The influence of micro-nutrient availability on the uptake of cadmium by cacao trees Authors: Geeroms Jonas (Vredeseilanden vzw) The following study was performed as part of a project performed by Vredeseilanden vzw and funded by the Belgium Directorate-General for Development Cooperation. The project was only possible with the help of many other actors. These organizations and people will however not be named due to a confidentiality agreement. Contact: jonas.geeroms@outlook.com Brussels, July 2016 Introduction Cadmium is a heavy metal, a group of natural occurring chemical elements that are widely spread in the environment due to their wide industrial, domestic, agricultural, medical and technological use. There have been serious concerns about their effect on human health and the environment (Tchounwou et al., 2014). The main source of human ingestion for none smoking people is through their daily diet. Renal dysfunction and liver failure are most common problems upon excessive intake. In heavy contaminated areas osteoporosis and respiratory problems have been documented with ItaïItaï disease as the most severe known case (ICdA, 2016; Mite, 2013). Cadmium has been ranked as a known carcinogen element and the provisional tolerable monthly intake was therefore indicated at 25 µg/kg body weight (WHO, 2010). Cacao (Theobroma cacao L.) and its derivatives have been known as products with high cadmium contents (Lee et al., 1985). Health risks due to chocolate intake are strengthened by the composition cacao which allows for a strong chemical binding of cadmium (Valiente et al., 1996). Furthermore chocolate is highly consumed by children (Dahiya et al., 2005). Because of the above reasons, the European commission established maximum levels for cadmium in cacao based products (see Table 1). These were introduced in the commission regulation No 488/2014 (UE, 2014). Table 1: Maximum allowed cadmium level in cacao based products: European Commision regulation No 488/2014. Specific cacao and chocolate products as listed below: Milk chocolate with < 30% total dry cocoa solids Chocolate with < 50% total dry cocoa solids; milk chocolate with ≥ 30% total dry cocoa solids Chocolate with ≥ 50% total dry cocoa solids Cocoa powder sold to the final consumer or as an ingredient in sweetened cocoa powder sold to the final consumer (drinking chocolate)
mg/kg 0,10 as from 1 January 2019 0,30 as from 1 January 2019 0,80 as from 1 January 2019 0,60 as from 1 January 2019
Cadmium has no known function in plant development, but it is easily absorbed by plant roots, while airborne contributions to plant absorption are generally insignificant in rural areas (Smolders, E., 2001). The soil medium therefore plays an important role in cadmium absorption. Main factors that known to influence this process are pH, organic matter and contents of different macro and micro nutrients (Crozier). Use of zinc fertilizers and liming are the most common forms of remediation studied in plants (Hart et al., 2002; Tlustoš et al. 2006 and Chaney et al., 2009). Liming immobilizes Cd in the soil (Tlustoš et al. 2006), while Zn is known to inhibit Cd uptake due to very similar chemical characteristics and therefore competition for a common transport carrier (Hart et al., 2002). The effect of Zn and liming in cacao are currently being investigated as a MSc. Thesis by the University of Zurich, but results have not yet been publicised. Some experiments using liming and ZnSO4 were also conducted by the national institute for agricultural research in Ecuador. Both experiments were able to reduce cadmium uptake in cacao significantly (Mite, F.). In several crops correlations have been established between cadmium levels in the soil and absorption by the plants (Xiangyang et al., 2010; Nylund, E., 2003 and Singh et al., 1998). For now, this type of relation has not been established in cacao. A study by CABI found no evidence of correlation (Crozier), while according to Mite et al. (2010) there is clear evidence that soil levels of cadmium influence the amount of cadmium absorbed by cacao trees.
