Concentration of Cadmium in Cacao Beans and its Relationship with Soil Cadmium in Southern Ecuador

Authors: CHAVEZ, E - University Of Florida; HE, H - University Of Florida; STOFFELLA, P - University Of Florida; MYLAVARAPU, R - University Of Florida; LI, Y - University Of Florida; MOYANO, B - Escuela Superior Politecnica Del Litoral; Baligar, Virupax

Submitted to: Science of the Total Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/24/2015
Publication Date: 7/11/2015
Publication URL: http://handle.nal.usda.gov/10113/63005
Citation: Chavez, E., He, H.E., Stoffella, P.J., Mylavarapu, R.S., Li, Y.C., Moyano, B., Baligar, V.C. 2015. Concentration of Cadmium in Cacao Beans and its Relationship with Soil Cadmium in Southern Ecuador. Science of the Total Environment. 533:205-214.

Interpretive Summary: Soils under cacao in Ecuador contain high levels of cadmium (Cd). Cadmium poses a threat to food safety and human and plant health in the world. Plants can tolerate Cd concentration at low levels without showing any symptoms of toxicity, but accumulation of Cd in edible parts often causes adverse effects on human health. Currently, in Ecuador, no information is available regarding Cd concentrations in soil, cacao plants, and cacao beans. Accumulation of Cd in cocoa beans results in considerable reduction in market value of this crop and drastically affects the economic wellbeing of resource poor cocoa farmers. In this paper we report the range of total and extractable Cd in soil profiles of 19 cacao farms and we assess the relationship between soil properties and level of total and extractable Cd in these soils. The results show that Cd has been substantially accumulated in the surface soils (0-15 cm) with 12 out of 19 sampling sites exceeding the critical level. The distribution of Cd in cacao plant generally decreased in the order of beans > shell >> leaves. This information will be of use to farmers and traders of cocoa beans in meeting the marked regulations for cadmium contaminations. Better understanding of soil properties and levels of cadmium will help to reduce the bioavailability of this toxic metal in cacao plants.

Technical Abstract: The concentration of cadmium (Cd) in cacao (Theobroma cacao, L.) beans above a critical level (0.6 mg kg-1 established by the European Union) has raised concerns of safety in the consumption of cacao-based chocolate (dark chocolate). Currently, little is available regarding Cd concentration in soil, cacao plant, and cacao beans in Ecuador. The objectives of this study were to: i) investigate the status of Cd in soils of representative cacao farms and potential source of pollution; ii) correlate Cd in cacao bean and plant tissues with available Cd in soils; iii) determine the major factors that may influence uptake of Cd by cacao plants; and iv) evaluate some commonly used chemical extraction methods for predicting soil Cd availability to cacao plants in Ecuador. Soil and plant samples were collected from a total of 19 representative small-scale cacao farms in Guayas and El Oro provinces (southern Ecuador). Soil samples were taken at the four depths: 0-5, 5-15, 15-30, and 30-50 cm. Four mature pods (4-month old) and 15 to 20 leaves were collected from trees located near to the soil sampling sites. Total recoverable Cd (EPA 3050B) and Mehlich 3 (M3) extractable Cd was measured for all the soil depths, whereas 0.1 M HCl-, 1 M NH4OAc-, and M3-extractable Cd were determined for the 0-5 and 5-15 cm depths. On average across the sampling sites, total recoverable Cd decreased from 1.54 to 0.85 mg kg-1, whereas M3-extractable Cd lowered from 0.40 to 0.09 mg kg-1, from 0-5 to 30-50 cm, respectively. The decrease in total recoverable or M3 extractable Cd from the surface to subsurface layer may indicate the anthropogenic source of soil pollution for most of the observed field sites. Concentrations of Cd in cacao beans ranged from 0.02 to 3.00 mg kg-1, with a mean value of 0.94 mg kg-1, Cacao beans from 12 sites contained Cd above the critical level (0.6 mg kg-1). Distribution of Cd in cacao tissues generally decreased in the order of beans > shell >> leaves. M3- and HCl-extractable Cd were highly correlated with bean-Cd concentration at both the surface and subsurface layers (0-5 and 5-15 cm), with similar correlation coefficients (r = 0.80 and 0.82 for M3, and r = 0.78 and 0.82 for HCl; P < 0.01). Cadmium has been substantially accumulated in the surface soils (0-15 cm) under cacao production in southern Ecuador, most likely associated with anthropogenic activities, and this, in turn, has resulted in Cd accumulation in cacao beans, with 12 out of 19 sampling sites exceeding the critical level. Of the three soil test methods, M3- and HCl-extractable Cd is superior to NH4OAc-extractable Cd for predicting Cd availability in the cacao-growing soils as they are more closely correlated with Cd concentration in cacao beans.