Plant metal concentrations in Theobroma cacao as affected by soil metal availability in different soil types

Association between bacterial community and cadmium distribution across Colombian cacao crops

Assessing the bacterial community composition across cacao crops is important to understand its potential role as a modulator of cadmium (Cd) translocation to plant tissues under field conditions; Cd mobility between soil and plants is a complex and multifactorial problem that cannot be captured only by experimentation. Although microbes have been shown to metabolize and drive the speciation of Cd under controlled conditions, regardless of the link between soil bacterial community (SBC) dynamics and Cd mobilization in the rhizosphere, only a few studies have addressed the relationship between soil bacterial community composition (SBCC) and Cd content in cacao seeds (Cdseed). Therefore, this study aimed to explore the association between SBCC and different factors influencing the distribution of Cd across cacao crop systems. This study comprised 225 samples collected across five farms, where we used an amplicon sequencing approach to characterize the bacterial community composition. The soil Cd concentration alone (Cdsoil) was a poor predictor of Cdseed. Still, we found that this relationship was more apparent when the variation within farms was controlled, suggesting a role of heterogeneity within farms in modulating Cd translocation and, thus, seed Cd content. Our results provide evidence of the link between soil bacterial communities and the distribution of Cd across Colombian cacao crops, and highlight the importance of incorporating fine-spatial-scale studies to advance the understanding of factors driving Cd uptake and accumulation in cacao plants.

IMPORTANCE

Cadmium (Cd) content in cacao crops is an issue that generates interest due to the commercialization of chocolate for human consumption. Several studies provided evidence about the non-biological factors involved in its translocation into the cacao plant. However, factors related to this process, including soil bacterial community composition (SBCC), still need to be addressed. It is well known that soil microbiome could impact compounds' chemical transformation, including Cd, on the field. Here, we found the first evidence of the link between soil bacterial community composition and Cd concentration in cacao soils and seeds. It highlights the importance of including the variation of bacterial communities to assess the factors driving the Cd translocation into cacao seeds. Moreover, the results highlight the relevance of the spatial heterogeneity within and across cacao farms, influencing the variability of Cd concentrations.

Interspecific variations in growth, physiology and Cd accumulation between Populus deltoides and P. × canadensis in response to Cd pollution under two soil types

Both acid and alkaline purple soils in China are increasingly affected by Cd contamination. The selection of fast-growing trees suitable for remediating different soil types is urgent, yet there is a severe lack of relevant knowledge. In this study, we conducted a controlled pot experiment to compare the growth, physiology, and Cd accumulation efficiency of two widely recognized poplar species, namely Populus deltoides and P. × canadensis, under Cd contamination (1 mg kg-1) in acid and alkaline purple soils. The objective was to determine which poplar species is best suited for remediating different soil types. Our findings are as follows: (1) the total biomass of both poplars remained largely unaffected by Cd pollution in both soil types. Notably, under Cd pollution, the total biomass of P. deltoides in acid purple soil was 1.53 times greater than that in alkaline purple soil. (2) Cd pollution did not significantly induce oxidative damage in the leaves of either poplar species in both soil types. However, in acid purple soil, Cd contamination led to a 21% increase in NO3- concentration and a 44% increase in NH4+ concentration in P. × canadensis leaves, whereas in alkaline purple soil, it led to a 59% increase in NH4+ concentration in P. deltoides leaves. (3) Cd concentrations in all root orders of P. × canadensis were significantly higher than those in P. deltoides, especially in the first three root orders, under alkaline purple soil. The total Cd accumulation by P. × canadensis in Cd-polluted alkaline purple soil was 2.18 times higher than that in Cd-polluted acid purple soil, a difference not observed in P. deltoides. (4) redundancy analysis indicated that the sequestration effect of higher soil organic matter on Cd availability in acid purple soil was more pronounced than the release effects caused by lower pH. In conclusion, P. × canadensis is better suited for remediating alkaline purple soil due to its higher capacity for Cd uptake, while P. deltoides is more suitable for remediating Cd-contaminated acid purple soil due to its better growth conditions and greater Cd enrichment capability.

Probabilistic health risk assessment of heavy metals (Cd, Pb, and As) in Cocoa powder (Theobroma cacao) in Tehran, Iran market

The concentrations of toxic elements were analyzed by using Flame Atomic Absorption Spectrophotometer (FAAS). Moreover, the human health risk was estimated by Total Target Hazard Quotient (TTHQ) and Cancer Risk (CR) in Monte Carlo Simulation (MCS) technique. The mean concentrations (mg/kg) of Cd (0.08 ± 0.08), Pb (0.23 ± 0.46), and As (0.06 ± 0.04), were in a good compliance with ISO (Iranian standard organization). TTHQ for adults and children was equal to 0.009 and 0.042, respectively and also mean CR in adults and children consumers was equal to 9.73E-7 and 9.08E-7, respectively. Consequently, the concentration of toxic elements (Cd, Pb, and As) in cocoa powder did not pose any safety concerns. Moreover, probabilistic health risk assessment revealed that both adults and children were not at considerable non-carcinogenic (THQ and/or TTHQ ≤ 1) and carcinogenic risk (CR ≤ 1E-6). Even though, seeking for mitigating solutions and applying them to suppress the dangers of food containing toxic elements is a critical subject.

