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The pH dependent cation exchange capacity (CEC) and pH buffering capacity (pH BC) are two crucial properties in soil fertility management of variable charge soils. However, they are rarely measured, since most of the existing methods are cumbersome and time-consuming. We propose a new method, based on the silver-thiourea (AgTU) method, for the simultaneous measurement of pH dependent CEC and the pH BC. In general, this method consists of first increasing the pH of the soil to about pH 7 and then gradually acidifying the soil by titrating with HN[O.sub.3], while measuring pH and CEC after each equilibration. This way, both CEC at different pH values and the pH BC are measured on the same sample, without the multiple washing steps needed in other procedures. Various aspects of the proposed method were tested and the results obtained were compared with the ion adsorption method. The modified AgTU method obtains field relevant results and is suitable for the routine analysis of large numbers of samples.

The use of ultrasonic energy for the dispersion of aggregates in studies of soil organic matter (SOM) fractionation entails a risk of redistribution of particulate organic matter (POM) to smaller particle-size fractions. As the mechanical strength of straw also decreases with increasing state of decomposition, it can be expected that not all POM will be redistributed to the same extent during such dispersion. Therefore, we studied the redistribution of POM during ultrasonic dispersion and fractionation as a function of (i) dispersion energy applied and (ii) its state of decomposition. Three soils were dispersed at different ultrasonic energies (750, 1500 and 2250 J g−1 soil) or with sodium carbonate and were fractionated by particle size. Fraction yields were compared with those obtained with a standard particle-size analysis. Undecomposed or incubated (for 2, 4 or 6 months)13C-enriched wheat straw was added to the POM fraction (0.25–2 mm) of one of the soils before dispersion and fractionation. Dispersion with sodium carbonate resulted in the weakest dispersion and affected the chemical properties of the fractions obtained through its high pH and the introduction of carbonate. The mildest ultrasonic dispersion treatment (750 J g−1) did not result in adequate soil dispersion as too much clay was still recovered in the larger fractions. Ultrasonic dispersion at 1500 J g−1 soil obtained a nearly complete dispersion down to the clay level (0.002 mm), and it did not have a significant effect on the total amount of carbon and nitrogen in the POM fractions. The 2250 J g−1 treatment was too destructive for the POM fractions since it redistributed up to 31 and 37%, respectively, of the total amount of carbon and nitrogen in these POM fractions to smaller particle-size fractions. The amount of13C-enriched wheat straw that was redistributed to smaller particle-size fractions during ultrasonic dispersion at 1500 J g−1 increased with increasing incubation time of this straw. Straw particles incubated for 6 months were completely transferred to smaller particle-size fractions. Therefore, ultrasonic dispersion resulted in fractionation of POM, leaving only the less decomposed particles in this fraction. The amounts of carbon and nitrogen transferred to the silt and clay fractions were, however, negligible compared with the total amounts of carbon and nitrogen in these fractions. It is concluded that ultrasonic dispersion seriously affects the amount and properties of POM fractions. However, it is still considered as an acceptable and appropriate method for the isolation and study of SOM associated with silt and clay fractions. [ABSTRACT FROM AUTHOR]

Soil organic matter (SOM) has a key role in maintaining soil fertility in weathered soils in the tropics. This study was conducted to determine the contribution of different SOM fractions to the cation exchange capacity (CEC) of a tropical soil as influenced by organic matter inputs of different biochemical composition. Soil samples were collected from a 16-yr old arboretum established on a Ferric Lixisol, under five multipurpose tree species: Leucaena leucocephala, Dactyladenia barteri, Afzelia africana, Pterocarpus santalinoides, and Treculia africana. Fractions were obtained by wet sieving and sedimentation after dispersion with Na2CO3. Fractions larger than 0.053 mm were separated into mineral and organic components by flotation on water. Relationships between CEC and pH were determined using the silverthioureum-method. For all treatments the organic fractions had the highest CEC, expressed on a dry matter basis, and the CEC of the fractions smaller than 0.053 mm was inversely related to theirparticle size: clay (< 0.002 mm) > fine silt (0.002-0.02 mm) > coarse silt (0.02-0.053 mm). A positive correlation (significant at the 0.01 probability level) existed between the slope of the fitted CEC-pH relationships and the organic C concentrations of the whole soil and both silt fractions. The clay and fine silt fractions were responsible for 85 to 90% of the CEC of the soil. Organic inputs with a high C/N and lignin/N ratio produced fine and coarse silt sized SOM fractions with the highest charge density. Therefore, inputs of slowly decomposing organic residues seem to be promising for increasing the CEC ofhighly weathered soils. [ABSTRACT FROM AUTHOR]