Methodology The study was performed in a cacao producing area characterized by a sub-humid, hot climate. The average temperature in the zone is 25°C with a relative humidity of 80% and annual precipitation between 1000 and 2000 mm. In the area 40 different cacao producing farms, all members of the same cooperative, were selected for thorough analysis through sampling. The location of the selected farms were recorded and can be used for further research purposes but cannot be shared due to the multiparty non-disclosure agreement. All selected cacao farms were subjected to soil analysis, leaf analysis an analysis of the cacao beans. Soil sampling was limited to the upper layer (0-15cm) due to the superficial distribution of lateral cacao roots (Woods et al., 1985). Sampling was performed according to the instructions of the laboratory in charge of the analysis, taking 15 sub-samples distributed across a representative part of the farm amounting to one composite sample. Leaf sampling was also done according to the prescriptions of the same laboratory. Fifteen sub-samples, each consisting out of four leaves from the same tree, were collected. All selected leaves were fourth fully developed ones. Sampling of the beans was conducted by harvesting and mixing 10 to 15 cacao pods across the farm. However due to known variation in cadmium content between pulp, skin and the actual beans (Lee et al., 1985 and Mite, 2013), bean samples were processed into nibs before analysis. This transformation was done at the cooperatives collection centre. All three types of samples were analysed by an autonomous institute for agricultural research. Soil samples were analysed for pH, electric conductivity and contents of NH4, P, K, Ca, Mg, S, Fe, Cu, Mn, Zn, B and Cd. Leaf samples were studied for N, P, K, Ca, Mg, Fe, Cu, Mn and Zn contents, while the cacao nibs were examined exclusively for Cd contents. Data analysis was done using R-project and QGis® software. Statistical tools used in the R software were pairwise Pearson correlation, scatter plots and simple regressions. These were used to
determine the relationship between soil characteristics and cadmium uptake and translocation to the cacao beans. QGisÂŽ software was utilized for geographic interpretation of collected data using GPS coordinates and interpolation functions.
Results and discussion 1. Cadmium concentrations in the soil Cadmium accumulation in the soils were found to be rather high, but without any indication of serious pollution. Results from the soil samples ranged from 0.52 ppm to 7.7 ppm. The average was 1.33 ppm, while the median resulted in a value of 0.94 ppm. Four of the samples contained possible outliers, which could not be verified due to budgetary restraints. When omitting dubious results all samples pertained to a range between 0.52 ppm and 1.92 ppm. Results that are similar to a study performed by Chavez et al. (2015). A geographical distribution throughout the area of cadmium contents in the soil are represented in Figure 1.
Figure 1: Geographical distribution of cadmium concentrations in the soils at the research area (ppm)
When comparing this illustration to the geographical distribution of clay contents in the soil (Figure 2), a strong resemblance was found. The influence of soil texture on cadmium was confirmed during correlation analysis. A pearson correlation factor of 0.506 with significance levels of p < 0.01 was found between clay (%) and cadmium levels in the soil (ppm).
Figure 2: Clay contents of the soils in the research area (%)
2. Cadmium concentrations in the cacao beans However, according to Wang, X-p., et al. (2004) heavy metals in clay soils are not always available to plants. In our case this was confirmed by comparing results from cadmium contents in bean samples and soil samples. No significant correlation could be determined between cadmium contents in the soil and cadmium content in cacao beans. Visually this can be observed from Figure 3. Furthermore it is clear that unlike cadmium contents in the soil, there is no geographic consistency in cadmium absorption by the cacao trees. Therefore analysis of cadmium contents in the beans can only be done at a farm-to-farm basis.
Figure 3: Cadmium absorption by cacao beans (ppm) in relation to cadmium in the soil (ppm)
The analysis of the cocoa nibs did demonstrate medium to high cadmium contents. An average value of 0.982 ppm of cadmium was found with a range between 0.24 and 1.81 ppm. These values are comparable to the ones in Chavez et al. (2015) but would pose a problem when the new EU regulation takes effect.
