Sensitivity of microbial bioindicators in assessing metal immobilization success in smelter-impacted soils.

While plant toxicity reduction remains the primary metric for judging the success of metal immobilization in soil, the suitability of microorganisms as universal indicators of its effectiveness in various contaminated soils remains a point of contention. This study assessed the sensitivity of microbial bioindicators in monitoring metal immobilization success in smelter-impacted soils. It compared plants and microorganisms as indicators of the efficiency of natural Fe-Mn nodules from the Gulf of Finland in immobilizing metals in soils contaminated by a Ni/Cu smelter, on the Kola Peninsula, Murmansk region, Russia. Perennial ryegrass (Lolium perenne) was grown on nodule-amended and control soils. Plant responses in the smelter-impacted soils proved to be sensitive and robust indicators of successful metal immobilization. However, microbial responses exhibited a more complex story. Despite the observed reductions in soluble metal concentrations, shoot metal contents in ryegrass, and significant improvements in plant growth, certain microbial bioindicators were unresponsive to metal immobilization success brought about by the addition of Fe-Mn nodules. Among microbial bioindicators studied, community-level physiological profiling, microbial biomass carbon, and basal respiration were sensitive indicators of metal immobilization success, whereas the number of saprotrophic, oligotrophic, and Fe-oxidizing bacteria and fungi, the biomass of bacteria and fungi, and enzymatic activity were less robust indicators. Interestingly, the correlations between different microbial responses measured were weak or even negative. Some microbial responses also exhibited negative correlations with plant biomass. These findings underscore the need for further research on comparative evaluations of plants and microorganisms as reliable indicators of metal immobilization efficacy in polluted environments.

Zinc shields against copper phytotoxicity in a contaminated soil.

While zinc protects plants from copper in hydroponics, its behavior in soil remains unclear. We investigated the potential of zinc sulfate to protect ryegrass from copper toxicity in contaminated soil. Twelve soil treatments combined varying levels of copper oxide (CuO) and zinc sulfate (ZnSO4). Increasing CuO significantly stunted ryegrass, but adding ZnSO4 mitigated the effects at each CuO level. ZnSO4 had no effect in unpolluted conditions. These results, supported by the Terrestrial Biotic Ligand Model, indicate that zinc competes with copper for binding sites, reducing copper uptake by ryegrass and mitigating its toxicity. Application of zinc sulfate to copper-contaminated soils appears promising for ryegrass growth, although field studies are critical to confirm real-world efficacy.

The downside of copper pesticides: An earthworm's perspective.

The widespread use of copper-based pesticides, while effective in controlling plant diseases, has been identified as a major source of copper contamination in soils. This raises concerns about potential adverse effects on earthworms, key players in soil health and ecosystem function. To inform sustainable pesticide practices, this study aimed to establish copper toxicity thresholds for earthworm avoidance in agricultural soils impacted by copper-based pesticides. We collected 40 topsoil samples (0-5 cm) from orchards and vineyards in the O'Higgins Region of central Chile, and 10 additional soils under native vegetation as background references. A standardized avoidance bioassay using Eisenia fetida assessed the impact of copper-based pesticides on the soils. Total copper concentrations ranged between 23 and 566 mg kg-1, with observed toxic effects on earthworms in certain soils. The effective concentration at 50% (EC50) for total soil copper, determined by Eisenia fetida's avoidance response, was 240 mg kg-1, with a 95% confidence interval of 193-341 mg kg-1. We further compared our EC50 values with existing data from agricultural soils impacted by mining activities. Interestingly, the results revealed a remarkable similarity between the thresholds for earthworm avoidance, regardless of the source of copper contamination. This observation underscores the universality of copper toxicity in agricultural ecosystems and its potential impact on soil biota. This study provides novel insights into copper toxicity thresholds for earthworms in real-world, pesticide-contaminated soils.

Extremely High Soil Copper Content, yet Low Phytotoxicity: A Unique Case of Monometallic Soil Pollution at Kargaly, Russia.