Ecological soil quality standards for lead (Pb) that account for soil Pb bioavailability have not yet been derived. We derived such standards based on specific studies of the long-term bioavailability and toxicity of Pb to soil organisms and a compilation of field data on the bioaccumulation of Pb in earthworms. Toxicity thresholds of Pb to plants, invertebrates, or microorganisms vary over more than 2 orders of magnitude, and the lowest values overlap with the range in natural Pb background concentrations in soil. Soils freshly spiked with Pb 2+ salts exhibit higher Pb bioavailability and lower toxic thresholds than long-term aged and leached equivalents. Comparative toxicity tests on leaching and aging effects suggest using a soil Pb threshold that is 4.0 higher, to correct thresholds of freshly spiked soils. Toxicity to plants and earthworms, and microbial N-transformation and bioaccumulation of Pb in earthworms increase with decreasing effective cation exchange capacity (eCEC) of the soil, and models were derived to normalize data for variation of the eCEC among soils. Suggested ecological quality standards for soil expressed as total soil Pb concentration are lower for Pb toxicity to wildlife via secondary poisoning compared with direct Pb toxicity to soil organisms. Standards for both types of receptors vary by factors of approximately 4 depending on soil eCEC. The data and models we have collated can be used for setting ecological soil quality criteria for Pb in different regulatory frameworks. Environ Toxicol Chem 2021;40:1950-1963. © 2021 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC., (© 2021 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.)

The validity of soil toxicity databases for predicting ecological impacts in the field is rarely explored. The present study was set up to test whether laboratory toxicity data and the combined concepts of metal availability and mixture toxicity can predict ecological impact in mining-affected soils. Metal and As contamination gradients were sampled approximately 5 different mines in Mexico where plant cover and abundances exhibited clear dose-related responses. Soils were analyzed for total and isotopically exchangeable (labile) concentrations of Ni, Cu, Cd, Pb, and As and for soil properties affecting the availability of these elements. Six different indices of toxic doses were compared to evaluate their accuracy in describing the field response expressed as relative abundance and cover. Each index was based on a different method to calculate the sum of toxic units ( Σ TUs) in soil, with 1 toxic unit equal to the concentration of the element in soil yielding 50% adverse effect on plants with median sensitivity as recorded in a recent database of salt-spiked soils. Toxic concentrations in the mine-impacted soils were dominated by Zn and As. In the field, 50% reduced cover or abundance was found at 10 to 13 Σ TUs if these were based on total soil concentrations and thresholds derived from freshly spiked soils, indicating a largely overestimated toxic effect. If thresholds were corrected for differences in availability among freshly spiked soils and spiked and laboratory-aged soils, the overestimation of field toxicity was 5- to 6-fold, irrespective of the consideration of soil properties. Finally, the Σ TU calculated only with labile metals and As overestimated the field toxicity by factors 1.1 to 1.6 (95% confidence interval 1-7; i.e., rather accurate and indicating some Zn-As antagonism as confirmed in experimental studies). That latter index of dose yielded a bell-shaped response on species richness peaking at approximately 1.6 Σ TU. Overall, the present study shows that the current toxicity databases of metals can predict the impact of metal contamination on plant communities within factor 2, expressing the dose as soil-labile concentrations and using the concentration addition concept in these mixed polluted environments. Environ Toxicol Chem 2020;39:1826-1838. © 2020 SETAC., (© 2020 SETAC.)

Silver (Ag) salts have been shown to be highly toxic to freshwater organisms. There is nevertheless still a high level of uncertainty as to the aquatic effects of Ag nanoparticles (AgNPs), and how these relate to the effects of soluble Ag salts. As part of the substance evaluation for Ag of the European Union Registration, Evaluation, Authorisation, and Restriction of Chemicals regulation, we have generated new data to justify read-across from soluble Ag salts to AgNPs. The aquatic toxicity to algae growth and Daphnia magna reproduction, fate, and behavior of AgNO 3 versus AgNPs were tested and compared. Chloride salts in the test media were replaced with equimolar concentrations of nitrate salts. Total Ag, "conventionally" dissolved Ag (0.45 µm), and "truly" dissolved Ag (3 kDa) were determined. Algae were the most sensitive test species to AgNO 3 (10% effect concentration [EC10] 0.10 µg Ag/L) when expressed as conventionally dissolved Ag. The corresponding value for AgNPs was 0.26 µg/L. For D. magna reproduction, the lowest EC10 values were 3.49 µg Ag/L for AgNO 3 and 33.4 µg Ag/L for AgNPs. Using measured Ag concentrations, AgNO 3 was experimentally shown to be more toxic than AgNPs for all Ag fractions. We explain these observations by a different dissolution behavior of AgNO 3 versus AgNPs. The results provide experimental confirmation that AgNO 3 can be used as a conservative estimate for the aquatic effects of AgNPs at comparable Ag concentrations. Environ Toxicol Chem 2019;38:1701-1713. © 2019 SETAC., (© 2019 SETAC.)

