Metal Concentrations In Soil Solution Research Articles

Impacts of atmospheric deposition on the heavy metal mobilization and bioavailability in soils amended by lime

Atmospheric deposition is an important source of heavy metal in agricultural soils, but there is limited research on the mobility of these metals in soil and their impact on soil amendment. Here, we performed a dust incubation experiment in soils in the laboratory and a factorial transplant experiment at three field sites with a gradient of atmospheric deposition to examine the impacts of atmospherically deposited heavy metals (Cu, Cd, and Pb) on the mobility and bioavailability in soils with and without lime applications. Results showed that the atmospherically deposited heavy metals showed high mobility and were primarily presented in the soluble ionic fractions in the wet part and acid-exchangeable and reducible fractions in the dry part of atmospheric deposition. Atmospheric dust addition caused the 2p3/2 and 2p1/2 electrons of Cu atoms in uncontaminated soils to transition the 3d vacant states, resulting in similar copper absorption peaks as atmospheric particles by the observation of X-ray absorption near-edge spectroscopy (XANES). In the field, atmospheric deposition can only increase the mobile fractions in the surface soils, but not in the deeper layers. However, the deposition can increase the soluble and diffusive gradients in thin films (DGT)-measured bioavailable fractions in profile along with the soil depth. Lime applications cannot significantly reduce the mobile fractions of heavy metals in the surface soils exposed to atmospheric deposition, but significantly reduce the heavy metal concentrations in soil solutions and the DGT-measured bioavailable concentrations, particularly in the deeper layer (6–10 cm). The major implication is that atmospherically deposited heavy metals can significantly increase their bioavailable concentrations in the plough horizon of soil and constrain the effects of soil amendments on heavy metal immobilization, thereby increasing the risks of crop uptake.

Effects of blast furnace slag on the immobilization, plant uptake and translocation of Cd in a contaminated paddy soil

The potential toxicity of Cd to soil and rice plant severely threaten human health. This study was conducted to investigate the remediation effects of blast furnace slag (BFS) on the bioavailability of Cd in a contaminated paddy soil from a perspective of soil solution chemistry. Batch experiments, pot culture experiments, and principal analysis (PCA) were used to study the effects and mechanisms of BFS addition changing Cd chemical behavior and Cd toxicity. Results indicated that BFS facilitated Cd adsorption in soils, increased pH, Eh, and EC values in soil solution, whereas reduced dissolved Cd content. BFS amendment was efficient in decreasing root Cd intake and Cd upward transport in rice plant, with the Cd translocation factor in brown rice decreased by ∼ 75% (BFS treatment, 6‰ wt) relative to Cd treatment, which in turn increased rice biomass and grain yield. PCA indicated that the dissolved Cd concentration had a close relationship with soil pH and metal concentration in soil solution. Results from this study indicated that BFS had potential ability for either immobilization or remobilization of Cd in soils, and the findings have important implications for Cd-polluted soil remediation or other resource utilization with slag-based materials.

Simulated Bioavailability of Heavy Metals (Cd, Cr, Cu, Pb, Zn) in Contaminated Soil Amended with Natural Zeolite Using Diffusive Gradients in Thin-Films (DGT) Technique

The aim of this study was to investigate the effect of natural zeolite amendment to contaminated soil on the heavy metal (Cd, Cr, Cu, Pb and Zn) simulated bioavailability over a three-month period of storage. Two pot experiments were created by amending soil with two different amounts of natural zeolite (3 and 6 wt.%). During the experiment, pH level, metal concentrations in soil solution (Csol) and metal concentrations in soil by the diffusive gradients in thin-films technique (CDGT) were determined. When the zeolite was added to the soil, a significant decrease (p = 95%, n = 3) for Cd and Pb concentrations, both in Csol and in CDGT, was observed. The Cu, Cr and Zn concentrations also showed a decreasing trend, but the changes in their concentrations were not statistically significant. The R ratio between CDGT and Csol was calculated for each metal in order to assess the resupply of metals from the soil solid phase to soil solution. The R values significantly decreased for Cd and Pb, showing a low resupply from the solid phase in the samples with added zeolite.

