Manganese and zinc toxicity thresholds for mountain and Geyer willow

Ecological risk thresholds for Zn in Chinese soils

The environmental risk threshold of a pollutant is a yardstick to measure soil environmental quality. The derivation of ecological risk thresholds of the heavy metal zinc (Zn) in soil environments based on up-to-date ecological risk assessments plays an important role in soil protection policy. According to regional soil classification, different representative soils with various degrees of acidity and alkalinity were selected, and a data set comprising ecotoxicities of Zn to 21 different test endpoints (plants, soil fauna, microorganisms, etc.) found in representative farmland soils of China was compiled based on new and published data to determine toxicological limits of Zn effects on endpoints. These limits were derived from fitted dose-response model parameters and indicated by EC₁₀ values (the effective concentrations of Zn that inhibit 10% of endpoint bioactivity and also represents the toxicity threshold of Zn in this study) ranging from 36 mg·kg^-1 to 682 mg·kg^-1. The hormesis effect appeared in the dose-response curve of Zn, for example, the relative Chinese cabbage growth reached more than 120% at most. Zn concentrations added in toxicity tests were also corrected for aging and leaching effects in order to more accurately reflect field conditions. The hazardous concentrations for 5% of the species affected (HC₅) were derived by the species sensitivity distribution (SSD) approach for four major types of Chinese soils: acidic (38 mg·kg^-1), neutral (106 mg·kg^-1), alkaline (217 mg·kg^-1), and alkaline calcareous soils (155 mg·kg^-1). Prediction models of ecological risk thresholds for Zn based on soil properties were generated, such as logHC₅ = 0.564 + 0.218pH + 0.097OC (R² = 0.790,p < 0.001). The predicted models based on lab test data were verified in the field, and the measured field data fell within two-fold of the prediction intervals. This work provides a scientific framework for developing soil-specific guidance on Zn toxicity thresholds.

Assessment of phytostabilization potential of two Salix L. clones based on the effects of heavy metals on the root anatomical traits

Willow species (Salix L.) are a useful tool for assessing phytostabilization of the sites polluted by heavy metals. Phytostabilization potential of two willow genotypes (Salix alba L. clone '68/53/1' and Salix nigra Marshall clone '0408') has been evaluated in a 45-day hydroponic experiment, using stem cuttings (diameter 12 to 14 mm, length 20 cm) exposed to two concentrations (10^-4 M and 10^-5 M) of individually applied Cd, Ni, and Pb. Metals were diluted in 25% Hoagland's solution, in forms of CdCl₂·H₂O, NiSO₄·6H₂O, and Pb-EDTA. The control group of cuttings was grown in 25% Hoagland's solution without heavy metals. High Cd concentrations in willow roots, 8637 mg/kg (clone '68/53/1') and 6728 mg/kg of dry weight (clone '0408'), have indicated a high phytostabilization potential. However, detailed analyses of cross-sectional area of the root cortex and the central cylinder revealed that the excess concentration of Cd led to a significant reduction of measured anatomical root's traits of clone '68/53/1' in comparison with the control samples. Excessive concentration of Ni and Pb in nutrient solution increased the values of quantitatively measured root's traits of clone '0408', implying stimulatory effects of the applied concentrations. Concentration of 10^-4 M of each metal had more negative effects on the roots' anatomical traits, notably on parenchymal and exodermal cells and vessels. Deposits of metals were observed in root tissues. Clone '0408' demonstrated an increased tolerance to heavy metals, which could potentially make this clone useful in phytostabilization.