The present study examined the effects of soil physical and chemical properties on the toxicity of lead (Pb) to earthworms (Eisenia fetida) and collembolans (Folsomia candida), and on bioaccumulation of Pb by earthworms, in soils amended with Pb salts. Toxicity tests were conducted in 7 soils varying in soil properties (pH 4.7-7.4, effective cation exchange capacity [eCEC] 4-42 cmol c /kg, organic carbon 10-50 g C/kg) that were leached and pH corrected after spiking with PbCl 2 . The median effect concentrations (EC50s) based on total soil Pb concentrations ranged from 35 to 5080 mg Pb/kg for earthworms and 389 to >7190 mg/kg for Collembola. Significant positive correlations were observed between log (EC50) for earthworm reproduction and log (eCEC, total C, exchangeable Ca and Mg, or clay content), but no significant correlations were observed between Pb toxicity to Collembola and soil properties. Expressing Pb dose as either the free ion (Pb 2+ ) activity in porewater or as the measured dissolved porewater concentration of Pb did not explain differences in toxicity among soils. The bioaccumulation factors (BAFs) for Pb in earthworms ranged up to >10-fold across 6 soil treatments, with a median of 0.16, and the BAF was significantly correlated with eCEC (p = 0.038, r = -0.84), but not with any other soil properties. Soil properties related to eCEC (total C, exchangeable Ca and Mg, clay content) had a significant effect on Pb toxicity and bioaccumulation in earthworms, but no relationship was found for Collembola. As a major soil property affecting the bioavailability of Pb, CEC should be incorporated into any soil hazard assessment of Pb as a modifying factor of toxicity and bioaccumulation for earthworms. Environ Toxicol Chem 2019;38:1486-1494. © 2019 SETAC., (© 2019 SETAC.)

Risk assessment of metallic nanoparticles (NPs) is critically affected by the concern that toxicity goes beyond that of the metallic ion. The present study addressed this concern for soils with silver nanoparticles (AgNPs) using the Ag-sensitive nitrification assay. Three agricultural soils (A, B, and C) were spiked with equivalent doses of either AgNP (diameter = 13 nm) or AgNO 3 . Soil solution was isolated and monitored over 97 d with due attention to accurate Ag fractionation at low (∼10 μg L -1 ) Ag concentrations. Truly dissolved (<1 kDa) Ag in the AgNO 3 -amended soils decreased with reaction half-lives of 4 to 22 d depending on the soil, denoting important Ag-aging reactions. In contrast, truly dissolved Ag in AgNP-amended soils first increased by dissolution and subsequently decreased by aging, the concentration never exceeding that in the AgNO 3 -amended soils. The half-lives of AgNP transformation-dissolution were approximately 4 d (soils A and B) and 36 d (soil C). The Ag toxic thresholds (10% effect concentrations, milligrams of Ag per kilogram of soil) of nitrification, evaluated at 21 or 35 d after spiking, were similar between the 2 Ag forms (soils A and B) but were factors of 3 to 8 lower for AgNO 3 than for AgNP (soil C), largely corroborating dissolution differences. This fate and bioassay showed that AgNPs are not more toxic than AgNO 3 at equivalent total soil Ag concentrations and that differences in Ag dissolution at least partially explain toxicity differences between the forms and among soils. Environ Toxicol Chem 2018;37:2123-2131. © 2018 SETAC., (© 2018 SETAC.)

