Rebecca Hamon

Review: A bioavailability-based rationale for controlling metal and metalloid contamination of agricultural land in Australia and New Zealand

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.

Partitioning and stability of engineered ZnO nanoparticles in soil suspensions using flow field-flow fractionation

Environmental context. Nanoparticulate materials are increasingly being used as catalysts and lubricants, for pollution control and drug delivery, and in electronics, fabrics, cosmetics and sunscreens. In spite of this growth, information about the fate and toxicity of nanoparticles in the environment is limited, partly due to the lack of techniques capable of measuring nanoparticles in complex environmental matrices. One emerging tool, flow field-flow fractionation, can be used to determine the presence and particle size distribution of engineered nanoparticles, for example in soil pore waters, thereby enhancing our understanding of their environmental fate and impacts. Abstract. This paper reports preliminary results for the determination of engineered ZnO nanoparticles using flow field-flow fractionation (FlFFF). This separation technique was used to determine the particle size distributions (PSDs) of ZnO nanoparticles spiked in soil suspensions. Before FlFFF analysis, the soil was prepared in suspension form, shaken overnight and gravitationally settled to extract the <1-µm fraction. The effect of aging was also investigated using soil samples incubated with known concentrations of ZnO nanoparticles for 7 and 14 days. The results show that FlFFF can be used to determine the PSD of engineered nanoparticles and monitor their partitioning and stability in soil suspensions.

Availability of zinc and cadmium to different plant species

A pot study was conducted to investigate whether differences in the amounts of Zn and Cd taken up by a number of plant species (canola, capeweed, subterranean clover, lettuce, swiss chard, ryegrass, and wheat) growing in a single soil could be explained as the result of plants having access to different soil pools of Zn and Cd. Prior to potting, the soil was spiked with carrier-free 65Zn and 109Cd. Total shoot concentrations of both Zn and Cd varied greatly across the plant species studied. The concentration of Cd in the shoots ranged from 0·075 mg/kg in wheat cv. RAC to 2·27 mg/kg in capeweed, while the concentration of Zn in the shoots ranged from 33 mg/kg in wheat cv. RAC to 259 mg/kg in swiss chard. However, the specific activity of Zn in the shoots was found to be the same in all cases. Hence the experiments showed that all plant species were accessing the same pool of Zn in the soil, despite differences in the total amounts of Zn taken up. In contrast, the specific activity of Cd in canola was found to be nearly double the specific activity of Cd in the other plants, suggesting that canola was unable to access a pool of soil Cd that was available for uptake by the other species. The percentage of total soil Zn and Cd that was in bioavailable pools was calculated to be approximately 12 and 36 (or 20% for canola), respectively, implying that significant percentages of these metals exist in non plant available forms in this soil.

Interferences in the determination of isotopically exchangeable P in soils and a method to minimise them

In soils with a high phosphate buffering capacity or in soils with large amounts of colloidal P in soil solution, E-values often overestimate P availability. These problems have been ascribed to a variety of causes including analytical difficulties in terms of measuring low concentrations of P, colorimetric interferences, and inadvertent measurement of non-isotopically exchangeable colloidal P. We investigated measurement of E-values in 8 soil types, of which 6 had properties likely to give overestimates of size of the available pool of P based on results of previous authors. The potential for overestimation of the E-value due to interferences described above was identified in several soil types. A simple anion exchange resin purification step was introduced to the E-value methodology. The resin adsorbs both 31P and 32P isotopes in proportion to their concentration in the solution and minimises transfer of colloids and Si into the analysed eluant, thereby providing a simple way to increase the eluant P concentration and to avoid analytical interferences.

Investigating chemical constraints to the measurement of phosphorus in soils using diffusive gradients in thin films (DGT) and resin methods

The effect of potential chemical constraints on the performance of two relatively new soil P testing methods, anion exchange membrane (AEM) and diffusive gradients in thin films (DGT), were evaluated. Exposures to ranges of anion (Cl(-), NO(3)(-), SO(4)(2-) and HCO(3)(-)) concentrations relevant to agricultural soils had minimal effect on P recoveries using DGT. It has also been shown previously that DGT P recoveries are unaffected by varying pH (3-9). In contrast, increasing NO(3)(-) and SO(4)(2-) concentrations in solution reduced the recovery of P using the resin method (anion exchange membrane, AEM) by 24% at 50mgL(-1) NO(3)(-) and by 47% at 12mgL(-1) SO(4)(2-) when the P concentration of the test solution was 2mgL(-1). Phosphorus sorption by the resin decreased with increasing Cl(-) concentrations until there was a 100% decrease at 300mgL(-1) Cl(-) when the P concentration of the test solution was 2mgL(-1) and a 92% reduction at 700mgL(-1) Cl(-) when the P concentration of the test solution was 0.2mgL(-1). There was also a small but significant effect of carbonate species on P sorption to the resin at carbonate concentrations that can occur in agricultural soils. Changing the pH of the solution had minimal effects on the resin P measurements in solutions above pH 4, but below pH 4, resin P measurements decreased dramatically. A poor coefficient of determination for the regression fit between DGT and resin measurements on eight agricultural soils suggested that these two methods are measuring different amounts of P for different soils. Resin P measurements increased significantly, but non-uniformly across soils, when the soil:water ratio was decreased but this did not result in an improved relationship with DGT P. There was a minimal effect on measured P using either Cl(-) or HCO(3)(-) as counter ions on the resin.

