Bioavailability Of Cd In Soil Research Articles

Effects of Cd-resistant bacteria and calcium carbonate + sepiolite on Cd availability in contaminated paddy soil and on Cd accumulation in brown rice grains.

A pot experiment was conducted to evaluate the effects of combined application of cadmium (Cd)-resistant bacteria (J) and calcium carbonate+sepiolite (G) on both Cd bioavailability in contaminated paddy soil and on Cd accumulation in rice plants. Adding the mixture (J+G) to the soils significantly increased soil pH, decreased extractable Cd contents, and increased Fe/Mn-oxide Cd and organic-bound Cd contents. The applying of J+G, J and G decreased Cd contents in various rice tissues (roots, stems and leaves, husks, and brown rice grains) to different degrees. Compared with those of the CK, Cd contents decreased by 17.8%-53.3% in the roots, 12.3%-27.4% in the stems and leaves, 25.4%-44.6% in the husks, and 28.8%-55.7% in the brown rice grains for the application of J+G; Cd contents decreased by 8.2%-28.5% in the roots, 11.5%-32.0% in the husks, and 27.8%-45.9% in the brown rice grains for the application of J; Cd contents decreased by 12.9%-26.5% in the roots, in the stems and leaves decreased by 4.6%-34.1% in the stems and leaves, 60.2%-79.7% in the husks, and 35.7%-47.6% in the brown rice grains for the application of G. The alone application of bacteria (J) could reduce the bioavailability of Cd in soil and the contents of Cd in brown rice grains to some extent. Moreover, when the bacteria were applied in combination with mineral (J+G), it was a more effective method than the alone application of J or G to reduce the soil Cd bioavailability. Under all the tested conditions, applications of J4+G4 (320mLkg-1 of J+8gkg-1 of G) resulted in the greatest reduction in Cd contents in brown rice grains. Overall, the results indicated that the combination of Cd-resistant bacteria and mineral material could effectively reduce Cd bioavailability in paddy soils and inhibit Cd accumulation in brown rice grains.

Influence of silicon on cadmium availability and cadmium uptake by rice in acid and alkaline paddy soils

The aim of this research was to study the inhibiting effects of cadmium uptake into plants by exogenous silicon between acid and alkaline paddy soils, especially, the effects of silicon without the differentiation of pH and accompanying ions. Therefore, without the interference of pH and accompany ions (Na+), the mechanisms of soil chemistry on cadmium (Cd) transformation and accumulation by Si application in the soil-plant system were examined. The acid and alkaline soils for the experiment were collected from the upper soil layer (0~20 cm) from two paddy fields. The Cd solution as CdNO3 and different levels of SiO2 as Na2SiO3 solutions were applied in the potted soils. To eliminate differences of nitrate and sodium among the treatments, 6.8, 6.8, and 1.7 g sodium nitrate was added into the CK, Si0Cd5, and Si90Cd5 treatments, respectively. The soil redox potential and pH during the experiments were determined. After harvest, changes in Cd concentrations in rice grains, stems, and roots were measured. Soil samples in close proximity to the rice roots were collected at harvest. The sequential extraction was employed to estimate the chemical speciation of Cd in the treated soils. The Si applications reduced Cd accumulation in rice plants, thereby increasing the rice yield. With increase of the dosages, the ratios of Cd contents in root, stem, and grain decreased significantly. The Eh and pH of the Si treatments were higher than application of Cd only. In the alkaline soil, Cd contents in the soil solutions of the Si treatments were higher than the Si0Cd5 treatment. In contrast, in the acid soil, the Cd contents in the soil solution of the Si treatments were lower than that Si0Cd5 treatment. In the acid soil, Si application improved the Cd adsorption by soil and alleviated Cd uptake by plants. Therefore, the alkaline soil reduced Cd phytoavailability by forming Si-Cd water-soluble complex, which could not be taken up by rice plants. The Si application significantly increased yield of rice and decreased Cd concentrations in the roots, stems, and grains of rice plants. In the alkaline soil, Cd and Si formed the Si-Cd water-soluble complex, which could not be taken up by the rice plants. In both soils, Si reduced the concentrations of acid extractable Cd and reducible Cd while promoting the formation of oxidable Cd and residual Cd, resulting in low soil Cd bioavailability.