Ecotoxicological studies on soil metal toxicity often rely on artificially contaminated soils. A major difficulty in using soils contaminated by anthropogenic activities (e.g., mining and agriculture) is the presence of multiple metals, which can make it impossible to distinguish the toxic effects of a particular metal. Therefore, sites with monometallic pollution have great potential for ecotoxicological research. One such site is an agricultural field in Kargaly, Orenburg region, Russia, where copper mining and smelting activities were carried out during the 18th-20th centuries. Samples of Mollisols (chernozems) were collected in the studied field. At several sampling points there were copper ore rocks on the surface, containing malachite (CuCO3 · Cu(OH)2 ). The soil samples had a high copper content, up to approximately 10 g kg-1 , compared with 75 mg kg-1 in the background soil. Importantly, the content of other elements in all soil samples was similar to that in the background soil, highlighting the uniqueness of the monometallic contamination in the study area. Despite the extremely high total copper content, exchangeable copper was relatively low, with a maximum of approximately 0.5 mg kg-1 . We performed a short-term (21-day) ecotoxicity assessment using perennial ryegrass as an indicator of copper toxicity. Contrary to expectations, plant growth was not affected by the high copper content in the studied soils. The low copper phytotoxicity may be explained by the low solubility of malachite. However, future long-term experiments may be warranted to determine copper toxicity thresholds for plants under field conditions. The site discovered in the present study could potentially acquire the same significance as the Danish Hygum site for the study of monometallic soil contamination. Environ Toxicol Chem 2023;42:707-713. © 2023 SETAC.

An Assessment of the Feasibility of Phytoextraction for the Stripping of Bioavailable Metals from Contaminated Soils.

Phytoextraction has been proposed in many papers as a low-cost method for remediating contaminated soil. However, if national regulation is based on total metal(loid) concentrations in soil, phytoextraction is generally infeasible because of the long time required for remediation. Assessing phytoextraction requires determination of the dynamic rate of metal removal from soil. Phytoextraction may be feasible if the main goal is to reduce the soluble fraction of the metal(loid) with the goal of reducing bioavailability. However, it has been reported that there is a large mass balance mismatch between the reduction of the soluble metal fraction in contaminated soil and metal uptake by plants. Several studies report that the decrease of soluble fraction of metals in soil is higher than can be accounted for by plant uptake. In other words, studies generally overestimate the feasibility of bioavailable contaminant stripping. Therefore, a more rigorous approach is advisable to ensure that papers on bioavailable contaminant stripping include relevant information on mass balances. Furthermore, to implement the concept of bioavailable contaminant stripping, regulations must distinguish between the bioavailable fraction and the total metal concentration in soil. Environ Toxicol Chem 2023;42:558-565. © 2022 SETAC.

Microbial responses are unreliable indicators of copper ecotoxicity in soils contaminated by mining activities.

Metal contamination of soil has become a serious environmental problem worldwide. Many studies have attempted to infer metal ecotoxicity from soil microbial responses. However, much of the data from these studies tends to be inconsistent and difficult to interpret. We hypothesized that microbial response would be a useful indicator of metal toxicity in soils contaminated by copper mining in Chile. Eighty-four topsoils (0-20 cm) were collected from three areas historically contaminated by copper mining (total Cu: 46-1106 mg kg-1, soluble Cu: 0.05-2.3 mg kg-1, pCu2+: 6.3-10, pH: 5.1-7.8, organic matter: 1.1-10%, clay: 0-28%). Based on soil metal concentrations and ecotoxicity thresholds, Cu was expected to be toxic to microorganisms in the studied soils, while the effects of other metals (total Zn: 79-672, As: 1.9-60, Pb: 19-220, Cd: 0.4-5.1 mg kg-1) were expected to be minor. Soil microbial responses (microbial biomass and numbers, nitrogen mineralization and nitrification, and community-level physiological profiles) were also measured. The results showed that the different responses of soil microbes were not correlated with each other. Furthermore, the soil microbial responses were mainly influenced by the physicochemical properties of the soil, not by the metal concentrations in the soil. The effect of copper on the microbial response was either stimulating (positive) or toxic (negative). Of the soil microbial responses measured in this study, only the microbial biomass was useful for calculating dose-response curves. However, the microbial biomass response was not consistent among the different soil copper pools (total copper, soluble copper, and activity of free Cu2+ ions). It is important to emphasize that the thresholds obtained for copper toxicity cannot be adopted in a robust manner because of the different microbial responses in different sampling areas. Thus, in the copper-contaminated soils under study, microbial response was found to be an unreliable indicator of metal toxicity.

Global issues in setting legal limits on soil metal contamination: A case study of Chile.