Although metal mixture toxicity has been studied relatively intensely, there is no general consensus yet on how to incorporate metal mixture toxicity into aquatic risk assessment. We combined existing data on chronic metal mixture toxicity at the species level with species sensitivity distribution (SSD)-based in silico metal mixture risk predictions at the community level for mixtures of Ni, Zn, Cu, Cd, and Pb, to develop a tiered risk assessment scheme for metal mixtures in freshwater. Generally, independent action (IA) predicts chronic metal mixture toxicity at the species level most accurately, whereas concentration addition (CA) is the most conservative model. Mixture effects are noninteractive in 69% (IA) and 44% (CA) and antagonistic in 15% (IA) and 51% (CA) of the experiments, whereas synergisms are only observed in 15% (IA) and 5% (CA) of the experiments. At low effect sizes (∼ 10% mixture effect), CA overestimates metal mixture toxicity at the species level by 1.2-fold (i.e., the mixture interaction factor [MIF]; median). Species, metal presence, or number of metals does not significantly affect the MIF. To predict metal mixture risk at the community level, bioavailability-normalization procedures were combined with CA or IA using SSD techniques in 4 different methods, which were compared using environmental monitoring data of a European river basin (the Dommel, The Netherlands). We found that the simplest method, in which CA is directly applied to the SSD (CA SSD ), is also the most conservative method. The CA SSD has median margins of safety (MoS) of 1.1 and 1.2 respectively for binary mixtures compared with the theoretically more consistent methods of applying CA or IA to the dose-response curve of each species individually prior to estimating the fraction of affected species (CA DRC or IA DRC ). The MoS increases linearly with an increasing number of metals, up to 1.4 and 1.7 for quinary mixtures (median) compared with CA DRC and IA DRC , respectively. When our methods were applied to a geochemical baseline database (Forum of European Geological Surveys [FOREGS]), we found that CA SSD yielded a considerable number of mixture risk predictions, even when metals were at background levels (8% of the water samples). In contrast, metal mixture risks predicted with the theoretically more consistent methods (e.g., IA DRC ) were very limited under natural background metal concentrations (<1% of the water samples). Based on the combined evidence of chronic mixture toxicity predictions at the species level and evidence of in silico risk predictions at the community level, a tiered risk assessment scheme for evaluating metal mixture risks is presented, with CA SSD functioning as a first, simple conservative tier. The more complex, but theoretically more consistent and most accurate method, IA DRC , can be used in higher tier assessments. Alternatively, the conservatism of CA SSD can be accounted for deterministically by incorporating the MoS and MIF in the scheme. Finally, specific guidance is also given related to specific issues, such as how to deal with nondetect data and complex mixtures that include so-called data-poor metals. Environ Toxicol Chem 2018;37:623-642. © 2017 SETAC., (© 2017 SETAC.)

Legacy soil data have been produced over 70 years in nearly all countries of the world. Unfortunately, data, information and knowledge are still currently fragmented and at risk of getting lost if they remain in a paper format. To process this legacy data into consistent, spatially explicit and continuous global soil information, data are being rescued and compiled into databases. Thousands of soil survey reports and maps have been scanned and made available online. The soil profile data reported by these data sources have been captured and compiled into databases. The total number of soil profiles rescued in the selected countries is about 800,000. Currently, data for 117, 000 profiles are compiled and harmonized according to GlobalSoilMap specifications in a world level database (WoSIS). The results presented at the country level are likely to be an underestimate. The majority of soil data is still not rescued and this effort should be pursued. The data have been used to produce soil property maps. We discuss the pro and cons of top-down and bottom-up approaches to produce such maps and we stress their complementarity. We give examples of success stories. The first global soil property maps using rescued data were produced by a top-down approach and were released at a limited resolution of 1km in 2014, followed by an update at a resolution of 250m in 2017. By the end of 2020, we aim to deliver the first worldwide product that fully meets the GlobalSoilMap specifications., (© 2017.)

There is an urgent need for sufficient knowledge to allow reliable assessment of the risks associated with engineered nanomaterials (ENPs). Significant advances in basic understanding of nano safety have been made, but there is still no clear systematic basis for risk-related research, and major uncertainties remain in the absence of uniform procedures. The following papers provide the guidance on how to proceed within the area of fate and hazard assessment, and how this links into grouping, testing and risk assessment of nanomaterials. This guidance is coupled with an industrial view on the most important research areas for nanomaterials., (Copyright © 2015 Elsevier Ltd. All rights reserved.)