Cadmium accumulation in rice (Oryza sativa L.) alleviated by basal alkaline fertilizers followed by topdressing of manganese fertilizer

Rice is a main source of dietary cadmium (Cd), thus, how to reduce the Cd concentration in brown rice has received extensive attention worldwide. In three acidic paddy soils slightly to moderately contaminated with Cd, a series of field experiments were conducted to evaluate the effects of different proportions of nitrogen-phosphorus-potassium (N-P-K) fertilizer (urea, calcium magnesium phosphate, and potassium carbonate, respectively) alone or coupled with a topdressing of manganese (Mn) fertilizer at the tillering stage on reducing Cd bioavailability in soil and uptake in rice. The rational application of N-P-K fertilizer not only provided the basic nutrients to promote the normal growth of rice but also increased soil pH and thereby reduced the Cd bioavailability in soil. The Mg(NO3)2-extracted Cd concentrations in the three soils were reduced by 26.46–56.53%, while TCLP-extracted Cd were reduced by 19.87–45.41%, with little influence on soil cation exchange capacity (CEC) and organic matter (OM). The application of Mn fertilizer at the tillering stage increased Mn and Cd sequestration in the iron plaque. The Mn content in iron plaque increased by 15.71–58.67% and a significant positive correlation between Cd and Mn was observed at the three sites. Collectively, this combined method of fertilization significantly reduced Cd accumulation in rice tissues, the Cd concentrations in roots of treated plants decreased by 11.18–37.78%, whereas the concentrations in straw decreased by 13.16–41.03%. Particularly to brown rice, in which accumulation decreased by 25.19–44.70%, 37.35–47.84%, and 38.00–60.88% in three typical paddy fields, but no significant effect was observed for the Cd translocation factors (TF) among rice tissues. Thus, the basal application of combined urea and alkaline inorganic fertilizers followed by topdressing of Mn fertilizer may be a promising and cost-effective tactics for the remediation of Cd-contaminated paddy soils.

Binding and adsorption energy of Cd in soils and its environmental implication for Cd bioavailability

AbstractThe interaction of Cd and other heavy metals with soil colloidal particles controls the sequestration, mobility and bioavailability of Cd in soils. In this study, the binding (△Gbi) and adsorption (△Gad) energies of Cd on colloidal particles of 18 soils were determined by the Wien effect method. The binding energy of Cd on soil colloidal particles varied from 5.3 to 9.9 kJ mol−1, depending on the soil characteristics including pH, Mn‐oxide content and dissolved organic carbon in the soil. The Cd adsorption energy correlated positively with Mn‐oxide content and pH. In parallel, the extended X‐ray absorption fine structure (EXAFS) spectroscopy was used to determine the speciation of Cd in Cd‐saturated soil samples, which revealed that the outer‐sphere Cd was the predominant species, accounting for 32.2–73.7% of the total adsorbed Cd, and positively correlated to the binding and adsorption energies. Humic acid‐Cd (10.4–42.2%) and montmorillonite‐Cd (2.5–51.2%) were also major species that were identified by EXAFS spectroscopy. The toxicity (log EC50) of Cd in soils to three organisms (earthworm, Collembola and Chinese cabbage) was found to correlate positively with the binding energies, indicating the predictive capability of using binding energies of Cd in different soils as an indicator for evaluating Cd bioavailability and toxicity in soils.

Evaluation of cadmium (Cd) transfer from paddy soil to rice (Oryza sativa L.) using DGT in comparison with conventional chemical methods: derivation of models to predict Cd accumulation in rice grains.

The consumption of rice contaminated with soil cadmium (Cd) threatens human health. It is essential to ensure the production of rice that meets food quality standards. Therefore, a large-scale field survey was conducted in Zhejiang province, southeastern China, to investigate the relationship between Cd accumulation in rice grains and Cd bioavailability in soil, and thus to establish a model to predict Cd contents in rice grains based on soil properties. For this purpose, a total of 156 paired rice and soil samples were collected. Pearson's correlation analysis revealed that Cd measurements obtained by diffusive gradient in thin films (DGT) had a higher correlation (r = 0.818, p < 0.001) with the Cd in rice grains as compared to the Cd measured by the DTPA, CaCl2, EDTA, and HCl extraction methods, which indicated that the DGT technique was a reliable method for the assessment of Cd bioavailability in soils. In addition, among the four extraction methods, the DTPA-extractable Cd showed the highest correlation with the Cd contents in rice grains. Therefore, we developed two predictive models (modelDGT and modelDTPA) to predict Cd levels in rice grains via Cubist multivariate mixed linear regression, using "soil DGT-measured Cd, pH, and oxide contents of Ca, Si, and Fe" or "soil DTPA-extractable Cd, pH, OM, and oxide contents of Ca and Fe" as explanatory variables, respectively. The overall modelDGT and modelDTPA had R2 values of 0.95 and 0.93, respectively, and relative error values of 0.30 and 0.33, respectively. Simple correlation analysis showed direct and close relationships between the measured Cd in rice grains and the Cd concentrations predicted by the Cubist modelDGT and modelDTPA, with R2 values of 0.979 and 0.922, respectively. Therefore, Cd levels in rice grains could be predicted very well based on the two prediction models, and thus, the two models derived in this study are effective in identifying soils in which the Cd in rice grains will exceed food safety standards, thereby helping to ensure safe rice production.