Transcriptomic, osmoregulatory and translocation changes modulates Ni toxicity in Theobroma cacao

Nickel is one of the most released trace elements in the environment and in the case of bioaccumulation in foods and beverages derived from cocoa beans can cause risk to human health. It is very important to understand how plants respond to toxic metals and which are the defense strategies they adopt to mitigate their effects. In the present study we used young plants of T. cacao, submitted to increasing Ni doses (0, 100, 200, 300, 400 and 500 mg Ni kg^-1 soil) and evaluated them for a period of 30 days. Doses of Ni, from 300 mg of Ni kg^-1 onwards in the soil, promoted changes in photosynthetic, antioxidant, osmoregulatory, transcriptomic and translocation levels, evidenced by the increase in the activity of antioxidant enzymes, proline, glycine betaine, upregulation of the metallothionein 2B gene (Mt2b), and lipid peroxidation of the cell membranes. Foliar gas exchange was severely affected at higher doses of Ni. In addition, reduced levels of stomatal conductivity and transpiration rate were observed from 300 mg Ni kg^-1 dose onwards in the soil, which consequently affected CO₂ assimilation. Phytostabilization and exclusion mechanisms control the translocation of Ni from the root to the shoot and reduce harmful effects on plant metabolism. Our results highlighted the toxicity of Ni, a trace element often underestimated in T. cacao. In particular, it was noted that doses of 100 and 200 Ni kg^-1 soil, although high, do not induce toxicity in T. cacao plants. But Ni toxicity is observed from 300 mg Ni kg^-1 soil onwards. This study contributed to the understanding of the harmful effects of higher doses of Ni in cacao plants and the biochemical processes the plant uses to mitigate the effects of this metal.

Divergent responses and ecological risks of wheat (Triticum aestivum L.) to cerium oxide nanoparticles in different soil types

Cerium oxide nanoparticles (nCeO₂), as a common component for sustainable agriculture, have been broadly investigated due to their potential threat to the soil biodiversity and health. However, few studies considered the impacts of soil types on response of ecotoxicity of nCeO₂ to plants. This study aimed to explore the effects of soil properties on ecological response of nCeO₂ to wheat (Triticum aestivum L.) and assess the ecological risks of nCeO₂ (0-1000 mg/kg) in red soil, yellow-brown soil, and brown soil by applying a multi-biomarker approach. The results showed that the clay content had the extremely significant correlation with acid solute fraction Ce in soil. Ce accumulation in wheat largely depended on acid-soluble fraction Ce, but not the total Ce. Both urease and invertase activities were highest in brown soil among the three soils, after exposure to diverse concentration nCeO₂. Although wheat has a stronger antioxidant capacity in red soil, integrated biomarker response index proved that nCeO₂ showed least toxicity to wheat in brown soil (IBRv2 = 34.3) among the three soils. These results indicated that the toxicity level of nCeO₂ to wheat was not only related to contaminated concentration, but also greatly depended on soil properties. The soil types are important factors governing ecological risk of nCeO₂ in soil, which needs to be adequately assessed and properly controlled.

Bioaccessibility and bioavailability evaluation of heavy metal(loid)s in ginger in vitro: Relevance to human health risk assessment

Ginger is a common spice in everyday diet. However, over time, it may absorb and accumulate heavy metal(loid)s (HMs) from the soil, posing a potential health risk to humans. In this study, we evaluated the pollution level, bioavailability, mobility evaluation, and health risk of Cr, Pb, Cd, and As in the soil-ginger system of five major ginger-producing cities in Shandong Province, China. Research indicated the concentrations of the Cr, Pb, Cd, and As in the soil were close to or even higher than background value, except Weifang. With the concentrations of Cr, Pb, Cd, and As in ginger being 0.08-0.47, 0.03-0.16, 0.002-0.03, and 0.006-0.028 mg/kg, the four HMs concentrations were within the limits of 0.1-0.5 mg/kg based on the Chinese health standard (GB2762-2017) of HMs in food. The bioavailability of HMs in soils was evaluated using diethylenetriaminepentaacetic acid (DTPA) and nitric acid (HNO3) extraction and the results showed that Pb and Cd have high bioavailability. Mobility evaluation of HMs from soil to ginger based on Pearson's correlation and the partial least squares-path method (PLS-PM) model showed that the soil's physicochemical properties influence the HMs migration process (especially Cd and Pb) in the soil-ginger system, and the PLS-PM model has good adaptability to Cd and Pb (gof > 0.5). The mean total carcinogenic risk (TCR) of bioaccessible gastric and intestinal HMs were 4.64 × 10-6 and 3.13 × 10-6, which were much lower than that of total HMs (2.60 × 10-5), indicating that existing models based on total HMs may overestimate the health risk of HMs. The bioaccessible concentrations should be considered to improve the accuracy of assessment results.