The fate and effects of toxic trace metals in soil freshly spiked soluble metal salts do not mimic those of metals in the field. This study was set up to test the magnitude of effects of salinity, acidification, and ageing on toxicity of lead (Pb) to plants, invertebrates and soil microbial processes. Three soils were spiked with Pb2+ salts up to a concentration of 8000 mg Pb/kg and were tested either after spiking, after soil leaching followed by pH correction, or after a 5-year outdoor ageing period with free drainage followed by pH correction. Soil solution ionic strength exceeded 150 mmol/L in soils tested directly after spiking and this decreased partially after leaching and returned back to background values after 5-year outdoor equilibration. Chronic toxicity to two plants, two invertebrates, and three microbial endpoints was consistently found in all spiked soils that were not leached. This toxicity significantly decreased or became absent after 5 years of ageing in 19 of the 20 toxicity tests by a factor 8 (median factor; range: 1.4->50), measured by the factor increase of total soil Pb dose required to induce 10% inhibition. The toxicity of Pb in leached soils was intermediate between the other two treatments. The lowest detectable chronic thresholds (EC10) in aged soils ranged 350-5300 mg Pb/kg. Correlation analysis, including data of Pb2+ speciation in soil solution, suggests that reduced ionic strength rather than acidification or true ageing is the main factor explaining the soil treatment effects after spiking. It is suggested that future toxicity studies should test fine PbO powder as a relevant source for Pb in soils to exclude the confounding salt effects., (Copyright © 2015 Elsevier B.V. All rights reserved.)

Although it is widely recognized that microorganisms are essential for sustaining soil fertility, structure, nutrient cycling, groundwater purification, and other soil functions, soil microbial toxicity data were excluded from the derivation of Ecological Soil Screening Levels (Eco-SSL) in the United States. Among the reasons for such exclusion were claims that microbial toxicity tests were too difficult to interpret because of the high variability of microbial responses, uncertainty regarding the relevance of the various endpoints, and functional redundancy. Since the release of the first draft of the Eco-SSL Guidance document by the US Environmental Protection Agency in 2003, soil microbial toxicity testing and its use in ecological risk assessments have substantially improved. A wide range of standardized and nonstandardized methods became available for testing chemical toxicity to microbial functions in soil. Regulatory frameworks in the European Union and Australia have successfully incorporated microbial toxicity data into the derivation of soil threshold concentrations for ecological risk assessments. This article provides the 3-part rationale for including soil microbial processes in the development of soil clean-up values (SCVs): 1) presenting a brief overview of relevant test methods for assessing microbial functions in soil, 2) examining data sets for Cu, Ni, Zn, and Mo that incorporated soil microbial toxicity data into regulatory frameworks, and 3) offering recommendations on how to integrate the best available science into the method development for deriving site-specific SCVs that account for bioavailability of metals and metalloids in soil. Although the primary focus of this article is on the development of the approach for deriving SCVs for metals and metalloids in the United States, the recommendations provided in this article may also be applicable in other jurisdictions that aim at developing ecological soil threshold values for protection of microbial processes in contaminated soils., (© 2014 The Authors. Integrated Environmental Assessment and Management Published by SETAC.)

Although aging processes are important in risk assessment for metals in soils, the aging of Ni added to soils has not been studied in detail. In this study, after addition of water soluble Ni to soils, the changes over time in isotopic exchangeability, total concentrations and free Ni(2+) activity in soil pore water, were investigated in 16 European soils incubated outdoors for 18 months. The results showed that after Ni addition, concentrations of Ni in soil pore water and isotopic exchangeability of Ni in soils initially decreased rapidly. This phase was followed by further decreases in the parameters measured but these occurred at slower rates. Increasing soil pH increased the rate and extent of aging reactions. Semi-mechanistic models, based on Ni precipitation/nucleation on soil surfaces and micropore diffusion, were developed and calibrated. The initial fast processes, which were attributed to precipitation/nucleation, occurred over a short time (e.g. 1h), afterwards the slow processes were most likely controlled by micropore diffusion processes. The models were validated by comparing predicted and measured Ni aging in three additional, widely differing soils aged outdoors for periods up to 15 months in different conditions. These models could be used to scale ecotoxicological data generated in short-term studies to longer aging times., (Copyright © 2013 Elsevier Ltd. All rights reserved.)

The free ion approach has been previously used to calculate critical limit concentrations for soil metals based on point estimates of toxicity. Here, the approach was applied to dose-response data for copper effects on seven biological endpoints in each of 19 European soils. The approach was applied using the concept of an effective dose, comprising a function of the concentrations of free copper and 'protective' major cations, including H(+). A significant influence of H(+) on the toxicity of Cu(2+) was found, while the effects of other cations were inconsistent. The model could be generalised by forcing the effect of H(+) and the slope of the dose-response relationship to be equal for all endpoints. This suggests the possibility of a general bioavailability model for copper effects on organisms. Furthermore, the possibility of such a model could be explored for other cationic metals such as nickel, zinc, cadmium and lead., (Copyright © 2013 Elsevier Ltd. All rights reserved.)