Identifying Controlling Factors of Bioaccumulation of Selected Metal(loid)s in Various Soil–Cereal Crop Systems within Cultivated Fluvisols

The paired Fluvisol and cereal samples in both the field screening and controlled experiments are reported to elucidate the soil–crop relationship for As, Cd, and Pb in relation to changing contamination levels. Significant varietal differences in plant uptake were observed for crop type (barley, triticale) and the harvested part of the crop (oat shoots and grain). When parametrizing the stepwise regression models, the inclusion of soil properties often improved the performance of soil–crop models but diverse critical soil parameters were retained in the model for individual metal(loid)s. The pH value was often a statistically significant variable for Cd uptake. For As and Pb, the more successful model fit was achieved using the indicators of quantity or quality of soil organic matter, but always with lower inherent model reliability compared to Cd. Further, a single correlation analysis was used to investigate the relationship between extractable metal concentrations in soil solution and their crop accumulation. For Cd, there were strong intercorrelations among single extractions, the NH4NO3 extraction stood out with perfect correlation with plant uptake in both experiments. For As and Pb, the CaCl2 and Na2EDTA solutions outperformed other single extractions and were the better choice for the assessment of depositional fluvial substrates.

Adsorption of Lead, Cadmium, and Nickel on Benchmark Soils of Pakistan

Adsorption and desorption reactions at the solid phase–solution interface, play a significant role in controlling metal concentrations in soil solution and metal translocation to plants. Five predominant benchmarks soils of Pakistan were characterized for their lead (Pb), cadmium (Cd), and nickel (Ni) adsorption parameters by constructing equilibrium adsorption isotherms. The adsorption parameters (maximum adsorption capacity and affinity coefficient) were calculated by fitting measured adsorption data to Langmuir, Freundlich, and Redlich–Peterson adsorption models. Thermodynamic properties (Gibbs free energy—ΔG°, enthalpy—ΔH°, and entropy—ΔS°) were measured by equilibrating soils with Pb, Cd, and Ni solution at 25 ± 2 and 45 ± 2°C. The results revealed that the Langmuir and Redlich–Peterson adsorption models described metal adsorption data equally good. Langmuir’s predicted Pb, Cd, and Ni adsorption demonstrated better correlation with measured adsorption than Freundlich’s model. According to the maximum Pb, Cd, and Ni adsorption calculated by Langmuir and Redlich–Peterson models and Freundlich sorbate affinity to sorbent, the studied soils were formed the following sequence: Kotli > Miranpur > Gujranwala > Shahdara > Rasulpur. The magnitude of metal adsorption (b and qmon) and affinity (Kf) was highly correlated with the clay, soil organic matter, and iron and aluminum contents and cation exchange capacity. The measured metal adsorption in different soils followed the order: Pb > Cd > Ni. The magnitude of maximum Pb sorption (calculated by Langmuir and Redlich–Peterson models) was approximately two and four times higher than that for Cd and Ni, respectively. Similarly, the Freundlich’s sorbate affinity was also higher for Pb than for Cd and Ni. Thermodynamic parameters revealed that the Pb, Cd, and Ni adsorption reactions were spontaneous and exothermic in nature, and the process was dominated by physical adsorption.

