4.1. 2D-ERT to Assess Cd Sources in Cacao Farms
The primary source of Cd in unpolluted soils is from parent material [21
], cretaceous sedimentary rocks (i.e., farms in Muzo) [22
] and shales, which are frequent rock types found in the central zone of Santander, especially in some municipalities in the southwest of the district [23
]. However, the contribution of these sources to Cd release could be low, less than 0.3 mg·kg−1
], compared to other sources due to human activities.
This work describes a resistivity analysis mainly due to the presence or absence of solid-state phase Cd-like material throughout the assessed pits that were geoelectrically assessed. Figure 7
shows the correlation between resistivity obtained by 2D-ERT and Cd determination at the same points in the assessed pits. There is a high correlation between resistivity and Cd in soils (R2
= 0.87), which demonstrates the accuracy of this geophysical technique to study Cd content in cacao soils following calibration. Regarding land use within the studied regions where cacao grows, it is possible to assess Cd-like material in both topsoil and subsoil, using the 2D-ERT technique. According to Figure 2
, one could associate Cd richness to anthropogenic sources, as the farms assessed are near to emerald and coal mines (i.e., in Muzo and San Vicente de Chucurí), and close to oil pipelines (i.e., in Arauquita) that could influence the Cd influx into cacao farms, as found in other cacao producing countries [25
]. However, such assumptions may require further exploration using other techniques such as Cd114
4.2. Physical and Biochemical Drivers for the Distribution of Soil Cd
Interestingly, previous studies on Cd distribution in cacao soils have focused only on the top soil to a depth of 15 cm [26
]. This is the first study exploring the Cd pool in cacao soils on Colombian farms at the subsoil root system levels that describes how Cd pools are distributed below the topsoil. These pools were observed in this study at a depth of 90 cm (secondary roots can be observed even at this depth). Hence, this study supports the idea that Cd studies of cacao should take into consideration the 2D-ERT analysis of soil profiles to a depth of at least 1 m. This is necessary to understand the vertical distribution of soil parameters such as phosphorus, SOM, pH, and calcium, and their relevance to Cd fluxes.
Regarding Cd fluxes, phosphorus is the second most important factor related to Cd. A higher content of this element (above 1000 mg·kg−1
) indicates a larger input, mainly at boundary B, at a depth of 40 cm. This could be related to the use of rock phosphate fertilizer, in some cases contaminated with more than 30 mg·kg−1
of Cd. This appears to be particularly relevant to Araucan farms where the SOM content is the lowest (1.13%) and the sand content is the highest (84%). It has been demonstrated in Bermuda grass that an increasing concentrations of available P in soils is an important ecological factor leading to increased Cd absorption and translocation to plant tissues [27
]. This could be the case in Araucan farms where higher available P concentrations were observed no Cd was detected using 2D-ERT, even though, high Cd contents were detected in the cacao beans.
Moreover, according to the Araucan soils survey, from the National Institute of Geography [28
], soil types in farms located in Arauquita, Saravena, and Tame were all classified as Typic Endoaquepts
, with low levels of clay content (1.15%), SOM (less than 3%), and parent material (resistivities below 200 Ohm·m). In contrast, according to the Boyacá soils survey [29
], Maripí, Muzo, and Pauna farms soils were classified as Lithic Udorthents
, featuring higher levels of parent material (resistivities greater than 1000 Ohm·m), clay content, and SOM (80% and 3.28%, on average, respectively). The area of Muzo is known to show the higher mineral content in the subsoil, as was confirmed in the pits. Furthermore, soils in cacao farms in El Carmen and San Vicente de Chucurí were classified as Typic Udorthents
], had high clay content and SOM (85% and 5.20%, on average, respectively). In all cases, the soil classification corresponds to the USDA soil taxonomy keys [31
Therefore, higher values of P in Arauca could cause the loss of the largest pores, pore size distribution variations, and water retention potential, as well as higher mechanical resistance to penetration affecting the development of plant root systems [32
]. This was observed in the root distribution in the Araucan trial-pits, where secondary roots were found to a depth of 40 cm, whereas in Boyacá and Santander roots were found to a depth of 90 cm. In this study, higher correlations were confirmed at farms in San Vicente de Chucurí (Santander), where higher rock material content and SOM was found across the pits (40–45% of rock material and 5–5.2% of SOM, respectively).
The primary cause of Cd enrichment in sedimentary environments has been reported to be the adsorption and complexation of Cd with SOM, followed by the accumulation of organic debris in a reduced depositional environment [33
]. This could be a source of Cd contamination for some farms in Muzo, San Vicente, as well as in Boyacá and Santander, but this is not the case for farms in Arauquita nor other municipalities in Arauca. Therefore, it is suggested that the farms assessed in this study could have geogenic or anthropogenic sources of Cd pollution at site-specific localities, although this does not affect an entire area or region. It also suggests a Cd presence in cacao beans due to specific conditions that may vary at the farm level (ranging from 0.02 to 1.3 ± 0.4 mg·kg−1
In this study, Fe was determined to be higher at all farms where the Cd content was higher as well, e.g., 8.80 ± 0.7 and 11.50 ± 0.2 mg·kg−1 Fe at farms in Muzo and San Vicente de Chucurí, respectively. At farms in San Vicente de Chucurí, less acidic pH values (5.4 on average) were found with a higher level of soil Cd (2.76 ± 0.2 mg·kg−1) and higher content of Fe (836 ± 4.4 mg·kg−1).
