By Eleonora Colzani, IAEA Office of Public Information and Communication
As import regulations of the European Union changed in 2019, Panama’s cocoa exports came under threat: exporters had to prove that the amount of cadmium in their products was below the EU’s legal threshold of 0.8 milligram per kilogramme. Isotopic techniques can help identify the source and movement of the metal and thereby help farmers prevent contamination in the fruit – increasing food safety and restoring exports.
The province of Bocas del Toro, in northwestern Panama, is home to 90% of the country’s gourmet-quality organic white cocoa. Here, farmers grow cocoa trees using traditional methods, in harmony with the surrounding nature. The soil of most cocoa-growing areas, however, is naturally rich in cadmium, putting at risk the quality of the beans and the livelihood of about 1,400 farmers and their families. “The export of cocoa beans represents an important source of income that helps alleviate poverty in the province,” said researcher at the Agricultural Research Institute of Panama (IDIAP) Jose Ezequiel Villarreal.
Experts from IDIAP are working to improve soil management practices and the overall quality of organic cocoa. The IAEA, in cooperation with the Food and Agriculture Organization of the United Nations (FAO), assists the Panamanian experts to characterize and map out the cadmium contents in soils where cocoa is cultivated. They then correlate the cadmium in the soils with the cadmium content in the cocoa fruits to calculate how much metal ended up in the fruits.
The Joint FAO/IAEA Centre of Nuclear Techniques in Food and Agriculture also provided training to local experts and farmers to ensure that more of the cadmium stays in the soil. This can be achieved by adding organic amendments to the soil; these help to increase organic matter and the pH of soils and lock in cadmium so that it is no longer available for absorption by the trees and their fruits.
“The isotopes help determine the extent to which cadmium is absorbable by the plant – a condition known as bioavailability – and trace its movement from the soil and through the plant,” explained Integrated Soil Fertility Management Specialist at the Joint FAO/IAEA Centre of Nuclear Techniques for Food and Agriculture, Joseph Adu-Gyamfi. Understanding this process is the first step towards reducing cadmium intake by the trees.
Engineer at the Agricultural Research Institute of Panama (IDIAP), Adolfo Santo,meets with a group of farmers to explain the purpose of sampling the soil, leaves and cocoa beans to measure cadmium concentration. With the help of isotopic techniques, experts are now making steps towards increasing the safety of Panamanian cocoa beans. (Photo: Ministry of Agricultural Development, Panama)
Cadmium isotopes, when added to the soil, allow the scientists to pinpoint the source of the metal to fertilizers and to the soil itself. This allows them to trace the metal to see how much of it moves from the soil to the fruit, or if it moves at all (see Using isotopes to trace heavy metals in the soil).
The IAEA has trained local experts on how to use cadmium isotopes as tracers to evaluate and select cadmium-avoiding cocoa variants. Moreover, about 150 farmers have already been trained on agricultural practices that prevent the cadmium in the soil from absorption by the plants.
In the following months, more farmers will be trained. As Karla Molina Diaz, Project Management Officer at the IAEA, said: “We are using science, nuclear science, to make a developmental impact and helping the participating farmers earn a stable and growing income.”
The Science
Using isotopes to trace heavy metals in the soil
Cadmium, a metal that is detrimental to the health of multiple organs when ingested, could be absorbed from the soil by plants, depending on its bioavailability. The passage of the metal from the soil to the various parts of the plant – roots, trunk, branches and fruits — does not always follow a specific pattern. Stable isotopes, which are non-radioactive forms of atoms, can be used as tracers to monitor the uptake of nutrients/metals from the soil and the transport to different parts, thanks to their unique characteristics: not only the isotopic signature of each nutrient can be a fingerprint of the source of cadmium, but it also allows the tracing of the metal’s movement in the tree. Using fertilizers labelled with cadmium-114 (114Cd) — compared with the ‘normal’ cadmium (cadmium-110) — scientists can track the isotopes to determine the movement of cadmium from the soil to the cocoa fruit. This is because they have the same chemical properties as “regular” atoms but have a heavier nucleus and therefore can be distinguished and followed. The technique is used to evaluate and select cocoa plants that that can be grown in soil with high cadmium content but with less of the metal moving to the fruit.