Controls on accumulation and soil solution partitioning of heavy metals across upland sites in United Kingdom (UK)

A significant body of knowledge suggests that soil solution pH and dissolved organic carbon (DOC) strongly influence metal concentrations and speciation in porewater, however, these effects vary between different metals. This study investigated the factors influencing soil and soil solution concentrations of copper (Cu), lead (Pb), nickel (Ni) and zinc (Zn) under field conditions in upland soils from UK having a wide range of pH, DOC and organic matter contents. The study primarily focussed on predicting soil and soil solution metal concentrations from the data on total soil metal concentrations (HNO3 extracts) and soil and soil solution properties (pH, DOC and organic matter content). We tested the multiple regression models proposed by Tipping et al. (2003) to predict heavy metal concentrations in soil solutions and the results indicated a better fit (higher R2 values) in both studies for Pb compared to the Zn and Cu concentrations. Both studies observed consistent negative relationships of metals with pH and loss on ignition (LOI) suggesting an increase in soil solution metal concentrations with increasing acidity. The positive relationship between Pb concentrations in porewater and HNO3 extracts was similar for both studies, however, similar relationships were not found for the Zn and Cu concentrations because of the negative coefficients for these metals in our study. The results of this study conclude that the predictive equations of Tipping et al. (2003) may not be applicable to the field sites where the range of DOC and metal concentrations is much lower than their study. Our study also suggests that the extent to which metals are partitioned into soil solution is lower in soils with a higher organic matter contents due to binding of these metals to soil organic matter.

Comparison of simple soil-solution extraction techniques for assessing transfer of metals from soil to plant in contaminated soils

An attempt was made to evaluate simple techniques for extraction of soil solution for predicting uptake of Zn, Cu, Ni, Pb and Cd by spinach (Beta vulgaris L. var. All Green) grown on metal contaminated soils under modelling framework. Twenty-eight surface soil samples (0-15 cm) with heterogeneous chemical properties were collected. A pot experiment was conducted using spinach as a test crop. The leachate from experimental pots was collected with the help of plastic container installed below the pots. The 1:2 and 1:5 soil: water ratio was used for soil solutionextraction. The total concentration of Zn, Cu, Ni, Pb, and Cd in soil solution was determined using ICP-MS. Harvested plant sample was analyzed for total Zn, Cu, Ni, Pb, and Cd using ICP-MS. For predicting metal uptake by spinach free ion activity model (FIAM) was used and total metal concentration in soil solution was used as an input of FIAM. Higher concentration of all metal in 1:5 soil-water ratio were recorded than that in 1:2 soil-water ratio. The free ion activity model as a function of total metal concentration in soil solution, as measured by 1:5 soil: water extraction, could explain the variability in metal content in spinach to the extent of 94% for Zn, 67% for Cu, 80% for Ni, 50% for Pb and 75% for Cd, respectively. Extraction of soil with 1:5 soil-water ratio is more efficient and suitable for predicting metal uptake by spinach as compared to that of 1:2 and leachate extraction.

Solubility of trace metals in two contaminated paddy soils exposed to alternating flooding and drainage

Uptake of trace metals by crops is determined by the solubility of trace metals. In paddy soils, flooding and drainage influence redox chemistry and consequently trace metal solubility and thus uptake by rice plants. Current knowledge on how the dynamics in redox chemistry affect the solubility of trace metals in contaminated paddy soils is still limited. The objectives of our study were to investigate (i) the effects of flooding and drainage on trace metal solubility in paddy soils and (ii) to what extent a multi-surface modeling approach can predict trace metal solubility under changing redox conditions. We performed a column experiment with two contaminated paddy soils with similar soil properties but contrasting pH. During two successive flooding and drainage cycles, dynamics in Eh, pH and dissolved organic matter concentrations greatly affected trace metal solubility for both soils. Multi-surface model predictions indicate that under aerobic conditions, the higher pH of the alkaline soil leads to a stronger complexation of trace metals by reactive surfaces of the soil and, consequently, to lower dissolved concentrations than in the acidic soil. Under anaerobic conditions, predictions shows that sulfide precipitates control trace metal solubility in both soils, but still the higher pH of the alkaline soil leads to lower trace metal concentrations in soil solution at equilibrium. Furthermore, model calculations showed that stoichiometry and solubility of copper sulfide minerals can substantially affect solubility of other trace metals especially when trace element concentrations exceed soil sulfate concentrations. This stoichiometry and solubility should be considered when predicting the solubility of trace metals under anaerobic conditions.

