Enhancing Soil Organic Carbon Storage to Meet the Objectives of “4 per 1000 Initiative” in Central and Western African Ecosystems: a review

Unsustainable land use practices in Central and West African Ecosystems (CWAEs) have resulted in a substantial decrease in soil organic carbon (SOC) and an increase in soil erosion in these ecosystems, with severe repercussions on climate, biodiversity and food security. However, the potential for SOC accretion as a nature-based solutions (NbS) strategy in CWAEs is scarcely understood. A systematic review of 77 articles published from 1990 to 2024, extracted from three electronic databases, was conducted to fill this knowledge gap while stressing other proven efficient SOC accretion land management strategies. SOC stocks at 0–25 cm depth increased by up to 0.8 Mg C ha−1 yr−1 in monocultures and 1.9 Mg C ha−1 yr−1 in mixed stands of nitrogen-fixing trees (NFTs) and non-NFTs compared to seven-year-old afforested stands of non-NFTs in central Africa. Tree-based plantations (i.e., Faidherbia albida, Piliostigma reticulatum) increased SOC stocks by 1–1.5-fold in Sahelian degraded lands of western Africa. Mixed stands of Cajanus cajan and Chromolaena odorata increased SOC stocks by 17 % at 0–10 cm depth after 2 years in Guinea savanna. SOC accretion was found to depend on tree presence and species, climatic and edaphic conditions, and management practices. In Sahelian ecosystems where tree density is low, tree planting should be recommended. In wetter and more forested zones, herbaceous legume-based fallows and planting mixtures of tree species, including NFS is recommended. Nature-based solutions have great potential for enhancing SOC stocks in the CWAEs. Despite biodiversity and ecological issues associated with afforestation of natural savanna ecosystems, the intervention proved to be an effective means of increasing SOC storage in the poorly structured savanna soils. All these strategies should be further evaluated under more diverse local socioeconomic conditions. The paper also summarizes knowledge gaps and suggests future research and policy actions to enhance the use of NbS in addressing societal challenges.