Uptake of Cd and Zn by intact seedlings of two contrasting ecotypes of the hyperaccumulator Thlaspi caerulescens was characterized using radioactive tracers. Uptake of Cd and Zn at 2 degrees C was assumed to represent mainly apoplastic binding in the roots, whereas the difference in uptake between 22 degrees C and 2 degrees C represented metabolically dependent influx. There was no significant difference between the two ecotypes in the apoplastic binding of Cd or Zn. Metabolically dependent uptake of Cd was 4.5-fold higher in the high Cd-accumulating ecotype, Ganges, than in the low Cd-accumulating ecotype, Prayon. By contrast, there was only a 1.5-fold difference in the Zn uptake between the two ecotypes. For the Ganges ecotype, Cd uptake could be described by Michaelis-Menten kinetics with a V(max) of 143 nmol g(-1) root FW h(-1) and a K(m) of 0.45 microM. Uptake of Cd by the Ganges ecotype was not inhibited by La, Zn, Cu, Co, Mn, Ni or Fe(II), and neither by increasing the Ca concentration. By contrast, addition of La, Zn or Mn, or increasing the Ca concentration in the uptake solution decreased Cd uptake by Prayon. Uptake of Ca was larger in Prayon than in Ganges. The results suggest that Cd uptake by the low Cd-accumulating ecotype (Prayon) may be mediated partly via Ca channels or transporters for Zn and Mn. By contrast, there may exist a highly selective Cd transport system in the root cell membranes of the high Cd-accumulating ecotype (Ganges) of T. caerulescens.

Metal pollution of agricultural land in Australia and New Zealand is less severe than that documented in many European countries, due to the lower density of urban developments and a lower level of industrialisation. However, Australia and New Zealand are highly dependent on plant production systems based on plant-microbial symbioses (e.g. Rhizobium, mycorrhizae) and other natural biogeochemical processes for maintaining nutrient status in soils that are generally low in nutrients and, in Australia, also low in organic matter. Data linking metal concentrations in soil to agricultural and ecological effects are sparse for Australia and New Zealand, and regulatory frameworks and guidelines to control metal contamination of soils rely heavily on data generated in countries of the northern hemisphere. Adoption of benchmark concentrations for metal contaminants from these countries has led to inappropriate levels being chosen for several elements. These problems could be avoided and metal contamination of soils could be more effectively controlled if instead of relying on total concentrations of metals in soil and soil amendments, regulations and guidelines considered the biologically active fractions. This review considers the advantages and disadvantages of a bioavailability-based approach to the control of metal contamination of soils and suggests improvements needed to avoid both over- and under-protective measures.

The use of soil amendments has been proposed as a low input alternative for the remediation of metal polluted soils. However, little information is available concerning the stability, and therefore the longevity, of the remediation treatments when important soil parameters change. In this paper we investigate the effect of pH changes on the lability of heavy metals in soils treated with lime, beringite, and red mud using a modified isotopic dilution technique in combination with a stepwise acidification procedure. Significant amounts of nonlabile (fixed) Cu and Zn were found to be associated with colloids <0.2 microm in the solution phase. The results obtained indicated that the mobility of fixed colloidal metals is significant and increases with soil pH. This must be considered because most of the soil amendments are alkaline and increase soil pH. All the soil amendments significantly decreased the lability of Cd, Zn, and Cu in the soils as a whole. However, when the soils were re-acidified, the labile pool of metals increased sharply and in the case of lime and beringite, the lability of the metals was similar, at equal pH, to the untreated soil. In contrast the lability of metals in the red mud treated soils was always smaller than that in the untreated soils across the range of pH values tested. These results suggest that the mechanism of action of lime and beringite is similar and probably related to increased metal adsorption and precipitation of metal hydroxides and carbonates at high pH. In the case of red mud, a combination of pH dependent and independent mechanisms (possibly solid-phase diffusion or migration into micropores) may be responsible for the metal fixation observed.

The performance of a mixed binding layer (MBL) for use in diffusive gradients in thin films (DGT) was investigated. The MBL consisted of ferrihydrite and Chelex-100 cation-exchange resin combined together in a binding gel in an attempt to allow measurement of anions and cations in a single assay. Results from the MBL were compared to experiments performed using individual Chelex gels and ferrihydrite gels that have been shown to work successfully for DGT methodology. To facilitate combined analysis of P and cations by ICP-MS, HCl (1 M) was used for gel elution to minimize interferences from 14N16OH or 15N16O on 31P. All elements tested (Cd, Cu, Mn, Mo, P, and Zn) were bound successfully to the MBL. An elution efficiency of 0.92 was obtained for all elements, apart from Mo (0.79). This is higher than the elution efficiencies obtained previously for pure Chelex or ferrihydrite gels using HNO3 (1 M) as the eluent. Uptake of cations by DGT using the MBL was consistent across the pH range 5-9, which compares well with results using pure Chelex. Below pH 5, accumulated masses were lower for Mn, Cu, and Zn. Uptake of P and Mo was unaffected by pH in the range 3-8, and the amount absorbed compared well with results obtained previously for pure ferrihydrite gels. Performance of the MBL at different ionic strengths (0.001, 0.01 M) was comparable to performance using the pure Chelex gel. DGT measurements obtained using the MBL on agricultural soils correlated well (r2 = 0.95) with separate measurements obtained using either pure Chelex or ferrihydrite binding layers. This suggests that the MBL could be used for simultaneous measurement of cationic and anionic element availability in soils.

