Solubility Of Metals In Soils Research Articles

A comparison of characterisation and modelling approaches to predict dissolved metal concentrations in soils

Environmental context It is useful to know the concentration of ‘labile’, or chemically active, metal in soils because it can be used to predict metal solubility and environmental impact. Several methods for extracting the labile metal from soils have been proposed, and here we have tested two of these to see how well the resulting data can be used to model metal solubility. Such mixed approaches can be applied to different soil types with the potential to model metal solubility over large areas. Rationale Predicting terrestrial metal dynamics requires modelling of metal solubility in soils. Here, we test the ability of two geochemical speciation models that differ in complexity and data requirements (WHAM/Model VII and POSSMs), to predict metal solubility across a broad range of soil properties, using differing estimates of the labile soil metal concentration. Methodology Using a dataset of UK soils, we characterised basic properties including pH and the concentrations of humic substances, mineral oxides and metals. We estimated labile metal by extraction with 0.05 mol L−1 Na2H2EDTA and by multi-element isotopic dilution (E-value). Dissolved concentrations of Ni, Cu, Zn, Cd and Pb were estimated in 0.01 mol L−1 Ca(NO3)2 soil suspensions using the total metal ({M}total), the EDTA-extracted pool ({M}EDTA) and the E-value ({M}E) as alternative estimates of the chemically reactive metal concentration. Results Concentrations of {M}EDTA were highly correlated with values of {M}E, although some systematic overestimation was seen. Both WHAM/Model VII and POSSMs provided reasonable predictions when {M}EDTA or {M}E were used as input. WHAM/Model VII predictions were improved by fixing soil humic acid to a constant proportion of the soil organic matter, instead of the measured humic and fulvic acid concentrations. Discussion This work provides further evidence for the usefulness of speciation modelling for predicting soil metal solubility, and for the usefulness of EDTA-extracted metal as a surrogate for the labile metal pool. Predictions may be improved by more robust characterisation of the soil and porewater humic substance content and quality.

Flooding and drainage induced abiotic reactions control metal solubility in soil of a contaminated industrial site

Intense industrialization has led to the increasing leaching risk of metals into groundwater at heavily polluted industrial sites. However, metal dissolution in polluted industrial soils has been neither fully investigated nor quantified before. In this study, the dissolution of Zn, Ni, and Cu in soil from a heavily contaminated industrial site during a flooding-drainage period was investigated by sequential extraction, geochemical modelling, and X-ray absorption near edge structure spectroscopy. The results showed a steady decrease in metal solubility during both reduction and oxidation stages. During reduction, with limited decrease in Eh (>100 mV), formation of carbonate precipitates rather than sulfide precipitates and adsorption on soil solids was responsible for Zn and Ni dissolution, whereas bound to soil organic matter (SOM) and iron oxides dominated Cu dissolution, due to its lower concentration and higher affinity to SOM and iron oxides compared to Zn and Ni. During oxidation, the acidity caused by ferrous oxidation was buffered by calcite dissolution, while metal precipitation ceased and adsorption on soil surface controlled metal solubility. The metal solubility and speciation during the flooding-drainage process were quantitatively predicted by geochemical model. The findings demonstrate that due to high metal concentrations and weak microbial effect in the industrial soil, metal release was largely regulated by abiotic reactions rather than biotic reactions, which is somehow different from that of the wetland or rice field soils.

Phosphorus depletion controls Cu and Zn biogeochemistry in canola and corn rhizosphere on a calcareous soil