Diffusive gradient in thin films technique for assessment of cadmium and copper bioaccessibility to radish (Raphanus sativus)

The aim of this study was to assess cadmium and copper uptake by radish (Raphanus sativus) and to test the capability of the diffusive gradient in thin films (DGT) technique to predict bioaccessibility of the metals for this plant. Radish plants were grown in pots filled with uncontaminated control and artificially contaminated soils differing in cadmium and copper contents. Metal concentrations in plants were compared with free ion metal concentrations in soil solution, and concentrations measured by DGT. Significant correlation was found between metal fluxes to plant and metal fluxes into DGT. Pearson correlation coefficient for cadmium was 0.994 and for copper 0.998. The obtained results showed that DGT offers the possibility of simple test procedure for soils and can be used as a physical surrogate for plant uptake.

Metal leaching in a highway embankment on field and laboratory scale

Increasing worldwide motor vehicle traffic leads to the question of the possible environmental consequences. This paper aims to analyse metal leaching in a highway embankment using both field and laboratory experiments. Soil, soil solution and road runoff were collected along one of the oldest highways in the world to characterize leaching of the metals Cd, Cr, Cu, Ni, Pb and Zn. Batch, column and adsorption experiments were carried out to study the reliability and transferability of laboratory approaches. Depending on the element, the ratio of particle-bound metals in road runoff varied between 15–90%. Metal levels in embankment soils were significantly higher compared to a reference site in a forest at 800m distance (up a factor of 30). High metal concentrations in soil solution at 50cm soil depth were not a direct result of road runoff but rather of elevated concentrations in the soil matrix. The use of batch S4 elution was found to be the best overall laboratory method to predict soil solution concentrations in field. Adsorption experiments showed a relative increase in retention capacity in roadside soil of up to a factor of 20 after nearly 100years of operation. The input of alkaline dust and organic carbon into roadside soils increases its retention capacity in the long term.

Variability and relationships between Pb, Cu, and Zn concentrations in soil solutions and forest floor leachates at heavily polluted sites

AbstractSeasonal variability of Cu, Pb, and Zn concentrations in litter leachates and soil solutions was examined in an afforested zone surrounding a copper smelter in SW Poland. Litter leachates (with zero‐tension lysimeters) and soil solutions (with MacroRhizon suction‐cup samplers, installed at a depth of 25–30 cm) were collected monthly at three sites differing in contamination levels in the years 2009 and 2010 (total Cu: 2380, 439, and 200 mg kg–1, respectively). Concentrations of Cu in the litter leachate were correlated with dissolved organic C (DOC), whereas Zn and Pb were mainly related to leachate pH. Metal concentrations in the soil solution were weakly influenced by their total content in soils and the monthly fluctuations reached 300, 600, and 700% for Cu, Pb, and Zn, respectively. Metal concentrations in soil solutions (Cu 110–460 μg L–1; Zn 20–1190 μg L–1; Pb 0.5–36 μg L–1) were correlated with their contents in the litter leachates. Chemical speciation, using Visual Minteq 3.0, proved organically‐complexed forms even though the correlations between metal concentrations and soil solution pH and DOC were statistically insignificant. The flux of organically‐complexed metals from contaminated forest floors is believed to be a direct and crucial factor affecting the actual heavy metal concentrations and their forms in the soil solutions of the upper mineral soil horizons.

