The Root Cause of Soiled Cacao – A mechanistic understanding of biochemical soil-root interactions for low-cadmium sustainable cacao production

The sustainable production of cacao (Theobroma cacao) in Latin America and the Caribbean (LAC) is threatened by stricter European Union food safety regulations on cadmium levels in chocolate. Cadmium, a non-essential element for plants with adverse human health impacts, occurs in LAC cacao beans at levels that regularly exceed limits acceptable to buyers (e.g. 50% of Ecuadorian beans). Cadmium threatens the sustainability of cacao production systems through its impacts on the livelihood of smallholder farmers and on the security in countries affected by drug-related conflict such as Colombia, where cacao is promoted as a viable alternative to replace illicit crops. The uptake of cadmium (Cd) from the soil by cacao is regulated by biological and chemical soil properties and soil-plant interactions, with an important role for soil organic matter (SOM) as a major factor determining available Cd. However, integrated knowledge of the biochemical mechanisms that govern metal uptake is lacking, especially for tropical perennial crops. Therefore, the overall aim of the proposed research is to obtain a fundamental understanding of soil-root mechanisms that govern Cd uptake by cacao. We hypothesize that the uptake depends on (1) SOM composition of the soil and organic amendments, (2) diffusion to the roots followed by internalization, and (3) root exudates that modify the dissolved organic matter (DOM) composition in the rhizosphere in response to nutrient deficiency. We will first quantify the carboxylic and phenolic functional groups in humic and fulvic acids of SOM and DOM in soils, amendments and soils + amendments. These data will be used to parameterize a multi-surface geochemical model to predict Cd availability. Second, the model will be extended to include Cd uptake by the roots, which can be limited by either diffusion or internalization; this will be measured in hydroponic experiments. Third, pot experiments will be used to demonstrate the effect of root exudates on Cd uptake as a result of phosphorous deficiency. Validation of the integrated Cd uptake model will be done by sampling roots in the field in a unique network of field trials in South America dedicated to studying cacao bean Cd reduction strategies. Ultimately the obtained results will allow us to design effective soil amendments to manipulate the Cd uptake by cacao roots, thus contributing to sustainable LAC cacao production.