Paddy periphyton reduced cadmium accumulation in rice (Oryza sativa) by removing and immobilizing cadmium from the water–soil interface

Periphyton plays a significant role in heavy metal transfer in wetlands, but its contribution to cadmium (Cd) bioavailability in paddy fields remains largely unexplored. The main aim of this study was to investigate the effect of periphyton on Cd behavior in paddy fields. Periphyton significantly decreased Cd concentrations in paddy waters. Non-invasive micro-test technology analyses indicated that periphyton can absorb Cd from water with a maximum Cd2+ influx rate of 394 pmol cm−2 s−1 and periphyton intrusion significantly increased soil Cd concentrations. However, soil Cd bioavailability declined significantly due to soil pH increase and soil redox potential (Eh) decrease induced by periphyton. With periphyton, more Cd was adsorbed and immobilized on organic matter, carbonates, and iron and manganese oxides in soil. Consequently, Cd content in rice decreased significantly. These findings give insights into Cd biogeochemistry in paddy fields with periphyton, and may provide a novel strategy for reducing Cd accumulation in rice.

Environmental and edaphic factors affecting soil cadmium uptake by spinach, potatoes, onion and wheat

The relative ease with which cadmium (Cd) in agricultural soils can transfer to crop plants can pose a potential health risk to consumers. However, efforts to predict and mitigate these risks are often confounded by the various factors that influence metal accumulation in the edible plant parts. The aim of this work was to identify key drivers that determine Cd concentrations in spinach leaves, potato tubers, onion bulbs and wheat grain grown in commercial horticultural operations across New Zealand (NZ). Paired soil and plant samples (n = 147) were collected from farms across different NZ growing regions. Cadmium concentrations in the edible parts were measured and four different tests were used to examine the potential bioavailability of soil Cd: pseudo-total and porewater concentrations, 0.05 M Ca(NO3)2-extraction and diffusive gradients in thin-films (DGT). Information on a range of soil and climatic variables was also collected. The methods' ability to represent Cd concentrations in the plant parts was assessed through single and multiple regression analysis that considered the different variables and the farm locations. Soil Cd concentrations determined by the different tests were positively related to plant concentrations and there were clear regional differences between these relationships. The Ca(NO3)2 extraction predicted over 76% of the variability in Cd concentrations in onion bulbs and spinach leaves, while DGT and porewater Cd provided the best estimates for potato tubers and wheat grains, respectively, once regional differences were considered, along with certain environmental and soil variables. The results show that certain soil and environmental factors can be a key influence for determining Cd accumulation in the edible parts of some plants and that regional differences are important for modulating the extent to which this occurs. These effects should be considered when trying to mitigate the potential risks arising from Cd in agricultural soils.

The application of ultrasonic treatment and a bis(2-ethylhexyl)sulfosuccinate-based novel ionic liquid for cadmium extraction

The application of [N4444]AOT can significantly decrease the bioavailability of Cd in soil without changing the soil properties.