The establishment of legal limits for soil contamination with trace elements is a global issue that has not yet been resolved. However, the resolution of any global problem begins at the national level. In this vein, we present the case of Chile, the world's leading copper producer, where soil contamination by trace elements in mining areas has been severe. We evaluated the magnitude of the ecological and human health risks from exposure to arsenic (As), copper (Cu), zinc (Zn), and lead (Pb) in soils of the La Ligua and Petorca basins, two important mining areas in Chile. Contrary to what might be expected in soils affected by Cu mining activities, As was identified as the most hazardous element in the studied soils, both in terms of ecological and human health risks. On the other hand, Chile does not currently have specific legislation establishing legal limits on soil contamination with trace elements. Since Chile is geochemically similar to New Zealand, Mexico, and Italy, we used the limits of these three countries as benchmarks. We determined the background concentrations of As, Cu, Zn, and Pb in the soils of the two river basins under study and found that they tend to exceed the limits established by foreign laws. We also found that the differences in background elemental concentrations in the studied soils were primarily due to the varied lithology of soil-forming rocks. This means that absolute "one-limit-fits-all" values of element concentrations may not be adequate to regulate the level of soil contamination in areas affected by mining. As a fundamental first step, it is necessary to establish background soil concentrations of trace elements in each river basin in Chile. It is clear that Chile urgently needs to move from rubber-stamping foreign laws to the development of national legislation on soil metal contamination.

Gypsum soil amendment in metal-polluted soils-an added environmental hazard.

Scientists around the world have long been searching for effective strategies to reduce the bioavailability of metals in contaminated soils. In case of metal-spiked soils, some studies have proposed gypsum as a soil amendment to alleviate metal phytotoxicity. However, for real field-collected soils, evidence on the efficacy of gypsum as a metal phytotoxicity amendment is limited. Therefore, the present study was designed to examine the effect of gypsum on plant growth in soils polluted by a copper smelter. We grew perennial ryegrass on untreated and gypsum-treated soils (at a dose of 3% by weight) under laboratory conditions. We found that gypsum had no effect on alleviating metal phytotoxicity in our soils. We also demonstrated - for the first time - that gypsum increased the concentrations of soluble metals in the soil, enhancing metal uptake by plants. The calcium ions from gypsum displace metals in the soil exchangeable complex; however, the metals do not get immobilized in soils because gypsum is a neutral salt. While our results contrast with the Terrestrial Biotic Ligand Model, that Model has never been tested on real industrially polluted soils but only on metal-spiked soils. Our main conclusion is that gypsum is ineffective in alleviating metal phytotoxicity in real industrially polluted soils and, moreover, its use is inappropriate as a soil remediation method, because it increases the environmental hazard rather than reducing it. Our study is the very first attempt to recognize that gypsum is a hazardous material when used to ameliorate soils polluted by metals.

The role of leaf litter as a protective barrier for copper-containing pesticides in orchard soils.

This study assessed the capacity of leaf litters to adsorb copper ions applied as a copper-based pesticide. Leaf litters of two fruit tree species with different lignin/N ratios were examined to determine their protective role against the incorporation of Cu into soil. A leaf litter Cu-adsorption capacity assay and a degradation assay were performed using table grape (lignin/N = 2.35) and kiwi (lignin/N = 10.85) leaf litters. Table grape leaf litter had a significantly (p = 0.001) higher Cu-adsorption capacity (15,800 mg kg-1) than kiwi leaf litter (14,283 mg kg-1). Following leaf litter degradation, significant differences (p = 0.011) were observed in the release of Cu from Cu-enriched leaf litter into soil, showing that kiwi litter has a greater protective effect against the incorporation of Cu into soil, regardless of the amount of Cu applied. This protective role is reflected in a significantly higher (p = 0.015) Cu concentration in table grape soil (41.71 ± 2.14 mg kg-1) than in kiwi soil (35.87 ± 0.69 mg kg-1). Therefore, leaf litter with higher lignin/N ratio has greater protective role against copper incorporation into soil.

Side effects of traditional pesticides on soil microbial respiration in orchards on the Russian Black Sea coast.

Agricultural use of pesticides has greatly increased worldwide over the last several decades, affecting soil microorganisms. Microbial basal respiration and substrate-induced respiration rates are commonly used to assess the detrimental effects of pesticides on soil quality. The goal of the present study was (1) to compare the impact of different pesticides on soil microbial respiration under field conditions, and (2) to characterize the recovery time of soil microbial respiration after pesticide application. The following pesticides were used in the present study: chlorpyrifos, phosalone, dimethoate (organophosphorus insecticides), λ-cyhalothrin (pyrethroid insecticide), and kresoxim-methyl (fungicide). The application of all the pesticides at commercial doses led to a decrease in soil microbial respiration. The inhibition of basal respiration and substrate-induced respiration rate decreased in the following order: chlorpyrifos > phosalone > dimethoate > λ-cyhalothrin ≈ kresoxim-methyl. Among all the pesticides assessed, chlorpyrifos showed the highest toxicity as well as the highest persistence. Several of the observed results differed greatly from previous studies; thus, local assessments are highly advisable. Given that environmental concerns can be a key decision factor for pesticide selection, assessment of different pesticides-such as undertaken in this study-could help farmers to choose the most appropriate pesticide.