Growth of the aquatic macrophyte Ricciocarpos natans (L.) Corda in different temperatures and in distinct concentrations of aluminum and manganese

One of the consequences of global mining is the exposure of metals into the environment, caused by the rupture of tailings dams. Excess of metals, such as aluminum (Al) and manganese (Mn) can cause serious damage to fauna and flora. The presence of these metals, associated with the temperature increase that occurs nowadays can potentially increase biochemical and metabolic rates in plant tissues and may affect growth. Therefore, the objective of this work was to evaluate the toxicity of the metals Al and Mn into the biomass' growth of the macrophyte Ricciocarpos natans, under two temperatures (25 and 27 °C). R. natans individuals (n = 10 ± 0.5 cm wide) were exposed during 30 days to Al (1.5; 2.5 and 5.0 mg L^-1) and Mn (0.7; 1.5 and 3.0 mg L^-1) at temperatures and photoperiod-controlled germination chambers. Fresh macrophyte masses were determined gravimetrically to determine the kinetic growth using a logistic model. With that, it was noticed that the presence of Al interfered negatively in the increase of the R. natans biomass, mainly in the highest concentrations and at 27 °C. Mn, on the other hand, affected the increase in biomass, mainly in the highest concentration. As a result, the growth coefficients (μ) changed, being up to 4 times lower in the Al bioassays and up to 2 times higher than the control in the Mn bioassays. However, the dry R. natans biomass individuals that were exposed to the treatments was reduced when compared to the control, except for the lower concentration of Mn. These results contribute to the understanding of the environmental changes that can occur due to metals contained in mining tailings in aquatic ecosystems and the influence of global warming on the metabolic processes of the growth of aquatic macrophytes.

Improving zinc phytoremediation characteristics in Salix pedicellata with a new acclimation approach

Some species of Salix sp. (willows) are a potential phytoremediator that can accumulate substantial contents of mineral elements and, therefore, to detoxify soils contaminated with pollutants and heavy metals such as the zinc (Zn). However, high concentrations of Zn inhibit plant growth and reduce biomass production in plants. In an attempt to overcome this inconvenience and to enhance plant tolerance to Zn toxicity, we tested a new tolerance induction approach by acclimation in two clones of Salix pedicellata, named SPK-12 and SP-K20. The approach comprises two successive phases. The first is a "tolerance induction phase" consisting of gradual exposure of plants to low concentrations of Zn sulfate (ZnSO₄) at regular intervals until reaching DI₁₀₀ (ZnSO₄ inhibitory concentration). And, the second is a "tolerance maintenance phase" to uphold the acquired tolerance to Zn toxicity. The SP-K20 clone was acclimated to DI₁₀₀ threshold over 33 days without noticeable symptoms of chlorosis or growth inhibition. Compared to controls, the SP-K20 clone was able to accumulate high concentrations of Zn, suggesting that phytoremediation abilities of S. pedicellata have been improved throughout the applied approach. Acclimated Salix plants might thus improve metal phytoextraction in heavily polluted soils without biomass growth inhibition.

Phytoremediation Efficacy of Salix discolor and S. eriocephela on Adjacent Acidic Clay and Shale Overburden on a Former Mine Site: Growth, Soil, and Foliage Traits

Plants regularly experience suboptimal environments, but this can be particularly acute on highly-disturbed mine sites. Two North American willows—Salix discolor Muhl. (DIS) and S. eriocephala Michx. (ERI)—were established in common-garden field tests on two adjacent coal mine spoil sites: one with high clay content, the other with shale overburden. The high clay content site had 44% less productivity, a pH of 3.6, 42% clay content, high water holding capacity at saturation (64%), and high soil electrical conductivity (EC) of 3.9 mS cm−1. The adjacent shale overburden site had a pH of 6.8, and after removing 56.5% stone content, a high sand content (67.2%), low water holding capacity at saturation (23%), and an EC of 0.9 mS cm−1. The acidic clay soil had significantly greater Na (20×), Ca (2×), Mg (4.4×), S (10×), C (12×) and N (2×) than the shale overburden. Foliar concentrations from the acidic clay site had significantly greater Mg (1.5×), Mn (3.3×), Fe (5.6×), Al (4.6×), and S (2×) than the shale overburden, indicating that these elements are more soluble under acidic conditions. There was no overall species difference in growth; however, survival was greater for ERI than DIS on both sites, thus overall biomass yield was greater for ERI than DIS. Foliar concentrations of ERI were significantly greater than those of DIS for N (1.3×), Ca (1.5×), Mg (1.2×), Fe (2×), Al (1.5×), and S (1.5×). There were no significant negative relationships between metal concentrations and growth or biomass yield. Both willows showed large variation among genotypes within each species in foliar concentrations, and some clones of DIS and ERI had up to 16× the Fe and Al uptake on the acidic site versus the adjacent overburden. Genetic selection among species and genotypes may be useful for reclamation activities aimed at reducing specific metal concentrations on abandoned mine sites. Results show that, despite having a greater water holding capacity, the greater acidity of the clay site resulted in greater metal mobility—in particular Na—and thus a greater EC. It appears that the decline in productivity was not due to toxicity effects from the increased mobility of metals, but rather to low pH and moisture stress from very high soil Na/EC.

