Cadmium uptake by cocoa trees in agroforestry and monoculture systems under conventional and organic management

摘要

Cadmium (Cd) uptake by cocoa has recently attracted attention, after the European Union (EU) decided to establish values for tolerable Cd concentrations in cocoa products. Bean Cd concentrations from some cocoa provenances, especially from Latin America, were found to exceed these values. Cadmium uptake by cocoa is expected not only to depend on a variety of soil factors, but also on plant and management factors. In this study, we investigated the influence of different production systems on Cd uptake by cocoa in a long-term field trial in the Alto Beni Region of Bolivia, where cocoa trees are grown in monocultures and in agroforestry systems, both under organic and conventional management. Leaf, fruits and roots of two cultivars were sampled from each production system along with soil samples collected around these trees. Leaf, pod husk and bean samples were analysed for Cd, iron (Fe) and zinc (Zn), the roots for mycorrhizal abundance and the soil samples for 'total' and 'available' Cd, Fe and Zn as well as DGT-available Cd and Zn, pH, organic matter, texture, 'available' phosphorus (P) and potassium (K). Only a small part of the variance in bean and pod husk Cd was explained by management, soil and plant factors. Furthermore, the production systems and cultivars alone had no significant influence on leaf Cd. However, we found lower Cd leaf contents in agroforestry systems than in monocultures when analysed in combination with DGT-available soil Cd, cocoa cultivar and soil organic matter. Overall, this model explained 60% of the variance of the leaf Cd concentrations. We explain lower leaf Cd concentrations in agroforestry systems by competition for Cd uptake with other plants. The cultivar effect may be explained by cultivar specific uptake capacities or by a growth effect translating into different uptake rates, as the cultivars were of different size.