Remediation Effects of Different Composite Materials on Cadmium-Contaminated Farmland Soil

A pot experiment was conducted to study the application effects of three composite materials, namely SC (lime:organic compound fertilizer=2:3), LS (ferrous sulfate:lime=1:1) and LB (ferrous sulfate:biochar in combinations of 1:1, 1:2, 1:3, 1:4 and 1:5), on soil Cd bioavailability, Cd cumulative distribution in different wheat organs, and wheat yield. The results indicated that:① Addition of composite materials all significantly decreased the soil available Cd content by 50.2%-81.8% (SC), 29.4%-48.1% (LS), and 18.7%-42.2% (LB). Composite materials significantly increased soil pH by 1.37-2.28 (SC), 0.41-0.86 (LS), and 0.14-0.17 (LB) units. ② The Cd cumulative distribution in different wheat organs were in the order of root > leaf > stem > glume > grain. The translocation abilities of Cd in different organs were in the order of root > glume > stem and leaf. ③ Compared with the control, 0.67% SC addition and 0.67% LS addition significantly increased the wheat yields by 56.4% and 51.2%; LB addition significantly increased wheat yield by 39.6% to 51.2%. ④ The correlation analysis showed that soil pH was significantly negatively correlated with soil available Cd and Cd contents in different wheat organs. There were significant positive correlations between soil available Cd and Cd contents in different wheat organs, and the correlation coefficients were 0.711 (grain), 0.817 (glume), 0.593 (stem), 0.630 (leaf) and 0.622 (root). Meanwhile, there is also a significant positive correlation between Cd content in different wheat organs. ⑤ Comprehensively, the addition of 0.93% SC increased soil pH by a maximum of 2.28 units, and the soil available cadmium content was decreased by a maximum of 81.8%. Therefore, adding 0.93% SC was the most suitable treatment for repairing and controlling the Cd pollution in farmland soil.

Evaluation of various approaches to predict cadmium bioavailability to rice grown in soils with high geochemical background in the karst region, Southwestern China

Evaluating the bioavailability of Cd to rice (Oryza sativa L.) was essential in the karst region, Southwestern China, where the soils have previously been shown to be anomalously enriched in Cd through geogenic processes. In this research, we examined the bioavailability of Cd to rice samples collected from 278 sites in Guangxi province, where rice is the most widely cultivated cereal crop that is responsible for the largest human dietary exposure to Cd. Both soil chemical extraction and soil-plant transfer modelling approaches were used to predict the bioavailability to rice. Some of the soil types were highly enriched in Cd, but their bioavailability was low, since the soil carbonates raised soil pH and remarkably reduced Cd bioavailability. In contrast, acidic soils (Ca was largely leached) with relatively low total Cd, the grown rice plants accumulated higher Cd in their grains. Results from CaCl2 extraction experiments provided good predictions for Cd in rice grain grown in soils of different types. Stepwise multiple regressions revealed soil pH and soil Ca content were the dominant factors that control the transfer of Cd from soil to rice. An extended Freundlich-type model and a polynomial surface model provided good prediction for Cd in rice grains. The diffusive gradients in thin films (DGT) technique gave the best estimation of soil Cd bioavailability, whereas water-extracted soil solution Cd provided relatively poor fits. Regional soil threshold that derived using the models, can avoid exceedance of Cd in rice and thereby enable local agricultural practitioners or authorities to develop appropriate management for croplands with high Cd background.

Cd accumulation and transfer in pepper (Capsicum annuum L.) grown in typical soils of China: pot experiments.

Food chain contamination by soil cadmium (Cd) through vegetable consumption poses a threat to human health. It is imperative to understand the Cd uptake and transfer in different soil-vegetable systems. The aims of this study were to understand the effect of soil characteristics on Cd accumulation and transfer in pepper and to derive regression models to predict Cd concentrations in the vegetable grown on a wide range of soils with different properties. The accumulation and transfer of Cd in the root, stem, and fruit of pepper (Capsicum annuum L.) grown in 18 typical soils of China were investigated through pot experiments. The bioavailability of Cd in soil was evaluated by using EDTA and HNO3 extraction methods. The pot experiments included a control and two concentration levels of Cd salt added to soils according to Soil Environmental Quality Standards of China. The results showed that the Cd content in pepper fruits ranged from 0.007 to 0.049 for the control, 0.045 to 0.260 for the low Cd treatment, and 0.076 to 0.345 mg/kg for the high Cd treatment, respectively. The concentrations of Cd in the different parts of pepper decreased in the order of root > stem > fruit, and there were significant correlations among the Cd concentrations in pepper root, stem, and fruit tissues. Bioaccumulation factor (BCF) and transfer factor (TF) of Cd in pepper fruits exhibited a low accumulation of Cd in the fruit of pepper. The Cd accumulation in pepper fruit could be quantitatively predicted by EDTA-extractable Cd content in soils. Multiple linear regression models proved functional in predicting Cd accumulation in different parts of pepper. The Cd content in pepper tissues was well predicted using EDTA-extractable Cd and soil variables, such as pH, EC, CEC, total phosphorus, and CaCO3 content. Soil pH and EC were major soil factors influencing Cd transfer from soil to pepper fruits, whereas total phosphorus content presented a negative effect on Cd accumulation in stem and root parts of pepper.

