Cadmium is a metal that has adverse human health impacts and is not essential for plant growth. New European Union food safety regulations for cadmium in cacao-based products presents a challenge to many cacao producers in Latin American and the Caribbean. Cadmium (Cd) levels in parts of this region are naturally high, rendering an estimated 50% of beans unsuitable for single-origin products. This threatens the sustainability of the system in terms of farmer livelihood and peace processes in countries like Colombia, where cacao is promoted as an alternative for illicit crops. Ideally, the Cd is bound to the soil particles so that it cannot be taken up by the plant. The design of strategies to achieve this, notably the use of soil amendments, is complicated by a lack of fundamental understanding of the chemical and biological processes in the soil-root interface that determine how much Cd is taken up under certain soil conditions. While ample studies exist on Cd uptake by temperate annual crops like wheat, tropical perennials are heavily understudied.

To close this knowledge gap, our research will elucidate three mechanisms that influence Cd uptake by cacao roots. First, the binding of Cd to the soil matrix in relation to soil properties will be quantified through laboratory analysis and geochemical modelling. Specifically, the soil organic matter is the major binding site for Cd and will be characterized for soils, amendments and soils + amendments. Second, we will study the actual uptake of Cd by cacao roots and the influence of prevalent soil conditions. Hence, the root surface will be conceptualized as an ‘adsorption site’ whose Cd uptake will be studied under different levels of acidity, organic carbon content and competing ions. Third, the plant roots actively influence the soil organic matter composition of their surroundings by producing root exudates as a strategy to overcome abiotic stress from nutrient deficiency. We will study the effect of phosphorous deficiency, which is common in tropical cacao-growing soils, on the uptake of Cd. Ultimately, the results of this study will contribute to the development of soil amendments that effectively lower bean cadmium levels in tropical woody perennials like cacao.