Trace element contamination in Ecuadorian soils and cacao beans: focus on Cd availability and soil-to-bean transfer

For several years now, cadmium (Cd) was found to be readily transferred from soil to cacao beans, especially in Andean countries. Cacao by-products can then contain excessive metal concentrations that do not comply with the European health standards. This research aims to assess trace elements (TE) contamination in cacao orchards, with a focus on Cd levels in three coastal Ecuadorian provinces to identify the drivers of availability and soil-to-beans transfer. In our study, 90 paired-samples of soil and cacao beans were collected in 15 farms of Esmeraldas, Manabí and Guayas provinces and analyzed by Inductively Coupled Plasma Mass Spectrometry to assess TE concentrations. Results show that Cd concentrations are heterogeneous among provinces with Esmeraldas and Guayas being Cd hot-spots but also at the farm scale with high coefficient of variation of 62 and 82% for soil and cocoa beans, respectively. In addition, the drivers of Cd availability in soil are site-dependent, and pH and cationic exchange capacity (CEC) are significant parameters in Esmeraldas and Guayas, respectively. Also, soil CEC, copper and zinc concentrations seem to control Cd accumulation in beans. Beyond Cd, the province of Guayas shows concerning Pb and As concentrations in soils while Esmeraldas appears as a Ni hot-spot for soil and beans samples highlighting the importance to monitor those elements, especially Ni as it is now under new European regulations. Finally, we propose that mitigation techniques through fertilization or amendment application can be applied, being site-specific, to ensure suitable and sustainable recommendations to cocoa farmers.