Copper (Cu) containing fungicides have been used for more than one century in Europe on agricultural soils, such as vineyard soils. Total Cu concentrations in such soils can exceed toxicological limits that are commonly derived using artificially spiked soils. This study surveyed Cu toxicity in vineyard soils with reference to soils spiked with CuCl(2). Soil was collected in six established European vineyards. At each site, samples representing a Cu concentration gradient were collected. A control (uncontaminated) soil sampled nearby the vineyard was spiked with CuCl(2). Toxicity was tested using standard ecotoxicity tests: two plant assays (Lycopersicon esculentum Miller (tomato) and Hordeum vulgare L. (barley) growth), one microbial assay (nitrification) and one invertebrate assay (Enchytraeus albidus reproduction). Maximal total Cu concentrations in the vineyard sites ranged 435-690 mg Cu kg(-1), well above the local background (23-105 mg Cu kg(-1)). Toxicity in spiked soils (50% inhibition) was observed at added soil Cu concentrations from 190 to 1039 mg Cu kg(-1) (mean 540 mg Cu kg(-1)) depending on the assay and the site. In contrast, significant adverse effects were only found for three bioassays in vineyard samples of one site and for two bioassays in another site. Biological responses in these cases were more importantly explained by other soil properties than soil Cu. Overall, no Cu toxicity to plants, microbial processes and invertebrates was observed in vineyard soil samples at Cu concentrations well above European Union limits protecting the soil ecosystem., (Copyright © 2012 Elsevier B.V. All rights reserved.)

To determine if long-term equilibration may alleviate molybdenum toxicity, earthworms, enchytraeids, collembolans and four plant species were exposed to three soils freshly spiked with Na(2)MoO(4).2H(2)O and equilibrated for 6 or 11 months in the field with free drainage. Total Mo concentrations in soil decreased by leaching, most (up to 98%) in sandy soil and less (54-62%) in silty and clayey soils. Changes in residual Mo toxicity with time were inconclusive in sandy soil. In the other two soils, toxicity of residual total Mo was significantly reduced after 11 months equilibration with a median 5.5-fold increase in ED50s. Mo fixation in soil, i.e. the decrease of soil solution Mo concentrations at equivalent residual total soil Mo, was maximally a factor of 2.1 only. This experiment shows natural attenuation of molybdate ecotoxicity under field conditions is related to leaching of excess Mo and other ions as well as to slow ageing reactions., (Copyright © 2011 Elsevier Ltd. All rights reserved.)

Metals in soils amended with sewage sludge are typically less available compared with those in soils spiked with soluble metal salts. However, it is unclear if this difference remains in the long term. A survey of copper (Cu) availability was made in soils amended with sewage sludge, manure, and compost, collectively named organic amendments. Paired sets of amended and control soils were collected from 22 field trials where the organic amendments had aged up to 112 yr. Amended soils had higher total Cu concentrations (range, 2-220 mg Cu kg; median, 15 mg Cu kg) and organic C (range, 1-16 g kg; median, 4 g kg) than control soils. All samples were freshly spiked with CuCl, and the toxicity of added Cu to barley was compared between amended and control soils. The toxicity of added Cu was significantly lower in amended soils than in control soil in 15 sets by, on average, a factor of 1.4, suggesting that aged amendments do not largely increase Cu binding sites. The fraction of added Cu that is isotopic exchangeable Cu (labile Cu) was compared between control soils freshly spiked with CuCl and amended soils with both soils at identical total Cu concentrations. Copper derived from amendments was significantly less labile (on average 5.9-fold) than freshly added Cu in 18 sets of soils. This study shows that Cu availability after long-term applications of organic amendments is lower than that of freshly added Cu salts, mainly because of its lower availability in the original matrix and ageing reactions than because of increased metal binding sites in soil., (Copyright © by the American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America, Inc.)

Total concentrations of metals in soil are poor predictors of toxicity. In the last decade, considerable effort has been made to demonstrate how metal toxicity is affected by the abiotic properties of soil. Here this information is collated and shows how these data have been used in the European Union for defining predicted-no-effect concentrations (PNECs) of Cd, Cu, Co, Ni, Pb, and Zn in soil. Bioavailability models have been calibrated using data from more than 500 new chronic toxicity tests in soils amended with soluble metal salts, in experimentally aged soils, and in field-contaminated soils. In general, soil pH was a good predictor of metal solubility but a poor predictor of metal toxicity across soils. Toxicity thresholds based on the free metal ion activity were generally more variable than those expressed on total soil metal, which can be explained, but not predicted, using the concept of the biotic ligand model. The toxicity thresholds based on total soil metal concentrations rise almost proportionally to the effective cation exchange capacity of soil. Total soil metal concentrations yielding 10% inhibition in freshly amended soils were up to 100-fold smaller (median 3.4-fold, n = 110 comparative tests) than those in corresponding aged soils or field-contaminated soils. The change in isotopically exchangeable metal in soil proved to be a conservative estimate of the change in toxicity upon aging. The PNEC values for specific soil types were calculated using this information. The corrections for aging and for modifying effects of soil properties in metal-salt-amended soils are shown to be the main factors by which PNEC values rise above the natural background range.