Phosphorus (P) deficiency may trigger rhizodeposition, including protons and organic compounds, with possible effects on metal solubility and speciation. To explore the relevance of this process, we investigated biogeochemical changes in the rhizosphere of P-deficient canola (Brassica napus L.) and corn (Zea mays L.) cultivars grown in a pot experiment on calcareous soil. Depletion of total soluble (0.005 mol/L Ca(NO3)2-extractable) P in the rhizosphere varied with crop species and cultivar but was generally strong and negatively correlated with dissolved organic carbon (DOC) in canola (R2 = 0.868) and corn (R2 = 0.844) rhizospheres, indicating rhizodeposition in response to limited P availability. DOC was correlated with dissolved Cu, explaining 86% of its variation in the rhizosphere and bulk soil solution of canola and corn cultivars, respectively, suggesting Cu mobilisation via the formation of Cu-organic complexes. In line with lower Zn-organic complex stabilities, the effect of rhizodeposition was less pronounced for Zn mobilisation. We show that the P nutritional status of plants and the related variation of rhizodeposition among crops and cultivars represents a major control of metal solubility in soil, with possible effects on micronutrient supply and toxicity. Hence, targeted P availability control should be considered in the management of polluted and micronutrient-deficient soils.

Characterization of dissolved organic matter in the rhizosphere of phytostabilizer Athyrium wardii (Hook.) involved in enhanced metal accumulation when exposed to Cd and Pb co-contamination

The characterization of DOM and its effect on heavy metal solubility in soils have been widely concerned, while few concerns on the phytostabilization of multi-metal contaminated soils. A pot experiment was performed to characterize dissolved organic matter (DOM) in the rhizosphere of the mining ecotype (ME) and non-mining ecotype (NME) of Athyrium wardii (Hook.) when exposed to Cd and Pb simultaneously, and investigate its effect on Cd and Pb solubility in soils. The ME presented more DOM in the rhizosphere when exposed to Cd and Pb simultaneously than that exposed to single Cd or Pb, and also than the NME. The acid fractions (hydrophilic acid, hydrophobic acid) and hydrophilic fractions (hydrophilic acid, hydrophilic neutral, and hydrophilic base) were the dominant parts of DOM in the ME rhizosphere. The ME presented more acid and hydrophilic fractions in the rhizosphere when exposed to Cd and Pb simultaneously. Meanwhile, there were more O–H, C–O, N–H and C–H, assigned to carboxylic groups, phenolic groups, hydroxyl groups, and/or amino groups, present in DOM from the rhizosphere of ME when exposed to Cd and Pb simultaneously. These results highlighted the acid characteristics of DOM in the rhizosphere of ME when exposed to Cd and Pb simultaneously. DOM in the rhizosphere of ME thereby showed greater complexation degree for Cd (68%) and Pb (77%), thus showing greater ability to enhance Cd and Pb solubility in soils when exposed to Cd and Pb simultaneously. This is thereby considered to be one of the key processes for enhancing Cd and Pb uptake by the ME when exposed to Cd and Pb simultaneously.

Enhancing the effectiveness of zinc, cadmium, and lead phytoextraction in polluted soils by using amendments and microorganisms.

For remediating polluted soils, phytoextraction of metals received considerable attention in recent years, although slow removal of metals remained a major constraint in this approach. We, therefore, studied the effect of selected organic and inorganic amendments on the solubility of zinc (Zn), cadmium (Cd), and lead (Pb) in polluted soil and enhancing the efficacy of phytoextraction of these metals by Indian mustard (Brassica juncea cv. Pusa Vijay). For this purpose, a greenhouse experiment was conducted using a metal-polluted soil to evaluate the effect of amendments, viz. green manure (T2), EDTA (T3), sulfur (S)+S oxidizing bacteria (Thiobacillus spp.) (T4), metal-solubilizing bacteria (Pseudomonas spp.) (T5), and green manure + metal-solubilizing bacteria (T6), on solubility and bioavailability of Zn, Cd, and Pb. Distribution of metals in different soil fractions revealed that Cd content in water soluble + exchangeable fraction increased to the extent of 34.1, 523, 133, 123, and 75.8% in T2, T3, T4, T5, and T6 treatments, respectively, over control (T1). Cadmium concentrations in soil solution as extracted by Rhizon sampler were recorded as 3.78, 88.1, 11.2, 6.29, and 4.27μgL-1in T2, T3, T4, T5, and T6, respectively, whereas soil solution concentration of Cd in T1 was 0.99μgL-1. Activities of Cd (pCd2+) in Baker soil extract were 12.2, 10.9, 6.72, 7.74, 7.67, and 7.05 for T1, T2, T3, T4, T5, and T6, respectively. Cadmium contents in shoot were recorded as 2.74, 3.12, 4.03, 4.55, 4.68, and 4.63mgkg-1 in T1, T2, T3, T4, T5, and T6 treatments, respectively. Similar trend in Zn and Pb content with different magnitude was also observed across the different amendments. Cadmium uptake by shoot of mustard was enhanced to the extent of 125, 62.5, 175, 175, and 212% grown on T2-, T3-, T4-, T5-, and T6-treated soil, respectively, over T1. By and large, free ion activity of metals as measured by Baker soil test proved to be the most effective index for predicting Zn, Cd, and Pb content in shoot of mustard, followed by EDTA and DTPA. Among the metal fractions, only water soluble + exchangeable metal contributed positively towards plant uptake, which explained the variation in shoot Zn, Cd, and Pb content to the extent of 74, 81, and 87%, respectively, along with other soil metal fractions. Risk to human health for intake of metals through the consumption of leafy vegetable (mustard) grown on polluted soil in terms of hazard quotient (HQ) ranged from 0.64 to 1.10 for Cd and 0.11 to 0.34 for Pb, thus rendering mustard unfit for the human consumption. Novelty of the study mainly consisted of the use of natural means and microorganisms for enhancing solubility of metals in soil with the ultimate aim of hastening the phytoremediation.