Effects of inorganic and organic amendments on physiological parameters and antioxidant enzymes activities in Zea mays L. from a cadmium-contaminated calcareous soil

In situ stabilization is a promising remediation technology to decrease the hazard potential of contaminants in soils through converting them into their least-soluble, -mobile or -toxic forms. In the present study, the effectiveness of various amendments (e.g., biochar 640 °C, and biochar 420 °C) and selected low cost amendments (e.g., leca, pumice, bentonite, and zeolite) was investigated on Cd stabilization through testing their potential to decrease soil Cd bioavailability, and their effects on photosynthetic pigments, antioxidant enzyme activities and Cd uptake by maize (zea mays L.) in a polluted calcareous soil. By the end of the experiment, all amendments resulted in increased soil pH value and electrical conductivity (EC) relative to the control soil. The Toxicity Characteristic Leaching Procedure (TCLP) Cd was supposed to indicate the potential bioavailability of Cd in the soils. Generally, application of the amendments decreased the Cd bioavailability in the soils compared to un-amended soil. This decreased bioavailability of Cd in soil was accompanied by an increased antioxidant enzyme activities and photosynthetic pigments, and decreased plant uptake of Cd. Plant antioxidant enzyme activities {superoxide dismutase (SOD), catalase (CAT), peroxidase (PX), and ascorbate peroxidase (ASP)} and photosynthetic pigments (chlorophyll a, chlorophyll b, total chlorophyll, and carotenoids) were significantly increased in the amended soils. These increases may be due to a lower Cd accumulation in leaves. Zeolitse (%5) was the most effective amendment in decreasing cadmium concentration in maize.

Mitigation of rice cadmium (Cd) accumulation by joint application of organic amendments and selenium (Se) in high-Cd-contaminated soils.

A pot experiment was conducted to investigate the possible mediatory effect of organic amendments (vermicompost and biochar) and selenium (Se) on Cd bioaccumulation in both rice cultivars (high-Cd accumulation rice: Yuzhenxiang (YZX) and low-Cd accumulation rice: Changliangyou772 (CLY)) in high-Cd-contaminated soils. The results showed that Cd sensitivity and tolerance were cultivar-dependent, and grain Cd contents for CLY accorded with the Chinese national food safety standards (0.2 mg kg-1), whereas grain Cd levels for YZX were 1.4-5.8 times higher than those for CLY. Soil applications of amendments decreased grain Cd levels by 3.5%-36.9% for YZX and 36.1%-74.4% for CLY. Moreover, vermicompost (VC) was more effective in reducing Cd bioaccumulation than biochar (BC). A combination of Se and organic amendments could significantly increase grain Se contents and help further reduce grain Cd levels by 5.8%-20.8%, compared to the single organic amendments. This mitigation progress could be attributed to the changes of Cd translocation and distribution among rice tissues and the inhibition of Cd bioavailability in soil through the alteration in soil properties. Organic amendments, especially high dose (5%), increased soil pH and organic matter contents, and correspondingly decreased soil Cd bioavailability. A sequential extraction analysis suggested that organic amendments and Se facilitated the transformation of soil Cd from the bioavailable form to the immobilized Cd form, and thus decreased grain Cd levels. Hence, co-applications of organic amendments and Se in combination with low-Cd accumulation cultivar could be an effective strategy for both Se needs of humans and safe utilization of Cd polluted soil.

In-Situ Immobilization of Cd-Contaminated Soils Using Ferronickel Slag as Potential Soil Amendment.

The current study investigated the efficiency and mechanisms of in situ immobilization of artificially Cd-contaminated soils with ferronickel slag (FNS). The available Cd content of soil was measured and the modified European Community Bureau of Reference (BCR) sequential extraction procedure (SEP) was adopted to quantify the evolutions of Cd chemical speciation after the immobilization by the FNS. The results showed that the addition of FNS (5%‒15%) remarkably reduced the available Cd content and increased the pH and cation exchange capacity of soils. The passivation rate of Cd increased from 58.13% to 73.25% as the spiked Cd content rose from 10 to 120mgkg‒1. The BCR SEP test revealed that the FNS addition substantially reduced the acid soluble fraction and increased the residual fraction of Cd, indicating the reduction of mobility and bioavailability of Cd in soils. The chemical precipitation, ion exchange and surface complexation might be involved in in situ immobilization of Cd-contaminated soils by the FNS.