Tolerance of nitrifying bacteria to copper and nickel

Evidence is mounting that soil microorganisms can become increasingly tolerant to metals on exposure. However, in situ investigations regarding the effects of metals, particularly Cu and Ni, on specific soil functions/communities are still limited in number. Here, we investigated whether preexposure of nitrifying bacteria to Cu or Ni can induce increased tolerance to these metals. We also investigated whether changes in the tolerance of populations exposed to Cu under field conditions (long term) or in a laboratory-spiked soil (short term) occur. The method used was specifically designed to avoid possible confounding factors because of aging of metals in soil. Sterilized soils were enriched with different concentrations of Ni or Cu and were inoculated with the same soil that was either uncontaminated or had been contaminated previously with metals. Nitrification was measured after 28 d. In the laboratory-spiked soil, the exposed nitrifier community showed an increased tolerance to Ni but not to Cu. However, we found an increased tolerance to Cu in the case of a nitrifying community exposed to Cu for nearly 80 years under field conditions. These results indicate that the capacity of nitrifying bacteria to adapt to at least some metals is a widespread phenomenon. However, acquisition of tolerance to Cu may be more difficult, or require more time, compared with tolerance to Ni.

Effect of toxic cations on copper rhizotoxicity in wheat seedlings

Copper pollution may occur in acidic soils where the low pH leads to release of Al and Mn in soil solution, which could interact with Cu toxicity. Very little information exists regarding the influence of toxic cations on the phytotoxicity of Cu. In this study, we tested the hypothesis that phytotoxicity of Cu2+ may be overestimated in acidic soils due to synergism between Al or Mn and Cu toxicity. Rhizotoxicity of Al, Mn, and Cu to wheat seedlings was studied in well-defined nutrient solutions, with these elements present singly or in combination. Toxicity was expressed on a solution metal-activity basis, with metal activities calculated using GEOCHEM-PC and verified using Donnan dialysis. Of the three ions, Cu2+ was the most rhizotoxic, with activities of Cu2+, Al3+, and Mn2+ resulting in a 25% reduction in root elongation of 0.12, 1.26, and 211 microM, respectively. Although there was no interaction between Mn2+ and Cu2+ toxicity, Cu2+ was significantly less toxic on a relative basis in the presence of Al3+. Thus, critical thresholds for soil solution Cu2+ activity determined in acidic soils will be underprotective compared to soils that contain low concentrations of soluble Al (e.g., limed or nonacidic soils).

Adaptation of Soil Microorganisms to Trace Element Contamination: A Review of Mechanisms, Methodologies, and Consequences for Risk Assessment and Remediation

The authors provide an updated and integrated view of the adaptation of soil microorganisms to elevated concentrations of trace elements. Starting with a summary of the occurrence of trace elements in soils and their effects on soil microorganisms, the scientific evidence underlying adaptation of microorganisms to trace elements from species to community level is discussed. Insights are given regarding the main physiological processes involved in the resistance of bacteria to toxic elements including the potential importance of horizontal gene transfer in the adaptation process. The review continues with a discussion of how new molecular and biotechnological techniques can enrich this field of study. Scientific evidence is utilized in constructing an illustration of microbial community responses with reference to ecological indicators during various adaptation stages, while the related effects on community biological functionality and resilience are discussed. The authors conclude with an evaluation of the importance of considering adaptation in risk assessment and possible remediation of trace element–contaminated sites.

Lead, antimony and arsenic in dissolved and colloidal fractions from an amended shooting-range soil as characterised by multi-stage tangential ultrafiltration and centrifugation

Environmental context The size of soil colloids is – among other characteristics – crucial for the mobility of associated contaminants. We analysed the effect of liming on the size of colloids mobilised from strongly contaminated shooting-range soils using multi-stage tangential ultrafiltration (MTUF) for the size fractionation of dispersed soil colloids. Our results indicate the high analytical potential of MTUF and show that liming induces the aggregation of colloids, thereby decreasing the mobilisation of colloid-bound Sb and As, but increasing colloidal Pb. Abstract The size and composition of colloids are important factors controlling their relevance as carriers of metal(loid)s in soils. Liming, which is often used to reduce the effect of heavy metal contamination in soil, can alter concentrations and characteristics of colloids in soil suspension. In batch studies, we compared the influence of changing pH and cation valency on the size distribution and composition of dispersed colloids and on the concentrations of Pb, As and Sb associated with colloids and in solution following the addition of Ca(OH)2 and KOH to soil samples from a contaminated-shooting range site. Multi-stage tangential ultrafiltration (MTUF) and centrifugation were used for the size fractionation of colloids in aqueous suspension. An increase in soil pH resulted in an increase in colloid-associated Pb, with much higher concentrations in the KOH than in the Ca(OH)2 treated samples. In contrast colloid-associated Sb and As increased only in the KOH treated samples. Addition of the monovalent K-ion induced the dispersion of small (~9–220 nm) organo(-mineral) colloids, whereas the divalent Ca-ion suppressed their dispersion and led to the formation of larger colloids (220–1200 nm). Whereas centrifugation underestimated contaminants (i.e. Pb) associated with organic colloids (density <2.6 g cm–3) MTUF gave a distorted distribution of inorganic colloids (i.e. needle-shaped sesquioxides).