Efficiency of biogas slurry and Burkholderia phytofirmans PsJN to improve growth, physiology, and antioxidant activity of Brassica napus L. in chromium-contaminated soil.

Contamination of soil is a major problem globally with colligated danger for ecosystem and human health. Chromium (Cr) is a toxic heavy metal and caused harmful effect on growth and development of plants. Phytostabilization reduced the mobility of heavy metals with addition of amendments which can significantly decrease metal solubility in soil. Phytostabilization can be achieved by application of biogas slurry (BGS) and endophytic bacteria as amendments in the contaminated soils. The present study revealed that the Burkholderia phytofirmans PsJN and BGS improved the growth, physiology, and antioxidant activity and reduced Cr uptake under a pot experiment spiked with Cr (20mgkg-1 soil). The experiment was designed under completely randomized design, four treatments with three replications in normal and Cr-contaminated soil. The inoculation of endophytic bacteria improved the growth and physiology of Brassica. This study showed that the inoculation of endophytic bacteria stabilized the Cr levels in soil and minimized the uptake by the plant shoots and roots in BGS-amended soil. Similarly, activity of antioxidants such as catalase (CAT), reduced glutathione (GSH), glutathione peroxidase (GSH-Px), and glutathione s-transferase (GST) was decreased to normal with combined treatment of BGS and endophytic bacteria in Cr-stressed soil. Overall, the best results were analyzed by combined treatment of BGS and endophytic bacteria to improve growth, physiology, and antioxidant activity of Brassica and immobilize Cr in soil. Moreover, results emphasized the need to use BGS alone or in combination with endophytic bacteria to optimize crop performance, stabilize Cr concentration, and improve environmental efficiency.

Solid phase speciation of Zn and Cd in zinc smelter effluent-irrigated soils

Solubility of metal in contaminated soils is a key factor which controls the phytoavailability and toxic effects of metals on soil environment. The chemical equilibria of metal ions between soil solution and solid phases govern the solubility of metals in soil. Hence, an attempt was made to identify the probable solid phases (minerals), which govern the solubility of Zn2+ and Cd2+ in zinc smelter effluent-irrigated soils. Estimation of free ion activities of Zn2+ (pZn2+) and Cd2+ (pCd2+) by Baker soil test indicated that metal ion activities were higher in smelter effluent-irrigated soils as compared to that in tubewell water-irrigated soils. Identification of solid phases further reveals that free ion activity of Zn2+ and Cd2+ in soil highly contaminated with Zn and Cd due to long-term irrigation with zinc smelter effluent is limited by the solubility of willemite (Zn2SiO4) in equilibrium with quartz and octavite (CdCO3), respectively. However, in case of tubewell water-irrigated soil, franklinite (ZnFe2O4) in equilibrium with soil-Fe and exchangeable Cd are likely to govern the activity of Zn2+ and Cd2+ in soil solution, respectively. Formation of highly soluble minerals namely, willemite and octavite indicates the potential ecological risk of Zn and Cd, respectively in smelter effluent irrigated soil.