Use of Zinc Carbonate Spiking to Obtain Phytotoxicity Thresholds Comparable to Those in Field-Collected Soils.

Several studies have reported the presence of smithsonite (ZnCO3 ) in soils polluted by zinc mining. The present study aimed to determine upper critical threshold values of Zn phytotoxicity in a substrate spiked with ZnCO3 and to compare them with those obtained in field-collected soils. We studied Zn toxicity to perennial ryegrass (Lolium perenne L.) grown in pots with unpolluted peat treated with increasing concentrations of ZnCO3 that produced nominal total Zn concentrations of 0, 0.7, 1.3, 2.0, 2.6, and 3.3%. To keep constant near-neutral pH value in all the treatments, we used decreasing concentrations of dolomitic lime. In the treatment with total soil Zn of 3.3% (pH 6.8), the foliar Zn concentration of L. perenne was 1914 ± 211 mg kg-1 , falling into the range of 2400 ± 300 mg kg-1 reported for Lolium species grown under similar laboratory conditions in a polluted soil (total soil Zn 5.4%, pH 7.3) collected near a Zn smelter. The value of 92 ± 98 mg kg-1 was obtained for the median effective concentration (EC50) values of 0.01 M KNO3 -extractable Zn using the responses of shoot dry biomass, shoot length, and total pigments. This value falls within the range of 95 ± 46 mg kg-1 reported in other studies for the EC50 values of salt-extractable Zn using field-collected soils. The application of ZnCO3 for spiking was able to mimic foliar Zn concentrations of Lolium species observed in field-collected soils. The effective concentrations of soil Zn obtained in the present study are comparable to those obtained in field-collected soils. Future research should determine effective concentrations of metals using soils spiked with metal-containing compounds that mimic a real source of contamination. Environ Toxicol Chem 2020;39:1790-1796. © 2020 SETAC.

Chilean regulations on metal-polluted soils: The need to advance from adapting foreign laws towards developing sovereign legislation.

Chile as a major international Cu producer faces serious soil contamination issues in mining areas. Currently Chile does not have any specific law governing the maximum permissible concentrations of metals in soils to protect ecosystems and human health. Chile heavily relies on the use of environmental laws of 14 foreign countries; the choice of the country depends on the similarity of its environmental conditions with those in Chile. In this study, we used an online database to compare the similarity of Chilean rocks to those in foreign countries. Likewise, we performed soil sampling and determined the background concentrations of Cu, As, Pb, and Zn in soils of the Aconcagua basin, the largest river basin in the Valparaiso Region of central Chile. The results showed that geochemical patterns in Chile have the greatest resemblance to New Zealand, Mexico, and Italy. The background Cu concentration in the Aconcagua basin (134 mg kg-1) exceeded the legislated limits of New Zealand (100 mg kg-1) and Italy (120 mg kg-1), whereas the background Zn concentration (200 mg kg-1) exceeded the legislated limit of Italy (150 mg kg-1). Due to the elevated natural abundance of Cu and Zn in Chile, international laws should not be applied in Chile for the assessment of soil contamination. In addition, we assessed ecological risk using the results of our previous studies obtained by analyzing native field-contaminated soils of the Valparaiso region. In the Aconcagua basin, Cu posed high risk for plants in 11% of the samples, whereas As posed high risk for earthworms in 48% of the samples. We suggest that future studies are required to search for other organisms that can serve as biomarkers of metal toxicity because our previous studies were limited to plants and earthworms. Importantly, As posed high risk to human health in 25% of the samples in our study. There is a need for future studies to demonstrate empirically an association between soil As and children's blood As in order to establish the national threshold values of soil As to protect human health. We conclude that there is an urgent need in Chile to advance from the current approach of adapting foreign laws to developing Chilean sovereign environmental legislation.

Advanced determination of the spatial gradient of human health risk and ecological risk from exposure to As, Cu, Pb, and Zn in soils near the Ventanas Industrial Complex (Puchuncaví, Chile).

