Shade tree diversity enhances topsoil fertility and deep soil nutrients drive aboveground carbon storage in cocoa-based systems in Côte d’Ivoire

This study investigated the impact of tree diversity on soil fertility and aboveground carbon biomass in cocoa-based land-use systems across Côte d'Ivoire's cocoa-producing regions. A total of 213 plots, each measuring 1,000 m² and representing cocoa monocultures, agroforestry systems, and fallows, were surveyed across six sites. These plots were analyzed for tree species diversity and aboveground carbon biomass. Soil samples were collected at depths of 0–20 cm and 20–50 cm from each plot and analyzed for various soil properties. Results showed that tree species richness and abundance were highest in fallows, followed by agroforestry systems, with the lowest diversity observed in monocropped cocoa farms. Aboveground carbon biomass was comparable between agroforestry systems and fallows, both significantly outperforming cocoa monocultures in carbon storage. Although 15 soil physicochemical parameters did not vary significantly across land-use systems, positive correlations were identified between tree abundance and soil potassium, aboveground carbon biomass and soil pH, and marginally between species richness and exchangeable magnesium. Negative correlations emerged between aboveground carbon biomass and soil aluminum content, as well as nitrogen and phosphorus in deeper soil layers. Cocoa monocultures, particularly in San-Pédro, displayed the lowest species diversity and aboveground carbon biomass. Agroforestry systems in Duékoué and San-Pédro exhibited moderate species diversity and carbon levels, with an abundance of exotic species and lower sand content. In contrast, fallows, predominantly in Oumé, recorded the highest tree species richness, aboveground carbon biomass, native tree dominance, and superior soil fertility (elevated pH, CEC, Mg, Ca, K) in the 0–20 cm soil layer. Tree diversity and aboveground carbon biomass significantly correlated with land-use system and their effects on soil properties varied with soil depth. These findings highlight the role of shade tree diversity in improving topsoil fertility, which benefits cocoa plants, while nutrients from deeper soil layers support shade trees, enhancing aboveground carbon biomass