Transition Metal Transport in Plants and Associated Endosymbionts: Arbuscular Mycorrhizal Fungi and Rhizobia.

Transition metals such as iron, copper, zinc, or molybdenum are essential nutrients for plants. These elements are involved in almost every biological process, including photosynthesis, tolerance to biotic and abiotic stress, or symbiotic nitrogen fixation. However, plants often grow in soils with limiting metallic oligonutrient bioavailability. Consequently, to ensure the proper metal levels, plants have developed a complex metal uptake and distribution system, that not only involves the plant itself, but also its associated microorganisms. These microorganisms can simply increase metal solubility in soils and making them more accessible to the host plant, as well as induce the plant metal deficiency response, or directly deliver transition elements to cortical cells. Other, instead of providing metals, can act as metal sinks, such as endosymbiotic rhizobia in legume nodules that requires relatively large amounts to carry out nitrogen fixation. In this review, we propose to do an overview of metal transport mechanisms in the plant–microbe system, emphasizing the role of arbuscular mycorrhizal fungi and endosymbiotic rhizobia.

Remediation of lead-, arsenic-, and cesium-contaminated soil using consecutive washing enhanced with electro-kinetic field

Purpose Extensive deposition of Pb, As, and Cs in soils may damage ecosystems and human’s health. Soil washing is the most conventional remediation method, and its efficiency depends on metal solubility in soil. This study aims to optimize operating variables of electro-kinetic field (EKF)-enhanced soil washing procedures.

Changes in the heavy metal solubility of two contaminated soils after heavy metals phytoextraction with Noccaea caerulescens

Hyperacumulator plant species, such as Noccaea caerulescens, have been deeply studied due to their use in phytoextraction techniques, although the fate of the metals remaining in the roots at the end of the remediation process is still uncertain. Here, germination and growth, metal accumulation in plant tissues and degradation of roots remaining in the soil after harvest have been studied in two contaminated soils from an area affected by a toxic pyritic sludge spillage. Specially designed pots allowed the separation of the bulk soil and rhizosphere, where heavy metals fractionation in soil was determined at the end of the growing period. High Cd and, especially, Zn concentrations in the aerial parts of the plants were found, although the bio-concentration factors (BCF) were higher for Cd (13–34) than for Zn (2.37–4.34). For both soils, the soluble and exchangeable (CaCl2-extractable) concentrations of Fe and Mn were higher in the rhizosphere than in the bulk soil, while Zn and Cd concentrations were greater in the bulk soil. After plant harvesting, the degradation of heavy metal enriched roots in the soil was studied as well as the effect of this process on soil metal solubility. The soluble concentrations of Cu and Mn were higher in the soils with roots that in the corresponding soils without roots. Nevertheless, the amount of heavy metals released to the soil after root degradation (8–14% of their organic C was decomposed) was rather low (below 0.1μgg−1), showing the feasibility of the use of N. caerulescens for phytoextraction.

Bioaugmentation with Endophytic Bacterium E6S Homologous to Achromobacter piechaudii Enhances Metal Rhizoaccumulation in Host Sedum plumbizincicola.