Phytoremediation of cadmium and lead-polluted watersheds

Abandoned hard rock mines and the resulting acid mine drainage (AMD) are a source of vast, environmental degradation that are toxic threats to plants, animals, and humans. Cadmium (Cd) and lead (Pb) are metal contaminants often found in AMD. In our mine outwash water samples, Cd and Pb concentrations were 300 and 40 times greater than EPA Aquatic Life Use water quality standards, respectively. We tested the phytoremediation characteristics, accumulation and tolerance of Cd and Pb contamination, for annual aboveground biomass harvest of three montane willows native to the Rocky Mountains: Salix drummondiana, S. monticola, and S. planifolia. We found S. monticola best suited for Pb remediation based on greater growth and tolerance in response to the low Pb treatment compared to the high Pb treatment. Salix monticola stems also contained higher Pb concentrations in control treatment compared to S. planifolia. We found S. planifolia and S. drummondiana best suited for Cd remediation. Salix drummondiana accumulated higher concentrations of Cd in stems than both S. monticola and S. planifolia. Salix planifolia accumulated nearly 2.5 times greater concentrations of Cd in stems in control treatment than did S. drummondiana. Salix planifolia also contained more total Cd in stems than did S. monticola in Cd treatments. Based on our results, S. drummondiana and S. planifolia could aid in reduction of Cd in watersheds, and S. monticola is better suited than is S. planifolia for aboveground accumulation and tolerance of Pb pollution.

Chemiluminescent examination of abiotic oxidative stress of watercress

Watercress (Nasturtium officinale) is an aquatic plant that readily bioaccumulates heavy metals that may be found in contaminated aquatic systems. Toxic effects of contaminants on the physiological processes cause changes in oxidase enzymatic activity in watercress, which can be measured with a luminometer. The luminometer uses the reaction produced when peroxidases break down hydrogen peroxide into water and an oxygen radical. The resulting oxyradical binds to and oxidizes phenolic groups, producing a measureable luminescent reaction. Nasturtium officinale plants were exposed to 3 different concentrations of heavy metals, including lead, nickel, copper, and manganese for 24 h, 48 h, and 72 h. Aquatic exposure to the 4 heavy metals caused a significant increase in oxidative enzyme production. Fluorometric and morphometric measurements were also conducted to compare plant stress with the oxidative enzyme analyses. Fluorometric measurements performed on plants stressed by exposure to heavy metals revealed no significant decreases in photosystem II efficiency. Morphometric measurements of root length showed decreased root growth resulting from exposures to Ni, Cu, and Mn.

Revegetation of non-Acid-generating, thickened tailings with boreal trees: a greenhouse study