One suite of in situ technologies for remediating metal contaminated soils involves the addition of reactive materials which lower metal availability. Until now it has been difficult to assess whether the amendment induced decrease in metal availability is due simply to increased sorption of the metal or whether it is the result of surface precipitation or other fixation mechanisms. This has made it difficult to predict the long-term stability of such remedial treatments. Using an isotopic dilution technique coupled with a stepwise acidification treatment, we examined changes in the labile pool of Cd and Zn in a polluted soil amended with either CaCO3, KH2PO4, red mud, or a kaolin byproduct. Fixation of both Cd and Zn was greatest in the KH2PO4 treated soil. The mode of fixation in this treatment was also found to be resistant to soil acidification. The results allowed a clear distinction between three classes of attenuation mechanisms which are hypothesized to increase in their resilience to environmental change as follows: reversible sorption < irreversible "fixation" at constant pH < irreversible "fixation" across a range of pH.

Summary• To examine whether root exudates of the Zn/Cd hyperaccumulator Thlaspi caerulescens play a role in metal hyperaccumulation, we compared the metal mobilization capacity of root exudates collected from two ecotypes of T. caerulescens , and from the nonaccumulators wheat ( Triticum aestivum ) and canola ( Brassica napus ).• Plants were grown hydroponically and three treatments (control, -Fe and -Zn) were later imposed for 2 wk before collection of root exudates.• On a basis of root d. wt, the total soluble organic C in the root exudates of T. caerulescens was similar to that of wheat, and significantly higher than that of canola. In all treatment, the root exudates of T. caerulescens and canola mobilized little Cu and Zn from Cu-or Zn-loaded resins, and little Zn, Cd, Cu or Fe from a contaminated calcareous soil. By contrast, the root exudates of wheat generally mobilized more metals from both resin and soil. In particular, the -Fe treatment, and to a lesser extent the -Zn treatment, elicited large increases in the metal mobilization capacity of the root exudates from wheat.• We conclude that root exudates from T. caerulescens do not significantly enhance mobilization of Zn and Cd, and therefore are not involved in Zn and Cd hyperaccumulation.

A pot experiment was conducted to study changes over time of Cd and Zn in soil solution and in plants. Radish was grown in a soil which had been contaminated with heavy metals prior to 1961. Constant amounts of a fertilizer solution (NH4N03, KN03) were added daily. Soil solution was obtained at intervals by displacement with water.The cumulative additions of small amounts of fertilizers were made equal to the plants' requirements at the final harvest but were found to exceed them during most of the experiment. Excess fertilizers caused substantial increases of major (K, Ca, Mg) and heavy-metal (Cd, Zn) ions in soil solutions and a decrease in soil pH, probably due to ion-exchange mechanisms and the dissolution of carbonates.Uptake of Cd and Zn into leaves was correlated with the mass flow of Cd (adjusted r2 = 0.798) and Zn (adjusted r2=0.859). Uptake of K, Ca and Mg by the plants was independent of their concentrations in solution.It is concluded that, in order to study effects of plants on heavy-metal availability and obtain soil solution that has not been altered by fertilizer ions, nutrients must be added according to the needs and growth of the plants. This could be achieved by linking fertilizer additions to the rate of transpiration, as nutrient uptake and transpiration were closely correlated in this experiment.

Isotope dilution techniques have been widely applied in research investigating the fate and availability of different elements in soils. One technique, known as the 'E-value' method, involves tracking the depletion of isotope from solution in a soil suspension to determine the amount of isotopically exchangeable element at a given time following addition of isotope. An assumption underlying this method is that the isotope behaves exactly as the cold element in soil and hence, the amount of depletion of isotope from solution relative to the quantity of cold element at a given time can be used to determine specific elemental compartments in the soil. However, there has been some confusion in the literature with many studies neglecting to consider that an E-value can be composed of either a solid-phase compartment (Ee) or a combination of a solid-phase and solution compartment (Ea) depending on how it is calculated. Selection of the correct E-value formula is crucial for meaningful interpretation of data in soils with low buffering capacities, high concentrations of the element of interest in solution, or methods which use very short incubation times of isotope with soil. Here we discuss the derivation of E-value equations and circumstances where each should be applied.