Application of hyperaccumulator-endophyte symbiotic systems is a potential approach to improve phytoremediation efficiency, since some beneficial endophytic bacteria are able to detoxify heavy metals, alter metal solubility in soil, and facilitate plant growth. The objective of this study was to isolate multi-metal resistant and plant beneficial endophytic bacteria and to evaluate their role in enhancing plant growth and metal accumulation/translocation. The metal resistant endophytic bacterial strain E6S was isolated from stems of the Zn/Cd hyperaccumulator plant Sedum plumbizincicola growing in metalliferous mine soils using Dworkin and Foster salts minimal agar medium with 1-aminocyclopropane-1-carboxylate (ACC) as the sole nitrogen source, and identified as homologous to Achromobacter piechaudii based on morphological and biochemical characteristics, partial 16S rDNA sequence and phylogenetic analysis. Strain E6S showed high level of resistance to various metals (Cd, Zn, and Pb). Besides utilizing ACC, strain E6S exhibited plant beneficial traits, such as solubilization of phosphate and production of indole-3-acetic acid. Inoculation with E6S significantly increased the bioavailability of Cd, Zn, and Pb in soil. In addition, bacterial cells bound considerable amounts of metal ions in the following order: Zn > Cd >Pb. Inoculation of E6S significantly stimulated plant biomass, uptake and bioaccumulation of Cd, Zn, and Pb. However, E6S greatly reduced the root to shoot translocation of Cd and Zn, indicating that bacterial inoculation assisted the host plant to uptake and store heavy metals in its root system. Inoculation with the endophytic bacterium E6S homologous to A. piechaudii can improve phytostabilization of metalliferous soils due to its effective ability to enhance in situ metal rhizoaccumulation in plants.

PHYTOACCUMULATION, COMPETITIVE ADSORPTION AND EVALUATION OF CHELATORS-METAL INTERACTION IN LETTUCE PLANT

Present study was envisaged to study chelators-metal interaction in phytoaccumulation of metals as a bioremediation technology to decontaminate metal pollution and adsorption of metals on soil. Lactuca sativa was used as model plant for phytoaccumulation to identify uptake of Cadmium (Cd) and Lead (Pb) and their subsequent accumulation in edible tissue of plant. Cadmium and lead were two selected metals for phytoaccumulation and copper was also studied for adsorption experiment. Green house experiments on Lettuce plant were conducted for 3 months and a significant difference of growth with metal chelator addition was observed. The whole study was divided into two experiments first enhanced phytoaccumulation with chelator addition in plants was conducted in green house and in second experiment batch studies were conducted to evaluate competitive adsorption of selected metals on agricultural soils in laboratory conditions. Three main trends were identified in the experiment. First, transfer of metals from root to shoot was increased after DTPA application as it helped to increase metal bioavailability. Second a negative trend was observed with increasing metal-chelator concentration and ˃ 50 % reduction in plant dry biomass. Thirdly, water solubility of metals in soil was significant after 3 months of DTPA which shows low degradation and higher bioavailability. Bioconcentration factor and metal transportation index were calculated to find relation with metal uptake and plant growth. Phytoaccumulationan and adsorption of Cd is higher than Pb and copper.

Gradient differences in soil metal solubility and uptake by shoots and roots of wheat (T. aestivum)

The aim of this research was to identify and quantify gaps in currents methods and models for predicting the plant availability of selected nutrient and contaminant metals (Cu, Ni, Zn, Cd) in soil. This study investigated relationships between the relative solubility of Cu, Ni, Zn, and Cd determined by six extraction methods with short-term uptake by shoots and roots of wheat (Triticum aestivum). For Cu, Ni, and Cd, relationships between solubility and plant uptake were found to be different for shoots and roots, with Cu and Ni solubility being more closely correlated with root uptake compared with shoot uptake. Correlation coefficients for Cd concentrations in shoot and root tissue for all six solubility methods were poor (r2 < 0.5), while corresponding results for Zn explained more than 50 % of shoot variation but less than 50 % of root variation. Soil Cu solubility explained up to 85 % of variation in root uptake compared with 42–55 % for shoot uptake. These results clearly demonstrated that purely chemical and passive diffusion mechanisms were inadequate predictors of Cd uptake by shoots and roots, together with Cu uptake by shoots. Thus further attempts at refining soil metal bioavailability assays based solely on chemical extraction without consideration of plant responses are unlikely to improve prediction of plant uptake.

