses to SOC and the uncertainty in SOC responses to increasing tree cover in tropical savannas.