Spatial and seasonal distribution of trace metals in floodplain soils. A case study with the Middle Elbe River, Germany

Field monitoring stations were used to assess selected soil parameters and trace metal concentrations in soil solution (As, Cd, Cu, Cr, Ni, Zn) to assess their behaviour under spatial as well as seasonal variations. We investigated three different plots within a floodplain research area varying in terms of their soil characteristics as well as their trace metal contents. Redox potential was found to be an important factor controlling the release of trace metals under field conditions on all plots coupled with a strong seasonal influence in almost all cases. However, by using generalised additive models it was possible to identify the specific courses of redox potentials related to the several trace metals depending on seasonal changes across the three plots. Whereas Cu and Cr concentrations increased with increasing redox potentials, Cd, Ni and Zn concentrations decreased. No clear pattern could be identified for As, although the data clearly reveal a seasonal dependency on flood events over the year and show increasing As concentrations even during smaller inundation events. The results of the generalised additive mixed models identified hot spots and hot moments of dissolved trace metal concentrations specific to soil horizons within the three plots. Thus we can provide data for further management implications that should be taken into account in an ecosystem that provides a whole host of ecosystem functions and services.

Soil parameters are key factors to predict metal bioavailability to snails based on chemical extractant data

Although soil characteristics modulate metal mobility and bioavailability to organisms, they are often ignored in the risk assessment of metal transfer. This paper aims to determine the ability of chemical methods to assess and predict cadmium (Cd), lead (Pb) and zinc (Zn) environmental bioavailability to the land snail Cantareus aspersus. Snails were exposed in the laboratory for 28days to 17 soils from around a former smelter. The soils were selected for their range of pH, organic matter, clay content, and Cd, Pb and Zn concentrations. The influence of soil properties on environmental availability (estimated using HF-HClO4, EDTA, CaCl2, NH4NO3, NaNO3, free ion activity and total dissolved metal concentration in soil solution) and on environmental bioavailability (modelled using accumulation kinetics) was identified. Among the seven chemical methods, only the EDTA and the total soil concentration can be used to assess Cd and Pb environmental bioavailability to snails (r²adj=0.67 and 0.77, respectively). For Zn, none of the chemical methods were suitable. Taking into account the influence of the soil characteristics (pH and CEC) allows a better prediction of Cd and Pb environmental bioavailability (r²adj=0.82 and 0.83, respectively). Even though alone none of the chemical methods tested could assess Zn environmental bioavailability to snails, the addition of pH, iron and aluminium oxides allowed the variation of assimilation fluxes to be predicted. A conceptual and practical method to use soil characteristics for risk assessment is proposed based on these results. We conclude that as yet there is no universal chemical method to predict metal environmental bioavailability to snails, and that the soil factors having the greatest impact depend on the metal considered.

Comparison of Extraction Procedures for Assessing Soil Metal Bioavailability of to Wheat Grains

AbstractAgricultural soils have been analyzed to elucidate whether the trace metal distribution changes in relation to agricultural activities and to predict environmental risk. In addition to the extractability of Cu, Mn, Ni, Pb, and Zn in soils was compared by single extraction (hydrochloric acid (HCl), ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA), calcium chloride (CaCl2), water (H2O)), and sequential extraction procedures. The modified Community Bureau of Reference (BCR) sequential extraction procedure (three‐step) used to extraction of metals in soil samples. The extraction capacity of the analyzed metals was found by using single extraction procedures in the order: HCl > EDTA > DTPA > CaCl2 > H2O. A single correlation analysis was used to investigate the relationship between extractable metal concentrations in soil solutions and metal accumulation in wheat grains. Simple correlation analyses indicated that the extractable Pb and Ni of soils by HCl, EDTA, and DTPA single extraction procedures were significantly correlated with the metal contents of wheat grains. For CaCl2, H2O, and BCR extraction procedures there was a relatively poor correlation between the extractable Cu, Mn, Ni, Pb, and Zn of soils and metal contents of wheat grains.