Using sequential extraction and DGT techniques to assess the efficacy of plant- and manure-derived hydrochar and pyrochar for alleviating the bioavailability of Cd in soils

Biochar has emerged as a useful tool to reduce the bioavailability of heavy metals in contaminated soils. However, limited research has been directed towards determining the effects of pyrolysis conditions and feedstock types of biochar on the bioavailability of heavy metals in biochar-treated soils. This work evaluated the efficacy of plant- and manure-derived hydrochar and pyrochar for alleviating Cd bioavailability in soils using conventional chemical extraction, diffusive gradients in thin films (DGT) and plant uptake. For each feedstock type, hydrochar showed lower maximum adsorption capacity than pyrochar. As determined by diethylenetriamine pentaacetic acid, BCR extraction and DGT measurement, pyrochar significantly decreased the theoretical bioavailability of Cd. Moreover, a slight decrease in the ability of soil to resupply Cd to the solution was detected. Also, pyrochar induced a substantial reduction in Cd uptake in ryegrass. Corresponding to the high sorption capacity, swine solid-derived 450 °C pyrochar was the most effective in alleviating Cd bioavailability in soils, indicating its great potential for remediating Cd-contaminated soils. Additionally, the bioavailable Cd amount determined by DGT had the best correlation with the Cd content in ryegrass, showing that DGT technology could better represent the effects of biochars on Cd phytoavailability in the studied soil.

Cadmium uptake in radish (Raphanus sativus L.) and surficial contamination: implications for food safety and local soil management

Soil management strategies for agricultural lands contaminated with potentially toxic trace elements, especially cadmium (Cd), are still inadequate and require a precise identification of soils that are not s afe for growing crops. Key soil variables need to be identified to connect soil safety with food safety by reliable models. Soil variables that affect concentrations of metals in different portions of radish, Raphanus sativus L., were examined as part of a greenhouse experiment. 0.01 M di-sodium-di-hydroxy-ethylenediamine-tretra-acetic acid (Na2H2EDTA) solution was used in a strong rinsing experiment. Cd soil-radish relationships were derived by different modeling approaches and were used to develop local risk screening values for Cd in soil. The current lab washing procedures readily remove surface Cd adherence but are not adequate to remove surface-deposited lead (Pb), thus overestimating bioaccumulation in plants by mean 111%. Shoot and root tissue Cd concentration in fresh weight basis do not present a significant difference and can be precisely predicted by regression models using different Cd pools and soil pH. Preferably a polynomial surface model can be used in developing local rick screening values that yield concentrations of Cd in radish at or below the Chinese food quality standard of 0.1 mg kg−1 (fresh weight). The bioaccumulation of Cd in radish depends on the Cd bioavailability in soil. But for Pb, surficial particle contamination masks the realistic bioaccumulation. We also demonstrated the usefulness of the polynomial surface model to develop local soil protection guidelines that are helpful to local farmers for proper soil management and avoidance of Cd exceedance in food.

Wheat straw biochar reduces environmental cadmium bioavailability

Cadmium contamination in waters and soils can lead to food chain accumulation and ultimately deterioration in human health; means for reducing bioavailable Cd are desperately required, and biochars may play a role. Long-term (240 d) lab incubation experiments were utilized to explain wheat straw-derived biochar effects on Cd sorption and decreasing Cd bioavailability in soils and solutions (0, 5, and 15% biochar as wt:wt or wt:vol, respectively), and to identify Cd forms present using both the European Community Bureau of Reference (BCR) chemical sequential extraction procedure and synchrotron-based X-ray absorption spectroscopy (XAS). Biochar Cd removal was up to ~90% from Cd-containing solutions and contaminated soil as compared to the control. Based on the wet chemical sequential extraction procedure in conjunction with XAS, biochar application promoted the formation of (oxy)hydroxide, carbonate, and organically bound Cd phases. As a material, biochar may be promoted as a tool for reducing and removing bioavailable Cd from contaminated waters and soils. Thus, biochar may play a role in reducing Cd bioaccumulation, trophic transfer, and improving environmental quality and human health.