Effect of Combined Soil Amendment on Immobilization of Bioavailable As and Pb in Paddy Soil

Heavy metal pollution in soil can have detrimental effects on soil ecosystems and human health. In situ remediation techniques are widely used to reduce the bioavailable fractions of heavy metals in soil. The main objective of this study was to examine the reduction of the bioavailable fractions of As and Pb in paddy soil with artificial lightweight material (ALM) manufactured from recycled materials. A total of four treatments, including a control (no amendment), ALM10 (10% of ALM in soil), ALM10+L (10% ALM combined with 0.5% lime), and ALM10+FeO (10% ALM combined with 0.5% FeO), were applied to paddy fields, and rice (Oryza sativa L.) was cultivated after 32 weeks. The highest reduction efficiencies for the bioavailable fractions of As and Pb in soil were observed in the ALM10+FeO (52.8%) and ALM10+L treatments (65.7%), respectively. The uptake of As decreased by 52.1% when ALM10+FeO was applied to paddy soil, and that of Pb decreased by 79.7% when ALM10+L was applied. Correlation analysis between bioavailable heavy metals in soil and soil chemical properties showed that soil pH, electrical conductivity (EC), P₂O_5, and soil organic matter (SOM) were the main factors controlling the mobility and bioavailability of As and Pb. Overall, the efficiencies of As and Pb reduction increased synergistically in both soil and plants when FeO and lime were combined with the ALM. In future studies, long-term monitoring is necessary to examine the longevity of soil amendments.

Effect of soil characteristics on cadmium absorption and plant growth of Theobroma cacao L. seedlings

BACKGROUND

Cadmium uptake by cacao plants can affect plant growth, consumer health and commercialisation. To develop mitigation strategies, it is essential to identify the soil characteristics that could influence this absorption. To determine the relationships between cadmium absorption and the soil characteristics of cacao areas, the responses at concentrations of 0, 2, 5, 10 and 20 μg g^-1 of cadmium in three soils of these areas and an andisol were evaluated, using 120-day-old seedlings of four cultivars of Theobroma cacao L.

RESULTS

In the present study, several relationships were found between chemical and physical soil characteristics and available cadmium, such as real and bulk densities, as well as contents of iron, sand, magnesium, potassium, sodium and copper. Additionally, moderate to strong correlations between potassium (r²  = -0.56) and real density (r²  = 0.42), with foliar cadmium, were found. Moreover, a differential deleterious effect on cacao growth in variables such as biomass was corroborated in cadmium concentrations from 5 μg g^-1 in soils. There were no statistical differences between cultivars with respect to cadmium uptake or plant growth. Finally, a multiple linear regression model is proposed to estimate the foliar cadmium content (r²  = 0.878).

CONCLUSION

Some soil characteristics such as density, as well as sand, clay, aluminium, potassium and iron contents, should be considered before establishing cacao crops to avoid cadmium accumulation. The correlation between potassium with foliar cadmium indicated that potassium could be significant in cadmium uptake mitigation strategies. The high correlation between available cadmium and foliar cadmium indicates that the quantification methodology developed using ethylenediaminetetraacetic acid extractant may be a useful diagnostic tool. © 2021 Society of Chemical Industry.

Bioavailability Assessment of Heavy Metals Using Various Multi-Element Extractants in an Indigenous Zinc Smelting Contaminated Site, Southwestern China

Despite recent studies have investigated the strong influences of smelting activities on heavy metal contamination in the soil environment, little studies have been conducted on the current information about the potential environmental risks posed by toxic heavy metals in smelting contaminated sites. In the present study, a combination of the bioavailability, speciation, and release kinetics of toxic heavy metals in the indigenous zinc smelting contaminated soil were reliably used as an effective tool to support site risk assessment. The bioavailability results revealed that the bioavailable metal concentrations were intrinsically dependent on the types of chemical extractants. Interestingly, 0.02 mol/L EDTA + 0.5 mol/L CH3COONH4 was found to be the best extractant, which extracted 30.21% of Cu, 31.54% of Mn, 2.39% of Ni and 28.89% of Zn, respectively. The sequential extraction results suggested that Cd, Pb, and Zn were the most mobile elements, which would pose the potential risks to the environment. The correlation of metal bioavailability with their fractionation implied that the exchangeable metal fractions were easily extracted by CaCl2 and Mehlich 1, while the carbonate and organic bound metal fractions could be extracted by EDTA and DTPA with stronger chelating ability. Moreover, the kinetic modeling results suggested that the chemical desorption mechanism might be the major factor controlling heavy metal release. These results could provide some valuable references for the risk assessment and management of heavy metals in the smelting contaminated sites.