Natural and Anthropogenic Sources of Cadmium in Cacao Crop Soils in Santander, Colombia


 Cadmium (Cd) levels in cacao products from Santander, the main producing area in Colombia, are well above those permitted for export of cocoa products and may pose a potential health hazard. High Cd in cacao is related to the high content of the metal in beans, which in turn is linked to high concentrations of Cd in soils. Geochemical and petrographic analyses of fertilizer, soil and rock samples from three farms were carried out to determine the sources of Cd and soil characteristics that can affect its bioavailability, in order to identify strategies that may reduce Cd in cocoa. Autochthonous natural sources determine the Cd concentration in soils, with a high correlation between elevated Cd in sedimentary parental rocks and soil metal levels. While no industrial or mining inputs were present, an organic fertilizer was identified as a great allochthonous source of Cd in soils. High levels in the fertilizer were probably due to bioaccumulation of the element, since it was sourced from animals in the same area. The addition of crop waste to fertilize the soil may further contribute bioavailable Cd. Even though the pH range, high OM content and presence of Mn and K all diminish bioavailability, the high metal content in the farm soils still results in significant uptake by the cocoa plants and accumulation in the beans. We suggest that phytoremediation and biological amendments, as well as testing of fertilizers before application, could all be cost-effective solutions to reduce Cd levels in the final product.


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Cadmium (Cd) is a non-essential heavy metal that can cause toxic effects in plants, animals    The Theobroma cacao plant is a bioaccumulator of Cadmium (Cd), which is easily absorbed by its roots 89 from soil and water in its available Cd 2+ form along with the other nutrients the plant needs, accumulating  The accumulation occurs preferentially in cacao beans, followed by fruit shells, and the smallest quantity     The anthropogenic sources of Cd are mainly related to the addition of both organic and inorganic 127 fertilizers, and to the potential contamination from mining or construction sites (e.g. Smolders & Mertens,128 2013). Cd can also be reinserted into the soil through the falling of the plant's leaves or branches, and in 129 some cases, farmers will leave plant debris as a fertilizer, thus recycling the metal into the ground (e.g.

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Our aim is to improve the understanding of the possible allochthonous and autochthonous sources of the 132 metal in the study area. We also analyzed soil parameters that may regulate the bioavailability of the 133 metal. Providing a sound baseline of Cd levels and sources sinks is a first step towards better management 134 practices and when needed, remediation strategies.  There is a noticeable enrichment of the metal in the west flank of the Eastern Andean Cordillera 151 corresponding to territories within the departments of Santander, Boyacá and Cundinamarca (Fig. 3). 152 Even though the study farms are within an area with no reported Cd values we can extrapolate high   167 A total of 37 samples (23 soil, 12 source rock and 2 fertilizers) were collected and analyzed at three 168 different farms in the study area. An organic fertilizer was used in Farm 1 and an inorganic one in Farm 169 2; no fertilizer was added at Farm 3. In Farms 1 and 2, which are plantations of less than 1 Ha, the sampling 170 was done in transects. In the first farm we collected a rock sample at each soil sampling point, while in 171 the second plantation only one rock sample was collected for each transect. In Farm 3, which is more 172 extensive (~15 Ha), but also more homogeneous, five soil samples were taken using randomized sampling.

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No rock samples were collected in this case since soil was homogenized in the first 2 meters and rock 174 fragments were not present. As the third farm is located over the same geological unit, we expect the 175 parental rock material to be of the same composition as both the other farms.

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A LAQUAact-PC110 probe was used to measure the pH in soil samples. To do so, 1g of humid soil was 188 placed in 9 ml of distilled water and the solution was mixed for 2 minutes in a vortex and left to rest for 189 30 min. The probe was placed in the supernatant and pH was measured 3 times.

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In order to quantify humidity, 100 g of each soil were placed in the oven at 45 °C for three days. The soil 191 was re-weighed and the difference in mass was assumed to be water content.

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To measure the organic matter (OM) content, the same dry soil samples were then placed in the oven at a    223 Santander is a well-known region for the cultivation of cacao and has been historically recognized for 224 having cacao crops of great extension and quality, representing 40 to 45% of the national production.

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Addressing elevated Cd content is crucial not only as a public health concern, but also for the potentially  shows significantly higher Cd values in sediments and soils (ranging from 1.8 to 74 mg/kg), than any other 238 region in the country, resulting in a potential health hazard (Fig. 3). 239 The parental rock in the study area presents unusually high Cd concentrations with values of up to 3 times  (Fig. 4).

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Total Cd content in cacao soils has been found to be mainly present as residual and oxidable fractions, that Cd in soils in the region is very elevated and represents a threat to cacao cultivation and 261 commercialization, which is the basis of the local economies of these municipalities.

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Of even more concern is that the vast majority of our samples do not comply with international regulations 263 for tolerable values for mammals' exposure, ecosystems or critical thresholds for human health risks (

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The organic fertilizer was sourced from chickens and pigs which were fed food residues and organic matter 365 from crops grown in the same soils. It is clear that this resulted in biomagnification of bioavailable Cd that 366 accumulated resulting in extremely high levels of the metal in the organic fertilizer (Fig. 4). This case is 367 particularly worrisome since the Cd introduced with the fertilizer will be easily absorbed by the plants and 368 result in further metal accumulation in cocoa beans. Although the addition of fertilizer may further enrich 369 the metal in soils, we found no significant correlation between soil and fertilizer Cd conecntrations, 370 indicating that this is not the main factor controlling total Cd in soils.

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The metal can also be reintroduced into the soil through the falling of the plant's leaves or branches, and

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Reducing the addition of organic fertilizers to minimize the amount of bioavailable Cd accumulation might 443 be an easy and effective strategy (Marini et al., 2020). But of course a better approach would be to test the 444 fertilizers of both chemical and organic nature to guarantee that they have low metal values and can be 445 applied as needed.

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While the addition of soil amendments that alter pH or soil organic matter content to reduce the 447 bioavailability of Cd for the cacao plants is widely used, it would not be useful in this particular case (e.g.

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In all soil, rock and fertilizer samples in our study area, high levels of Cd were found, with the majority