Prediction of the solubility of zinc, copper, nickel, cadmium, and lead in metal-contaminated soils

Risk assessment of metal-contaminated soil depends on how precisely one can predict the solubility of metals in soils. Responses of plants and soil organisms to metal toxicity are explained by the variation in free metal ion activity in soil pore water. This study was undertaken to predict the free ion activity of Zn, Cu, Ni, Cd, and Pb in metal-contaminated soil as a function of pH, soil organic carbon, and extractable metal content. For this purpose, 21 surface soil samples (0-15 cm) were collected from agricultural lands of various locations receiving sewage sludge and industrial effluents for a long period. One soil sample was also collected from agricultural land which has been under intensive cropping and receiving irrigation through tube well water. Soil samples were varied widely in respect of physicochemical properties including metal content. Total Zn, Cu, Ni, Cd, and Pb in experimental soils were 2,015 ± 3,373, 236 ± 286, 103 ± 192, 29.8 ± 6.04, and 141 ± 270 mg kg(-1), respectively. Free metal ion activity, viz., pZn(2+), pCu(2+), pNi(2+), pCd(2+), and pPb(2+), as estimated by the Baker soil test was 9.37 ± 1.89, 13.1 ± 1.96, 12.8 ± 1.89, 11.9 ± 2.00, and 11.6 ± 1.52, respectively. Free metal ion activity was predicted by pH-dependent Freundlich equation (solubility model) as a function of pH, organic carbon, and extractable metal. Results indicate that solubility model as a function of pH, Walkley-Black carbon (WBC), and ethylenediaminetetraacetic acid (EDTA)-extractable metals could explain the variation in pZn(2+), pCu(2+), pNi(2+), pCd(2+), and pPb(2+) to the extent of 59, 56, 46, 52, and 51%, respectively. Predictability of the solubility model based on pH, KMnO4-oxidizable carbon, and diethylenetriaminepentaacetic acid-extractable or CaCl2-extractable metal was inferior compared to that based on EDTA-extractable metals and WBC.

Spatial distribution of metals in smelter-impacted soils of woody habitats: Influence of landscape and soil properties, and risk for wildlife

The spatial distribution of total and CaCl 2-extracted Cd, Pb and Zn concentrations in smelter-impacted soils was investigated over a polluted site (40 km 2) in Northern France. The study was conducted on 262 soils sampled in woody habitats. Total and extracted concentrations of trace metals (TMs) rose up to 2402 and 59.5 mg kg −1 for Cd, 41 960 and 13.7 mg kg −1 for Pb, 38 760 and 143.0 mg kg −1 for Zn, respectively. The spatial dependence of both total and extracted concentrations showed a high spatial auto-correlation and ordinary kriging was used to predict soil concentrations. Investigating which variables influenced metal concentrations and their spatial distribution, we found that total concentrations mostly depended on the distance from the smelter, the wind and the organic carbon, while extracted concentrations were mainly modulated by the pH. Conditionally to those variables, other soil properties and landscape characteristics influenced both total and extracted concentrations. We conclude that total and extracted TM concentrations are governed by different processes which act at various spatial ranges: total concentrations are mainly related to input and retention of metals (large scale) whereas extracted concentrations were mainly explained by factors controlling metal solubility in soils (local scale). Spatial distributions of total and extracted TMs differed over the area, which should be considered for risk assessment. Maps of risk based on the US EPA’s Eco-SSLs (for plants, invertebrates and vertebrates) were realized, showing that wildlife may be at risk but that the relevance of Eco-SSL values is questionable.

The effects of exogenous plant growth regulators in the phytoextraction of heavy metals

The term “assisted phytoextraction” usually refers to the process of applying a chemical additive to contaminated soil in order to increase the metal uptake by crop plants. In this study three commercially available plant growth regulators (PGRs) based on cytokinins (CKs) were used to boost the assisted phytoextraction of Pb and Zn in contaminated soil collected from a former manufactured gas-plant site. The effects of EDTA treatment in soil and PGR treatment in leaves of Helianthus annuus were investigated in terms of dry weight biomass, Pb and Zn accumulation in the upper parts of the plants, Pb and Zn phytoextraction efficiency and transpiration rate. Metal solubility in soil and its subsequent accumulation in shoots were markedly enhanced by EDTA. The combined effects of EDTA and cytokine resulted in an increase in the Pb and Zn phytoextraction efficiency (up to 890% and 330%, respectively, compared to untreated plants) and up to a 50% increase in foliar transpiration. Our results indicate that exogenous PGRs based on CKs can positively assist the phytoextraction increasing the biomass production, the metal accumulation in shoots and the plant transpiration. The observed increase in biomass could be related to its action in stimulation of cell division and shoot initiation. On the other hand, the increase in metal accumulation in upper parts of plant could be related to both the role of PGRs in the enhancement of plant resistance to stress (as toxic metals) and the increase in transpiration rate, i.e. flux of water-soluble soil components and contaminants by the regulation of stomatal opening.