Modelling trace metal extractability and solubility in French forest soils by using soil properties

Soil/solution partitioning of trace metals (TM: Cd, Co, Cr, Cu, Ni, Sb, Pb and Zn) has been investigated in six French forest sites that have been subjected to TM atmospheric inputs. Soil profiles have been sampled and analysed for major soil properties, and CaCl2‐extractable and total metal content. Metal concentrations (expressed on a molar basis) in soil (total), in CaCl2 extracts and soil solution collected monthly from fresh soil by centrifugation, were in the order: Cr > Zn > Ni > Cu > Pb > Co > Sb > Cd , Zn > Cu > Pb = Ni > Co > Cd > Cr and Zn > Ni > Cu > Pb > Co > Cr > Cd > Sb , respectively. Metal extractability and solubility were predicted by using soil properties. Soil pH was the most significant property in predicting metal partitioning, but TM behaviour differed between acid and non‐acid soils. TM extractability was predicted significantly by soil pH for pH < 6, and by soil pH and Fe content for all soil conditions. Total metal concentration in soil solution was predicted well by soil pH and organic carbon content for Cd, Co, Cr, Ni and Zn, by Fe content for Cu, Cr, Ni, Pb and Sb and total soil metal content for Cu, Cr, Ni, Pb and Sb, with a better prediction for acidic conditions (pH < 6). At more alkaline pH conditions, solute concentrations of Cu, Cr, Sb and Pb were larger than predicted by the pH relationship, as a consequence of association with Fe colloids and complexing with dissolved organic carbon. Metal speciation in soil solutions determined by WHAM‐VI indicated that free metal ion (FMI) concentration was significantly related to soil pH for all pH conditions. The FMI concentrations of Cu and Zn were well predicted by pH alone, Pb by pH and Fe content and Cd, Co and Ni by soil pH and organic carbon content. Differences between soluble total metal and FMI concentrations were particularly large for pH < 6. This should be taken into account for risk and critical load assessment in the case of terrestrial ecosystems.

Changes in Cu, Ni, and Zn availability following simulated conversion of biosolids-amended forest soils back to agricultural use

In this study we examined the bioavailability and chemistry of Cu, Ni, and Zn in metal-spiked, biosolids-amended forest (Pinus radiata) soils that had undergone a simulated conversion back to agricultural use. Mixing of the biosolids-treated forest litter into the underlying mineral soil resulted in high concentrations of each metal in easily extractable and soil solution forms. There was also very little change in these concentrations during a subsequent 2-year incubation period of the samples. Chemical speciation of the soil solutions using WHAM 6 showed that Cu was dominated by organic complexes, whereas most Ni and Zn was present as Ni2+ and Zn2+, with generally <5% of these elements present as organic complexes. Addition of lime to the soils substantially decreased both readily extractable and soil solution metal concentrations. However, even in their unlimed state, although plant metal concentrations were increased by the original biosolids treatments, there were no adverse effects due to the metals on plant growth as determined in a wheat germination and seedling growth test. In this study, the DGT technique showed considerable promise for assessing metal availability to plants. However, the study suggests that conversion of biosolids-treated forest soils back for agricultural use is unlikely to result in any substantial problems related to the metal loadings built up in the forest litter layer.

Feasibility of phytoextraction to remediate cadmium and zinc contaminated soils

A Cd and Zn contaminated soil was mixed and equilibrated with an uncontaminated, but otherwise similar soil to establish a gradient in soil contamination levels. Growth of Thlaspi caerulescens (Ganges ecotype) significantly decreased the metal concentrations in soil solution. Plant uptake of Cd and Zn exceeded the decrease of the soluble metal concentrations by several orders of magnitude. Hence, desorption of metals must have occurred to maintain the soil solution concentrations. A coupled regression model was developed to describe the transfer of metals from soil to solution and plant shoots. This model was applied to estimate the phytoextraction duration required to decrease the soil Cd concentration from 10 to 0.5 mg kg(-1). A biomass production of 1 and 5 t dm ha(-1) yr(-1) yields a duration of 42 and 11 yr, respectively. Successful phytoextraction operations based on T. caerulescens require an increased biomass production.