Tree planting presents clear advantages for mine reclamation that is aimed at achieving rapid reclamation of forested landscapes. A greenhouse study was conducted to evaluate the capacity of non-acid-generating, thickened tailings to support six boreal tree species during two growing seasons. One treatment was thickened tailings alone fertilized with inorganic N, P, and K fertilizer or chicken () manure. A thin layer of overburden topsoil was used to cover the tailings and was compared with topsoil alone, where normal tree growth was expected. Two amendments were also tested: overburden topsoil and vermicompost from food wastes. The presence of alkaline thickened tailings under the thin layer of acidic topsoil had a positive effect on tree height and root biomass (broadleaved and jack pine [ Lamb.]) by increasing topsoil pH and available Ca concentrations, which decreased Al, Zn, and Mn phytoavailability to trees; however, root contact with the tailings also increased their Cu concentrations. In thickened tailings that were mixed with topsoil, C/N ratios increased along the experiment from 21 to 40, a value where N immobilization by microorganisms occurred, as suggested by low N concentrations in tree tissues. In consequence, tree height growth (broadleaved) and biomass (conifers) were reduced. Amendment with compost raised the electrical conductivity (3.4 dS cm) to thresholds limiting broadleaved survival, while conifers showed a generalized decrease in biomass production. No trace metal contamination of the trees occurred in the mixtures, probably due to the near-neutral pH conferred by the tailings.

Hydroponic screening of willows (Salix L.) for lead tolerance and accumulation

Lead tolerance and accumulation in five willow clones were investigated using a nutrient film technique. Plants were exposed to 0, 48, 121, 169, or 241 microM Pb for 14 days. Tolerance indices (TI) and critical toxicity thresholds (EC50) were determined for five willow clones. SX61 had the highest TI values (92%) in the 48 and 121 microM Pb treatments, as well as the highest EC50 threshold values (70.5 microM for roots, 155.9 microM for aboveground tissue), indications of a high degree of tolerance to Pb. This clone also developed the highest biomass of all the clones tested. We found significant variation in willows' lead accumulation. The highest Pb content in roots (24 mg plant(-1)) and aboveground tissue (7.6 mg plant(-1)) was recorded in the 48 microM Pb treatment in SX61. Based on high biomass, TI, ECso, and Pb content in plant tissues, SX61 holds promise for phytoextraction of lead.

Zinc accumulation potential and toxicity threshold determined for a metal-accumulating Populus canescens clone in a dose-response study

The effect of increasing soil Zn concentrations on growth and Zn tissue concentrations of a metal-accumulating aspen clone was examined in a dose-response study. Plants were grown in a soil with a low native Zn content which was spiked with Zn salt solutions and subsequently aged. Plant growth was not affected by NH(4)NO(3)-extractable soil Zn concentrations up to 60 microg Zn g(-1) soil, but it was completely inhibited at extractable concentrations above 90 microg Zn g(-1) soil. From these data an effective concentration of 68.5 microg extractable Zn g(-1) soil was calculated at which plant growth was reduced by 50%. The obtained information on toxicity threshold concentrations, and the relation between plant Zn accumulation and extractable soil Zn concentrations may be used to assess the suitability of the investigated Populus canescens clone for various phytoremediation strategies. The potential risk of metal transfer into food webs associated with P. canescens stands on Zn-polluted sites may also be estimated.

Establishment and growth of two willow species in a riparian zone impacted by mine tailings

A field study was initiated to determine survival, growth characteristics, and metal uptake of two montane riparian willow species, Geyer (Salix geyeriana Andersson) and mountain (S. monticola Bebb) willow, grown in amended fluvial mine tailing deposits. Revegetation was done with staked and previously rooted cuttings to determine if planting method had an effect on successful establishment of willows. A second planting was done the following growing season which tested cuttings of different ages. The addition of lime increased the soil pH from 5.0 to 6.5 and effectively reduced bioavailability of most heavy metals below phytotoxic levels. However, both willow species, regardless of planting method, concentrated Cd, Mn, Pb, and Zn in their leaf tissue above levels considered toxic to agronomic plants. Over the course of four growing seasons, prerooted mountain willows had a consistently higher survival rate compared to staked willows. At the end of the fourth growing season, mountain willow had a higher survival rate and produced greater aboveground growth for both planting methods, irrespective of year planted, compared with Geyer willow. Based on growth characteristics, the use of prerooted mountain willows would be recommended for successful revegetation of amended fluvial mine tailing deposits in riparian zones. However, because of the high Cd uptake into aboveground tissues, care should be taken in restoration efforts where wildlife and domestic livestock are likely to browse on the willows.