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Dryad

Soil carbon is mostly grass-derived in tropical savannas, even under woody encroachment

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Sep 28, 2022 version files 176.25 KB
Feb 17, 2023 version files 146.30 KB

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. Because savanna SOC is also derived from grasses, this assumption may not reflect real changes in SOC under afforestation, but grass contributions to SOC and changes in SOC with increasing tree cover remain poorly synthesized. Here, we combine a case study from Kruger National Park, South Africa, with data synthesized from tropical savannas globally to show that grass-derived C constitutes more than half of total SOC to a soil depth of 1-meter, even in soils directly under trees. The largest SOC concentrations were associated with the largest grass contributions (> 70% of total SOC). Regionally and 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 1-meter profile only increased by a negligible and non-significant 6% (SE = 4%, n = 44). These results underscore the substantial contribution of grasses to SOC and the considerable uncertainty in SOC responses to increasing tree cover, challenging the widespread assumption that afforestation universally and substantially enhances SOC storage across tropical savannas.