Although a large body of evidence indicates that metal toxicity to soil organisms is affected by physicochemical soil properties, use of this knowledge in ecological risk assessments is limited because of the lack of a model applicable to a wide range of soils. To study the effect of soil characteristics on the toxicity of copper to terrestrial invertebrates, chronic toxicity tests with Eisenia fetida and Folsomia candida were performed in 19 European field soils. These soils were carefully selected to cover the range of toxicity-influencing parameters encountered in the European Union. Toxicity values varied greatly among soils, with 28-d median effect concentrations ranging from 72.0 to 781 mg Cu/kg dry weight for E. fetida and from 45.4 to 2,270 mg Cu/kg dry weight for F. candida. For both species, variation in copper toxicity values was best explained by differences in the actual cation-exchange capacity (CEC) at soil pH. Using the obtained regression algorithms, the observed toxicity could, in most cases, be predicted within a factor of two for E. fetida and within a factor of three for F. candida. The developed models were validated in three additional European field soils, a standard artificial soil and a standard field soil. The presented regression equations, based on the actual CEC, offer an easy-to-apply method for taking the influence of soil properties on metal toxicity into account.

Antimony trioxide (Sb2O3) is a widely used chemical that can be emitted to soil. The fate and toxicity of this poorly soluble compound in soil is insufficiently known. A silt-loam soil (pH 7.0, background 0.005 mmol Sb kg(-1)) was amended with Sb2O3 at various concentrations. More than 70% of Sb in soil solution was present as Sb(V) (antimonate) within 2 days. The soil solution Sb concentrations gradually increased between 2 and 35 days after Sb2O3 amendment but were always below that of soils amended with the more soluble SbCl3 at the lower Sb concentrations. The soil solution Sb concentrations in freshly amended SbCl3 soils (7 days equilibration) were equivalent to those in Sb2O3-amended soils equilibrated for 5 years at equivalent total soil Sb. Our data indicate that the Sb solubility in this soil was controlled by a combination of sorption on the soil surface, Sb precipitation at the higher doses, and slow dissolution of Sb2O3, the latter being modeled with a half-life ranging between 50 and 250 days. Toxicity of Sb to plant growth (root elongation of barley, shoot biomass of lettuce) or to nitrification was found in soil equilibrated with Sb2O3 (up to 82 mmol Sb kg(-1)) for 31 weeks with 10% inhibition values at soil solution Sb concentrations of 110 microM Sb or above. These concentrations are equivalent to 4.2 mmol Sb per kg soil (510 mg Sb kg(-1)) at complete dissolution of Sb2O3 in this soil. No toxicity to plant growth or nitrification was evident in toxicity tests starting one week after soil amendment with Sb2O3, whereas clear toxicity was found in a similar test using SbCl3. However, these effects were confounded by a decrease in pH and an increase in salinity. It is concluded that the Sb(V) toxicity thresholds are over 100-fold larger than background concentrations in soil and that care must be taken to interpret toxicity data of soluble Sb(III) forms due to confounding factors.

For most trace elements, the technique of phytoextraction needs significant improvements to become practically feasible. Calculations for Cd revealed that the amount of Cd taken up by Thlaspi caerulescens or Salix spp. needs at least to be the double of the present amount to slightly decrease the Cd concentration in the upper 0.5m of the soil within a period of 10 years. Additionally, metals taken up by the plants might pose an important risk. Alternatives as bioavailable contaminant stripping and phytostabilization might be more appropriate.

