Mitigating cadmium uptake in cacao (Theobroma cacao L.) with biochar made of local feedstocks in the Peruvian Amazon

Latin America and the Caribbean (LAC) is the world’s leading producer of fine flavour cocoa and contributes 17% of the global cocoa production, which is mainly produced by resource-poor smallholder farmers in Andean countries (Peru, Ecuador, Colombia). Recent surveys have shown that cacao originated in LAC have relatively high concentrations of cadmium (Cd,) which is unrelated to soil pollution.  New restrictions on Cd content in chocolate are limiting farmers access to important international markets, hence threatening their livelihoods. One of the possibly most promising mitigation methods for reducing Cd uptake in cacao beans is the use of biochar, that is the organic material which is incinerated through pyrolysis. Biochar can be used as a soil amendment in existing and new cacao plantations. Biochar has been used for remediation of Cd contaminated soils due to its sorption capacity and liming effect. There is, however, little understanding under which conditions of soils or feedstock the biochar has a beneficial effect on reducing Cd uptake beyond is mere effect of liming. Indeed, biochar properties are largely dependent on the feedstock material used for its preparation and the methods used. For an effective use of biochar as soil amendment it is important to investigate and identify the most appropriate feedstocks available in local waste streams and that biochar production methods are fine-tuned to maximize their sorption capacity for Cd. Possibly, the presence of sufficient functional groups, in turn related to its oxidation and to the presence of Sulphur groups, is a key property. Among possible feedstock are cacao pods, leaf and petioles of oil palm (Elaeis guineensis) and asai (Euterpe oleracea Mart), outer shell of sacha inchi (Plukenetia volubilis), husk of rice (Oryza sativa), and different types of animal biowaste. The main objective of this research is to identify the most cost-efficient and effective biochar preparations to mitigate Cd accumulation in cacao in Peruvian soils and to understand the mechanisms of Cd immobilization in soil. To this end, this work will evaluate bioavailable Cd in factorial combinations of different soils amended with different biochar preparations using Diffusive Gradient in-Thin film (DGT) devices, DGT has been shown to indicate Cd availability to cacao. Additionally, biochar charge, acid neutralizing capacity and functional groups will be characterized by potentiometric titration and FTIR and its elemental composition determined with standard methods, physical and chemical properties analysis, major and minor nutrients and heavy metal content. The most promising biochars based on DGT and chemical analysis will be subsequently tested in pot experiments with a fast growing indicator plant (tobacco). Finally, the efficiency of selected biochars in reducing Cd uptake and accumulation in cacao beans will be investigated in cacao plantations located in a representative set of agroecological zones. The field experiments are part of a larger project, this thesis will explore the mitigation success in relation to soil and biochar properties.