A field experiment investigating the effects of olive husk and cow manure on heavy metal availability in a contaminated calcareous soil from Murcia (Spain)

Abstract The transfer of heavy metals from the former sites of mining activity in La Union (Murcia) to the nearby agricultural soils is a serious environmental risk. The effect of two organic wastes, used as soil amendments, on the bioavailability of heavy metals in an agricultural soil and on their accumulation in Beta vulgaris L. var. Nomonta and Beta maritima L. was studied in a field experiment. The soil was a calcareous Xeric Torriorthent and the total metal levels were (mg kg−1): 2706 Zn, 3235 Pb, 39 Cu. The treatments were: fresh cow manure, olive husk and inorganic fertiliser as a control. Two successive crops (B. vulgaris and B. maritima) were grown. The soil was sampled before each planting and after each harvest. B. vulgaris behaved as a metal indicator plant, as its concentrations of heavy metals (Cu, Mn and Pb) in all treatments reflected the extractable soil levels. The high concentrations of metals, especially Pb (5.1–16.8 times the EU limit for plant foodstuffs), revealed a health risk for human and livestock due to the spread of the metal pollution from mining sites to agricultural areas. Cow manure did not alter the DTPA-extractable concentrations of metals in the soil or their absorption by plants in comparison with the control. But olive husk favoured solubility of metals in soils and their accumulation in plants, due to the reduction of Mn oxides during degradation of phenolic compounds. So, this material could be a useful amendment for phytoextraction of metals by accumulator species, while manure can be very useful for phytostabilisation.

Changes in water extractable metals, pH and organic carbon concentrations at the soil-root interface of forested soils

Soluble metals are of nutritional and ecotoxicological interest as they are the most readily available form to the biota. Metal solubility in soils is mostly controlled by pH and the organic matter content. The rhizosphere is generally considered as an environment enriched in organic matter and often more acidic (depending on nutritional status of the plant) than the bulk soil. Yet, there is a lack of consensus on the distribution of metals at the soil-root interface. Consequently, the specific objectives of this paper are to compare the chemical properties and the water extractable metal concentrations of the rhizosphere and the bulk soil of forest soil (1) along a gradient in soil contamination and (2) under different tree species. Two study areas were used: (1) Rouyn-Noranda (Canada) where samples were collected along a gradient in metal contamination at a distance of 0.5, 2 and 8 km downwind from a copper smelter; (2) Saint-Hippolyte (Canada) where the effect of three tree species (Abies balsamea, Acer saccharum and Betula papyrifera) was studied. In the field, the rhizosphere was operationally defined as the soil adhering to the roots after agitation, soil falling from the roots and the rest of the soil composing the bulk soil. Once in laboratory, a second agitation was performed to separate the rhizosphere into an inner and an outer component. Water extractable metal concentrations (Al, Ca, Cd, Co, Cr, Cu, Fe, Li, Mg, Mn, Ni, Pb and Zn) were quantified either with an ICP-AES or a GFAAS. Measurements of pH, electrical conductivity (EC), water-extractable organic carbon (WEOC) and solid phase organic carbon (SPOC) were performed. Results systematically indicate that EC, WEOC and SPOC follow the sequence inner rhizosphere > outer rhizosphere > bulk soil. The pH is always lower in the inner rhizosphere than in the bulk soil, while the outer rhizosphere frequently shows an inconstant behaviour. The results also show a clear gradient following inner rhizosphere > outer rhizosphere > bulk soil for water extractable Al, Ca, Cd, Cu, Fe, Mg, Mn, Ni, Pb and Zn. Li, Co and Cr levels were below method detection limit in all cases. WEOC seems to be the main variable related to the water-extractable metals concentrations. The gradient in metal contamination at Rouyn-Noranda was not as expected in the water extracts with the site at 2 km frequently presenting higher metal concentrations than the sites at 0.5 and 8 km. Moreover, a tree species effect did not clearly immerge for any of the chemical properties studied. However, the water extractable Ca concentrations were higher in the soil under Acer saccharum. The effects of the metal gradient and of the tree species may be more pronounced if stronger extractants are used. The addition of an outer rhizosphere component is useful as its behaviour is not consistently intermediate between the inner rhizosphere and bulk soil.