Nickel is a trace metal that exhibits pronounced long-term immobilization reactions in soil. It is unknown if the slowly decreasing solubility of Ni in soil on aging correlates with decreased toxicity to soil biota. Three uncontaminated soils (pH 4.5-7.6) were contaminated with NiCl2 and experimentally leached or incubated outdoors with free drainage for up to 15 months. Nickel toxicity was measured for three microbial processes (potential nitrification rate, glucose-induced respiration, and C mineralization of maize residue). Results for leached and aged samples were compared with results for these soils tested immediately after spiking. Experimental leaching increased Ni ED50s (Ni dose to inhibit process by 50%) with a median factor of 2.0, whereas Ni ED50s in soils aged 15 months were a factor 1 to 23 (median, 4.6) larger compared to freshly spiked soils. Changes in soil Ni toxicity on aging generally were largest in the soil with the highest pH, consistent with the largest relative decreases of soil solution Ni concentration or predicted Ni2+ activity. Soil solution Ni concentrations explained part, but not all, of the reduction in Ni toxicity. The predicted soil solution Ni2+ activity also did not fully explain the reduced toxicity, which was ascribed to the variable concentrations of ions competing with Ni2+ at biological membranes (e.g., H+, Mg2+, or Ca2+) among treatments. It is concluded that testing Ni toxicity to soil microbial processes immediately after spiking soils in the laboratory overestimates Ni toxicity compared to aged soils. Soil solution composition in freshly spiked soils clearly is different from that in leached or aged soils; therefore, soil spiked with metal salts should be leached before toxicity tests begin.

The Terrestrial Biotic Ligand Model (TBLM) is applied to a number of noncalcareous soils of the European Union for Cu and Ni toxicities using organisms and endpoints representing three levels of terrestrial organisms: higher plants, invertebrates, and microbes. A comparison of the TBLM predictions to soil metal concentration or free metal ion activity in the soil solution shows that the TBLM is able to achieve a better normalization of the wide variation in toxicological endpoints among soils of disparate properties considered in this study. The TBLM predictions of the EC50s were generally within a factor of 2 of the observed values. To our knowledge, this is the first study that incorporates Cu and Ni toxicities to multiple endpoints associated with higher plants, invertebrates, and microbes for up to eleven noncalcareous soils of disparate properties, into a single theoretical framework. The results of this study clearly demonstrate that the TBLM can provide a general framework for modeling metals ecotoxicity in soils.

A systematic comparison of Cu toxicity thresholds was made between freshly spiked soils and soils in which elevated Cu concentrations have been present for various times. Three uncontaminated soils were spiked and experimentally leached or incubated outdoors for up to 18 months. Additionally, five field-contaminated soils with a 6- to 80-year-old Cu contamination were sampled, and corresponding uncontaminated soils were spiked to identical total concentrations. All soil samples were subjected to three microbial assays (nitrification potential, glucose-induced respiration, and maize residue C-mineralization). Experimental leaching or soil incubation after spiking reduced Cu toxicity (1.3- or 2.3-fold increase of dose, respectively, to inhibit process by 50% [ED50]). No significant effects of soil type, aging time (6, 12, or 18 months), or bioassay on the factor change of ED50 were found. Significant reductions of microbial activity in field-contaminated soils were only identified in 2 of the 15 series (three assays in five soils), whereas freshly spiking the corresponding control soils significantly affected these processes in 12 series. Soil solution Cu concentrations significantly decreased on leaching at corresponding total soil Cu, and smaller decreases were found during additional aging. Soil solution Cu concentrations largely explain changes in Cu toxicity on leaching and aging, although additional variation may be related to changes in the sensitivity of microbial populations. It is concluded that total Cu toxicity thresholds are lower in freshly spiked soils compared to soils in which Cu salts have equilibrated and leaching has removed excess soluble salts. The large variability of soil microbial processes creates a large uncertainty about the magnitude of the factor by which aging mitigates Cu toxicity.

It generally is unknown to what extent the toxicity of Cu and Ni for soil microbial processes varies among different soils. A comparative study was made using three different microbial assays (nitrification potential, glucose-induced respiration, and C-mineralization of a plant residue) in 19 (for Cu) or 16 (for Ni) soils with contrasting soil properties. Each soil was spiked with CuCl2 or NiCl2 at seven different concentrations, and the bioassays were started after a 7-d equilibration period. The Cu and Ni toxicity thresholds varied 19- to 90-fold among soils. The differences in both Cu and Ni toxicity among soils were neither explained by soil solution metal concentrations nor by free ion activities calculated from soil solution composition. Copper toxicity thresholds (total concentrations) increased with increasing organic matter content or cation exchange capacity (CEC) and, surprisingly, decreased with increasing pH depending on the assay. Nickel toxicity thresholds consistently increased with increasing CEC, background Ni, and clay content for all three assays. Thresholds expressed as soil solution free ion activities all significantly decreased with increasing soil solution pH (r2 = 0.57-0.93), consistent with a decreased H+:M2+ competition at the biological membrane. That competition largely counteracts the H+:M2+ competition for sorption, effectively explaining the insignificant or weak effect of pH on total Ni or Cu toxicity thresholds. It is concluded that free metal-ion activity alone does not explain variation in metal toxicity among soils.