3. Factors influencing cadmium absorption in cacao beans In an attempt to pinpoint possible mitigations for high cadmium levels, a Pearson correlation analysis was done on the complete data set. Statistical significance was established at p<0.05. Due to the established relation between Zn and Cd (Hart et al., 2002) a ratio of the soil contents of Zn and Cd (Zn/Cd) was also included in the correlation analysis. Because it was speculated that a same logic might be true for other micronutrients ratios of Mn/Cd, Fe/Cd, Cu/Cd and B/Cd were also included. The results from the correlation analysis are shown in Table 2. Because of the extent of the full data set, only significant values were represented. According to these results, higher availability of nitrogen may decrease uptake of cadmium by cacao beans. This directly contradicts a theory that due to acidification of the soil, root growth and increase of ionic strength of the soil solution, nitrogen fertilizers might make Cd more available (Roberts, TL., 2014). At the same time the results do support a recommendation from CABI that nutrient deficiencies might introduce an increased cadmium uptake, due to the influence on general plant health. Since almost all soil samples showed very little available nitrogen and organic matter, these results seem to confirm the latter theory. Influence of soil phosphor on the other hand was found to be as expected. Higher P contents in the soil are correlated with higher Cd uptake by the cacao. Influence of phosphor on cadmium uptake can
be explained because of its inhibiting characteristics on Zn availability in the soil, which results in higher Cd uptake (Roberts, T., 2014 and experimental data presented below). Furthermore phosphate rock has been used in the research area as a fertilizer. It is, however, known as an anthropogenic source of cadmium in agricultural areas due to innate Cd (Roberts, TL., 2014). Table 2: Pearson correlation factors of all significant (p<0.05 and p<0.01) correlations with Cd absorption by cacao beans
Cdbeans
Mnsoil NH4_soil PSoil -0.3946* -0.3485* 0.3447*
Znsoil/Cdsoil -0.4233**
Mnsoil/Cdsoil -0.4914**
* p < 0.05 ; ** p < 0.01
The results with the highest correlation factors and moreover with the highest significance levels were the correlations between cadmium absorption and the soil ratio of Zn/Cd and Mn/Cd. As earlier mentioned, cadmium contents in the soils did not show any correlation with the absorbed amount of cadmium in the beans. However, it seems soil cadmium content might play an important role in absorption rates through its relation towards some essential micro-nutrients. Zinc contents in the soil neither showed a significant correlation to cadmium absorption, but the relationship between Zn and Cd in the soil was proven to be highly significant (p < 0.01). Manganese levels on the other hand were found to be significantly correlated (p < 0.05) to cadmium absorption, nevertheless the significance was even higher (p < 0.01) in relationship to cadmium levels in the soil. A graphical representation of both relationships can be seen in Figure 4 and Figure 4.
Figure 4: Relationship between cadmium absorption (ppm) and the ratio of Zn/Cd in the soil
Figure 5: Relationship between cadmium absorption (ppm) and the ratio of Mn/Cd in the soil
Both relationships can be explained through the fact that Zn and/or Mn carriers might be the same ones transporting cadmium from the soil into the plant. An important observation can be made that if this is the case, these carriers show higher affinity to Cd than to Zn or Mn. A similar finding was done for zinc by Hart et al. (2002). Based on the above regressions, availability of Zn should be 4 times higher than amounts of cadmium in the soil to reduce cadmium uptake levels below the new European regulation. Manganese should be 20 times higher. Zinc and manganese levels in almost all soil samples were found to be low. All Zn levels were in a range between 1.2 and 4.7ppm, with an average of 2.62 ppm. Mn levels were ranging from 5.8 to 18.4 ppm with an average of 9.3 ppm. Since at the same time it was already observed that Cd levels in the soil were found to be high, this makes for a very plausible explanation why cadmium contents in cacao beans from the investigated area were found to be high.
Lastly it was deemed worth mentioning that no indication was found that pH had an effect on Cd uptake, although it is generally regarded as a main factor in bio-availability of cadmium (Smolders, E., 2001; Roberts, TL., 2014 and Chavez et al., 2015). This might be explained because of the high average pH value of 6.9 and small differences among the samples ranging from 6.3 to 7.2.
Conclusion Cadmium contents in both soil and bean samples in cacao growing farms in the area under investigation were rather high, but similar to other available research. No geographical consistency was observed in cadmium contents of cacao beans, analysis can therefore only be done on a farm-tofarm basis. Soil characteristics had a major impact on cadmium contents in the soil medium and the absorption of cadmium by cacao trees. Cadmium contents in the soil were found to be correlated to clay contents, but at the same time have proven to be a poor indicator for cadmium absorption by the cacao trees. They were however found to be of main importance when put into relationship to Zn and Mn soil contents. A ratio of 4:1 – Zn:Cd in the soil and a ratio of 20:1 – Mn:Cd would theoretically reduce cadmium uptake below the threshold for the new European regulation on cadmium contents in cacao based products. Research on the influence of micronutrient availability in the soils on cadmium absorption would be highly recommended. A balanced fertilization scheme, high in nitrogen but low in phosphor might also help reduce cadmium absorption by cacao trees.
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