The townships of Puchuncaví and Quintero, on the coast of central Chile, have soils contaminated by atmospheric deposition of sulfur dioxide and trace elements from the nearby Ventanas Industrial Complex. The purpose of this study was to evaluate potential human health and ecological risks, by determining the spatial distribution of soil total concentrations arsenic (As), copper (Cu), lead (Pb), and zinc (Zn) in these townships. Total concentrations of these elements were determined in 245 topsoil samples, used to generate continuous distribution maps. The background concentrations of Cu, As, Pb, and Zn in the studied soils were 100, 16, 35, and 122 mg kg-1, respectively. The concentrations of Cu, As, and Pb were positively correlated with each other, suggesting that their source is the Ventanas copper smelter. On the other hand, correlations for Zn were weaker than for other trace elements, suggesting low impact of the Ventanas copper smelter on spatial distribution of Zn. Indeed, only 6% of the study area exhibited Zn concentrations above the background level. In contrast, 77, 32 and 35% of the study area presented Cu, As, and Pb concentrations, respectively, above the background level. The carcinogenic risk due to exposure to As was above the threshold value of 10-04 in the population of young children (1-5 years old) on 27% of the study area. These risk values are classified as unacceptable, which require specific intervention by the Chilean government. Based on the estimated concentrations of exchangeable Cu, 10, 15, and 75% of the study area exhibited high, medium, and low phytotoxicity risk, respectively.

Which soil Cu pool governs phytotoxicity in field-collected soils contaminated by copper smelting activities in central Chile?

Several studies have attempted to predict the so-called "phytoavailable" fraction by correlating plant responses with different soil metal pools. Most of the data derived from these studies tend to be inconsistent, making interpretations difficult. Thus, the main objective of this study was to determine which soil Cu pool (free Cu2+, salt-exchangeable Cu or total Cu) controls Cu phytotoxicity in soils near a Cu smelter in central Chile. We studied the following traits of the local plant community grown spontaneously on the study site: species richness, shoot biomass, and plant cover. The site was dominated by four early plant colonizers: Eschscholzia californica Cham., Hirschfeldia incana (L.) Lagr.-Fossat, Lolium perenne L., and Vulpia bromoides (L.) Gray. We determined exchangeable soil Cu and activity of free Cu2+ in 0.1 M KNO3 extracts using soil/solution ratio of 1/2.5. The effect of total soil Cu on plant responses was not significant (p > 0.05). In our field-collected soil series, exchangeable Cu was a better indicator of soil phytotoxicity than either total soil Cu or free Cu2+ in the soil solution. We determined upper critical threshold values for Cu exposure using the three plant traits cited above. The mean values of EC10, EC25, and EC50 (effective concentration at 10%, 25%, and 50%, respectively) of exchangeable soil Cu (in µg L-1) were 255, 391, and 533, respectively. The mean EC10, EC25 and EC50 values of pCu2+ were 7.5, 6.8, and 5.9, respectively. We highlight the importance of further studies on Cu phytotoxicity using actual field-contaminated soils.

Evaluation of connected clonal growth of Solidago chilensis as an avoidance mechanism in copper-polluted soils.

Plant resistance to metals can be achieved by two strategies, tolerance and avoidance. Although metal tolerance has been broadly studied in terrestrial plants, avoidance has been less considered as a strategy to cope with soil metal pollution. Avoidance may be an effective alternative in herbaceous plants with connected clonal growth in environments having high heterogeneity in soil micro-spatial distribution of available metals and other soil conditions (i.e. organic matter). In this study, we performed a laboratory experiment on clonal growth of Solidago chilensis when exposed to copper-spiked soils (800 mg kg-1) at different depths (0, 2, 5 and 8 cm depth), with (20%) and without addition of organic matter to mimic contrasting microhabitats found at smelter hinterlands (i.e. open bare ground and microhabitats below shrubs). Results showed that plants grown in the 2 cm-depth Cu-spiked soils were able to growth and produce ramets and rhizomes. However, increased Cu uptake of plants determined phytotoxic effects and a reduction in clonal spread in the 5 cm- and 8 cm-depth Cu-spiked soils. Addition of organic matter to the Cu-spiked soil layers allowed clonal spread. Considering that ramet and rhizome production is decreased but not inhibited when copper pollution is restricted to the uppermost soil layer (2 cm depth) and that organic matter eliminated soil copper toxicity allowing normal clonal spread, connected clonal growth may be an effective avoidance mechanism of Solidago chilensis, particularly in environments with high heterogeneity in micro-spatial distribution of metals and organic matter in the soil profile and between microhabitats.