Cadmium bioavailability in acidic soils under bean cultivation: role of soil additives

Continuous application of phosphate fertilizer and bio-solids leads to cadmium (Cd) accumulation in agricultural soil. Acidic soil environment makes the situation worsen due to the conversion of higher soil Cd to plant available form. Therefore, identifying effective and economical methods to reduce the bioavailability of soil Cd is imperative. This pot experiment was conducted in an acidic soil (pH 5.69) with three different soil additives [biochar, plant growth-promoting rhizobacteria (PGPR) and ZnSO4] under three levels (6, 12 and 18 mg kg−1) of Cd on common bean (Phaseolus vulgaris L. cv. Falguni). Biochar and ZnSO4 were mixed with soil at 5.5 and 0.1 Mg ha−1, respectively, before seed sowing, while PGPR was applied as a seed coating. A significant reduction in plant morpho-physiological and biochemical parameters and soil chemical and biological properties was recorded with increasing Cd levels. Biochar, PGPR and ZnSO4 addition resulted in substantial improvement in the studied plant and soil parameters in Cd-treated soil by significantly reducing the bioavailability of soil Cd. ZnSO4 addition recorded improved (51%) plant growth in terms of total biomass with lower (57% shoot and 42% reduction in root) plant accumulation of Cd. The highest reduction in bioavailability of soil Cd was recorded with the application of biochar (34–97% reduction) compared with ZnSO4 (14–89%). Though application of PGPR reduced the Cd bioavailability to 94% at the lowest contamination, it increased Cd bioavailability by 2% at the highest level of contamination. Thus, biochar application (5.5 Mg ha−1) as soil additive is an effective means to mitigate Cd toxicity in acidic soils.

Effects of grape seedlings intercropping with post-Grafting generation of Cosmos sulphureus on different fractions of cadmium content in soil

A pot experiment was conducted to study the effects of grape seedlings intercropping with post-grafting generation of Cosmos sulphureus on different fractions of cadmium (Cd) in soil. The results showed that when grape seedlings intercropped with the generation of ungrafted C. sulphureus (I CK), the content of exchangeable Cd increased by 25%, and the content of residual Cd decreased by 17% compared to the monoculture, respectively. Compared to the monoculture, grape seedlings intercropping with the post-grafting generation of self-rooted grafting by two uniform plant seedlings of C. sulphureus (I UG) significantly increased the residual Cd content (37%) while the exchangeable Cd content increased not significantly (11%). In conclusion, the treatment of I CK was not conducive to the reduction of the bioavailability of Cd in soil, while the treatment of I UG was better.

Root-induced soil acidification and cadmium mobilization in the rhizosphere of Sedum plumbizincicola: evidence from a high-resolution imaging study

Plant roots can significantly alter soil pH and the chemical concentration and distribution of different elements in the rhizosphere environment. Here we ask whether cadmium (Cd) bioavailability in the rhizosphere of Cd-hyperaccumulator Sedum plumbizincicola can be influenced by root-induced effects on soil pH. The Cd-hyperaccumulator S. plumbizincicola and the Cd non-hyperaccumulator ecotype Sedum alfredii were both grown in four different Cd-contaminated soils. We used the planar optode imaging technique to produce two-dimensional and high-resolution measurements of soil pH. Shoot excess cation concentration, root architecture and Cd concentrations ([Cd]) in soil pore water were also measured. Spatial analyses based on kernel density estimate of roots (KDE) and a Moran’s I correlogram were performed to assess spatial patterns and potential relationships among root distribution, soil pH and [Cd]. Both Sedum species showed root-induced increases in soil acidification (i.e. soil pH decreases of 0.1 to 0.62 units), which were clearly associated with greater root density of these plants. Remarkable excess cation uptakes by both Sedum species were detected and likely a driving factor for the root-induced acidification. The presence of the roots of S. plumbizincicola were then related to higher [Cd] in the rhizosphere than in bulk soil in Orthic Acrisol (+342%) and in Hydragric Antrosol soils (+296%). The hyperaccumulator S. plumbizincicola had larger root systems, higher acidification ability, and was associated with greater soil [Cd] than S. alfredii. Spatial patterns of root distribution and soil pH were similar between Sedum plants, however, spatial patterns of [Cd] differed across polluted soils. Rhizosphere acidification induced by S. plumbizincicola plants can play an important role on soil Cd mobilization, but overall effects on soil Cd bioavailability will depend on intrinsic soil biogeochemical properties.