Heavy metal bioavailability and chelate mobilization efficiency in an assisted phytoextraction process

The heavy metal bioavailable fraction of a soil is a core parameter to verify the potential risks of contaminant exposure to organisms or plants. The purpose of the present work is to identify the bioavailable metal fraction in soils treated with chelates. This fraction was evaluated directly by analyzing metal concentrations in soil solution and indirectly using sequential extraction procedures. The metal bioavailable fraction was compared with metal accumulated in plant leaves, grown in both untreated and chelate-treated reactors. In order to verify the effect of the readily and slowly biodegradable chelates [S,S]-ethylenediaminedisuccinic acid (EDDS), methylglycine diacetic acid (MGDA), and ethylenediaminetetraacetic acid (EDTA) on metal speciation in soils, a simulation of chelate treatment was made and metal concentrations in different soil compartments before and after the simulation were compared. Lead concentration in the soil solution was positively correlated with metal concentration in the test plants. The soluble fraction showed the best correlation with metal concentration in soil solution. The simulation of the chelate treatment demonstrated that EDTA and EDDS were able to extract part of the organic- and sulfide-bound fraction, which are less available to plants.

Phytoextraction and fractionation of heavy metals in soil after multiple applications of natural chelants

Chelate-assisted phytoextraction of heavy metals is a promising approach to clean up polluted soils. However, the most successful chelants tested so far are synthetic agents that barely degrade in soil, increasing the metal leaching risks. Natural organic acids have been proposed to enhance phytoextraction due to their higher biodegradability, but they can also be a drawback for efficient phytoextraction. This work was carried out to compare the effectiveness of multiple applications of citric and gallic acids on the availability in soil and accumulation of Cd, Pb, Cu, and Zn by velvet bean (Stizolobium aterrimum) plants. The organic acids were added as follows: 5 mmol kg-1 in the 28th cultivation day; two doses of 5 mmol kg-1 at the 28th and 31st day; and three applications at the same rate at the 28th, 31st, and 34th day of velvet bean cultivation. Soil samples were sequentially extracted and soil solution metal concentrations assessed. Neither citric acid nor gallic acid was efficient for metals phytoextraction. In general, low molecular weight organic acids (LMWOA) application increased the metal concentrations in the water soluble and exchangeable fractions. Zinc and Cu were retained mostly in the organic matter fraction. Zn was remobilized from the organic matter fraction through LMWOA application into the water soluble and exchangeable fractions. LMWOA mobilized Pb and Cu from iron oxides, but such an increase in solubility was not high enough to affect phytoextraction.

Soil factors controlling the toxicity of copper and zinc to microbial processes in Australian soils

Abstract-Two soil microbial processes, substrate-induced nitrification (SIN) and substrate-induced respiration (SIR), were measured in the topsoils of 12 Australian field trials that were amended separately with increasing concentrations of ZnSO4 or CuSO4. The median effect concentration (EC50) values for Zn and Cu based on total metal concentrations varied between 107 and 8,298 mg kg(-1) for Zn and 108 and 2,155 mg kg(-1) Cu among soils. The differences in both Zn and Cu toxicity across the 12 soils were not explained by either the soil solution metal concentrations or CaCl2-extractable metal concentrations, because the variation in the EC50 values was larger than those using total concentrations. Toxicity of Zn and Cu decreased with increasing soil pH for SIN. For Cu, also increasing cation exchange capacity (CEC) and percent clay decreased the toxicity towards SIN. In contrast to SIN, soil pH had no significant effect on toxicity values of SIR. Significant relationships were found between the EC50 values for SIR and background Zn and CEC for Zn, and percent clay and log CEC for Cu. Relationships such as those developed in this study will permit Australian environmental regulation to move from single-value national soil quality guidelines to soil-specific quality guidelines and permit soil-specific risk assessments to be undertaken.