Soil pH could also influence the distribution of Cd in the subsoil of cacao systems. Soil-solution pH greatly influences diffusion rates because of the strong pH effect on Cd solubility in soil [34
], generating a reciprocity between the pH and Cd/resistivity ratio. In farms in Arauquita, Muzo, and San Vicente de Chucurí (Figure 8
A–C) similar patterns of distribution were described. Such reciprocity between pH and soil Cd content has also been observed in previous studies [35
]. Soils assessed in this study have pH ranging from 3.5 to 7. Acidic pH is associated with lower Ca and P content (<1 and <15 mg·kg−1
, respectively), and higher Fe and Cd content, as also observed in other studies [36
Critical non-biotic factors influencing Cd debris, according to the literature, includes soil pH, clay content, carbonates, and SOM content [37
]. Factors directly controlling Cd mobility, as reported in literature, are pH and soil type [21
]. In the present survey of cacao soils, the order of factors related to soil Cd were resistivity > P > SOM > Fe > pH > Ca, as shown on the PCA plot in Figure 9
Acidic soil pH was observed in Muzo (3.5), followed by San Vicente de Chucurí and Arauquita (4 and 4.6, respectively). Interestingly, farms in these municipalities have medium to high soil Cd content (0.54, 0.84, and 1.3 mg·kg−1
at Arauquita, Muzo, and San Vicente de Chucurí, respectively). At the same boundaries where acidic pH was found, a higher Fe content was also found (i.e., 16.54, 76, and 200 mg·kg−1
in Arauquita, Muzo and San Vicente de Chucurí, respectively). Calcium, as shown in Figure 9
, is the least significant nutrient related to Cd distribution in soils of farms assessed in this study. However, its presence was enriched at the same boundaries where the level of Cd was found to be high. Thus, the flux of calcium and its role in Cd distribution should be addressed in further studies.
4.3. 2D-ERT Technique As a Proxy of Cd Dynamics
shows several methods developed during the past two decades to analyze Cd content in agricultural soils. As illustrated, the 2D-ERT technique is accurate for the detection of Cd in both topsoil and subsoil in cacao farms. In comparison to other techniques, such as the dynamic-based diffusive gradients in thin-films (DGT) or the portable X-Ray diffraction methods, which are also in situ non-invasive techniques, the accuracy of the 2D-ERT is greater. With exception of the DGT method [38
], the mentioned below techniques have not yet been applied in cacao farm soils to our knowledge.
Cd has an excellent electric conductivity [44
]. Therefore, the study of Cd distribution in soils under cacao trees requires non-destructive and non-invasive methods to assess its presence and its relationship with mobile fractions of Cd in cacao beans. We highlighted the use of 2D-ERT to assess both the horizontal and vertical distributions of Cd in geomorphs and their locations underground. The accuracy of the 2D-ERT technique (R2
= 0.84) to describe Cd distribution with Cd counts by spectrometry is also highlighted, even when no (phyto)available Cd was described [44
In cacao farms from Boyacá and Santander districts, resistivities related to Cd were greater than 103
Ohm·m (Figure 5
and Figure 6
). Predicting the existence of underground rock or parent material was successfully accomplished according to our hypothesis that reservoirs of solid-state phase soluble and non-soluble Cd aggregates, close to CdCO3
, when present at higher concentrations, can induce outcroppings of carbonates with higher resistivity values [45
]. Therefore, solid-state phase Cd compounds can occur during soil formation in an agricultural soil system such as cacao [46
], where geological and anthropogenic Cd interchange might occur frequently.
Moreover, the biology of the system plays a key role in Cd dynamics. Since bacterial carbonate-genesis relies on the relation with soil pH and is highly correlated with the distribution of SOM, it is possible that microbial activity influences the secondary formation of otavite in neo-tropical acidic soils like those assessed; however, this needs to be studied in more detail. At the farms surveyed in this work, acidic pH, and higher SOM content, even at a 68 cm soil depth, might lead to an ideal scenario for bioweathering activity mediated by Cd-tolerant bacteria [1
Furthermore, the 2D-ERT profiling was an accurate technique in monitoring in situ and in real-time parent material and rock aggregates related to soil Cd, addressing the site-specific sampling in the trial pits. However, it is not yet clear if Cd selectivity is addressed by the bioweathering of a calcite system. Therefore, further research is necessary to examine Cd dynamics through resistivity when both otavite, and hydroxy- and fluorapatite, are present, and the bioweathering process occurs due to Cd-tolerant bacterial activity in interaction with ligands disposed in clays. Regardless of the specific mechanisms of Cd speciation in soils, an estimation of Cd levels was possible using the 2D-ERT technique.