Metallic trace element concentrations, vertical patterns and environmental implications in soils under the "cabruca" agroforestry system of Brazil's largest cocoa-producing region.

The occurrence of elevated concentrations of metallic trace elements (MTEs) in chocolate and cocoa-derived products is a growing global concern. Although agroforestry systems such as cocoa-cabruca (AS-CC) are widely regarded as sustainable, their soils have rarely been evaluated regarding metal-related risks and geogenic or anthropogenic origins. This study was conducted in the largest cocoa-producing region of Brazil, a high-productivity area where monitoring MTEs is essential due to their persistence and potential toxicity in soils. The objectives were to quantify MTE concentrations and vertical distribution in AS-CC soils, assess contamination and ecological risks using geochemical and ecological indices (Igeo, CF, EF, ERᵢ, and PERI), and evaluate potential anthropogenic influences. Cd, Co, Cr, Cu, Mo, Ni, Pb, V, Zn, and Fe were analyzed across AS-CC farms distributed among ten municipalities. A total of 50 composite soil were collected at depths of 0-5, 5-10, 10-20, 20-40, and 40-60 cm. Soil fertility and texture were determined, MTEs were extracted following EPA Method 3051A, and concentrations were measured by ICP-OES. Mean concentrations of Co, Cu, Mo, Pb, and Zn slightly exceeded local soil quality reference values, whereas Cd, Cr, Ni, V, and Fe remained below established thresholds. The Igeo classified 88% of surface samples (0-5 cm) as unpolluted to moderately polluted, and EF values indicated negligible anthropogenic enrichment. Moderate ecological risk was observed only for Cd, while PERI values reflected low overall ecological risk, although elevated Mo and Cd warrant targeted monitoring. Comparable or higher concentrations of Cd, Cr, V, Zn, and Fe at depth reflect the influence of parent materials such as granite, diorite, and basalt. Lithological diversity thus provides a key geochemical framework explaining vertical and spatial variability, reinforcing the importance of integrated, multi-index approaches to guide sustainable soil management in tropical cocoa agroforestry systems.