Dynamics of copper fractions and solubility in a savanna soil under continuous cultivation

An understanding of the influence of soil management practices on the distribution, form and solubility of trace metals in soils is required to formulate appropriate guidelines for the applications of organic manures and inorganic fertilizers for sustainable soil fertility and environmental protection. The distribution, forms and solubility of Cu were investigated in a savanna soil under long-term cultivation and varying management practices which included fertilization with NPK, farmyard manure (FYM), NPK+FYM and a control which received neither NPK nor FYM for 50 years. Total Cu varied between 5 and 13 mg kg−1. Long-term application of FYM and FYM+NPK significantly increased DTPA extractable Cu in the surface horizon, and in the subsurface horizon in FYM fertilized fields. Averaged across the sampling depths, cultivation and management history did not affect the concentration of DTPA extractable Cu, water-soluble Cu, organically complexed Cu and residual Cu. But fertilization with FYM and FYM+NPK significantly increased organically complexed Cu in the surface layer as compared to NPK fertilization. Solubility equilibria studies indicated that Cu2+ activities in soil solution approached an apparent equilibrium with cupric ferrite (CuFe2O4), suggesting that the control on Cu2+ solubility was either a CuFe2O4 or a Cu solid with solubility similar to cupric ferrite. The soil management practices involving combined application of inorganic fertilizers and farmyard manure did not seem to pose any risk of inducing Cu deficiency, but sole application of inorganic NPK poses such a risk.

Enhancing phytoextraction: the effect of chemical soil manipulation on mobility, plant accumulation, and leaching of heavy metals.

For heavy metal-contaminated agricultural land, low-cost, plant-based phytoextraction measures can be a key element for a new land management strategy. When agents are applied into the soil, the solubility of heavy metals and their subsequent accumulation by plants can be increased, and, therefore, phytoextraction enhanced. An overview is given of the state of the art of enhancing heavy metal solubility in soils, increasing the heavy metal accumulation of several high-biomass-yielding and metal-tolerant plants, and the effect of these measures on the risk of heavy metal leaching. Several organic as well as inorganic agents can effectively and specifically increase solubility and, therefore, accumulation of heavy metals by several plant species. Crops like willow (Salix viminalis L.), Indian mustard [Brassica juncea (L.) Czern.], corn (Zea mays L.), and sunflower (Helianthus annuus L.) show high tolerance to heavy metals and are, therefore, to a certain extent able to use the surpluses that originate from soil manipulation. More than 100-fold increases of lead concentrations in the biomass of crops were reported, when ethylenediaminetetraacetic acid (EDTA) was applied to contaminated soils. Uranium concentrations could be strongly increased when citric acid was applied. Cadmium and zinc concentrations could be enhanced by inorganic agents like elemental sulfur or ammonium sulfate. However, leaching of heavy metals due to increased mobility in soils cannot be excluded. Thus, implementation on the field scale must consider measures to minimize leaching. So, the application of more than 1 g EDTA kg(-1) becomes inefficient as lead concentration in crops is not enhanced and leaching rate increases. Moreover, for large-scale applications, agricultural measures as placement of agents, dosage splitting, the kind and amount of agents applied, and the soil properties are important factors governing plant growth, heavy metal concentrations, and leaching rates. Effective prevention of leaching, breeding of new plant material, and use of the contaminated biomass (e.g., as biofuels) will be crucial for the acceptance and the economic breakthrough of enhanced phytoextraction.