Soil carbon in tropical savannas mostly derived from grasses

Abstract

Tropical savannas have been increasingly targeted for carbon (C) sequestration from afforestation, assuming large gains in soil organic C (SOC) with increasing tree cover. This assumption may not reflect real changes in SOC under afforestation, because savanna SOC is also derived from grasses, but grass contributions to SOC and SOC changes with increasing tree cover remain poorly synthesized. Here, we combine a case study from Kruger, in South Africa, with data synthesized across tropical savannas from 148 soil profiles to 1-meter depth to show that grass-derived C constitutes more than half of total SOC, even in soils directly under trees. The largest SOC concentrations were associated with the largest grass contributions (> 70% of total SOC). Regionally or 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 only increased by a negligible 6%. These results underscore the substantial contribution of grasses to SOC and the considerable uncertainty inherent in SOC responses to increasing tree cover, challenging the widespread assumption that afforestation universally and substantially enhances SOC storage across tropical savannas.