Chemical Characterization of Plant-Available Cadmium in Soils From Ecuadorian Cacao Farms.

Cadmium (Cd) is one of the most troublesome soil contaminants, as it is very mobile and readily available to plants.  Cacao (Theobroma cacao) is the most grown crop in Ecuador, with an area of approximately 470.000 hectares throughout the coastal and amazon region.  Cacao beans are the final products for commercialization; studies have reported high concentration of in cacao beans from southern Ecuador.  Anthropogenic contamination was suspected to be responsible for the abnormal metal concentration. Plant and soil samples were collected at 20 sites in El Oro and Guayas provinces (Southern Ecuador). Soils were sampled at four depths: 0-5, 5-15, 15-30 and 30-50 cm, whereas plant samples (leaves and pods) were collected at the same sites. The spatial distribution of Cd was assessed comparing soil depth and total recoverable Cd (USEPA 3050B); plant-available Cd was determined with Mehlich 3 extraction. Extractable and total recoverable Cd varied among the 20 profiles, the average concentration was 0.21 mg kg^-1 and 0.57 mg kg^-1 for extractable and total recoverable Cd respectively. Four sites had Cd concentration > 0.5 and > 1 mg kg^-1 for extractable and recoverable Cd accordingly, these values might be considered high according to available soil surveys and reports. Moreover, total recoverable Cd was highly correlated (r= 0.57) with extractable Cd, indicating high availability of Cd in the studied sites for plant uptake. Additionally, statistical differences (p<0.0001) were reported for total recoverable and extractable Cd with regard of soil depth, higher Cd concentrations were found at surface (0-15 cm) rather than subsurface (15-50) layers, suggesting that Cd inputs could come from anthropogenic source rather than parent material. Cd concentration at leaves were statistical different (p<0.001), nonetheless, most of the sites showed very low Cd concentration in leaves, thus, no correlation could be established between Mehlich 3 extractable Cd and tissue. Further analysis should be carried out in different cacao tissues, such as pods and beans, to pinpoint Cd accumulation thoroughly.