Finalist SRA 2019: Hester – Lime (CaCO3): A way to prevent cadmium in our beloved cacao?

Hester Blommaert

We, Belgians, are known for our large consumption of chocolate. Some concerns have arisen on the intake of chocolate because the so-called ‘food of the gods’ may contain cadmium (Cd), a toxic heavy metal. To protect chocolate consumers, the European Commission entered new threshold limits into force for Cd in chocolate. These limits may largely affect cacao producers in Latin America as the natural background concentrations of cadmium in the young soils of the Andes are high, resulting in the presence of cadmium in cacao beans. Hence, a major threat looms over the Latin-American cacao sector. Consequently, a great urgency has risen to find strategies to reduce cadmium in cacao. A promising method is the application of lime (CaCO3) on the soil, which is the focus of this thesis research.

Lime is a traditional soil amendment which increases soil pH, making Cd less available for the plant. In the past, this soil amendment has shown its efficiency in reducing cadmium in various crops. However, in cacao plantations, lime was often not capable to establish this reduction. The relationship between root morphology and Cd uptake is unknown in cacao, so it was hypothesized that the deep penetrating roots of cacao were increasing the absorption of Cd when lime was applied on the surface soil. Hence, to find a liming strategy for Latin-American farmers, it was of striking importance to treasure which rooting structures (the surface roots or the deeper penetrating roots) are most responsible for the uptake of cadmium, being the ambition of this thesis research.

The thesis research investigated, with the stable isotope 108Cd, the effect of the application of lime (CaCO3) on the uptake of Cd, taking into account the vertical heterogeneity of the cacao roots (surface versus deep roots) and soil types (surface versus subsoil).

The first –and most exciting- part of the research was performed in Ecuador. We set up 72 (!) pots with cacao seedlings that were grown for four months. With the use of a stable isotope of cadmium, we could trace where the cadmium in the seedlings was originating from. We figured out that there is indeed a difference in the uptake of cadmium from the two rooting structures. In normal conditions, the roots settle more in the nutrient-rich surface soil and consequently, the uptake of Cd is higher in this surface soil. When lime is applied to the surface soil, the total Cd in the plant decreases, which is the desired result. Nonetheless, the decrease is less than expected since the uptake from the deeper roots slightly increases.

Hence, lime can be a way to reduce Cd in cacao, however the efficiency may not always be as desired because it induces more uptake of Cd by the deeper roots. Consequently, the development of other strategies to reduce Cd in the ‘food of the gods’ is crucial to sustain the Latin American cacao sector.

Wil je Hester aan het werk zien? Kom dan naar de finale van de VBI Student Research Awards op 14 oktober in de Aula van de 2e Hoofdwet in Leuven en help haar de hoofdprijs te bemachtigen! Meer info