Soil properties and metal accumulation by earthworms in the Siena urban area (Italy)

This paper reports the results of a study focused on the metal (Cd, Co, Cr, Cu, Ni, Pb, Sb, U and Zn) distribution in soils and uptake and accumulation by earthworms Nicodrilus caliginosus (Savigny) from urban, peri-urban, green-urban and non-urban zones of Siena municipality (central Italy). The main goal was to define the influence of soil properties and metal soil contents on the uptake of these contaminants by earthworms. Data indicated that Cd, Cu, Pb, Sb and Zn soil contents increased in the following order: non-urban < green-urban < peri-urban < urban soils, suggesting that vehicular traffic affects the distribution of these metals. Pb and Sb were the main soil contaminants and their highest enrichments were found in urban sites where stop-and-go traffic occurs. Concentrations of these traffic-related metals in earthworms showed a distribution pattern similar to that in soil, suggesting that soil contamination influenced the uptake of Cd, Cu, Pb, Sb and Zn by N. caliginosus. There were significant positive correlations between Cd, Pb and Sb earthworm concentrations and their soil contents. The lack of correlation for Cu and Zn could be due to the physiological regulation of these elements by earthworms. Statistical analysis pointed out that the uptake and accumulation of Cd, Cu, Pb, Sb and Zn by earthworms were affected by some soil physicochemical properties such as the organic carbon and carbonate contents that are able to rule the bioavailability of metals in soils.     

Effects of earthworms on decomposition and metal availability in contaminated soil: Microcosm studies of populations with different exposure histories

Population-specific differences in the responses of earthworms to simultaneous exposure to Cu and Zn were studied in microcosm experiments. Two populations of Aporrectodea caliginosa tuberculata (Eisen) with different metal exposure histories were chosen for the studies. Microcosms were prepared containing either uncontaminated soil or soils with low or high combined Cu/Zn -concentrations (79/139 or 178/311 mg kg⁻¹ dry mass of soil, respectively). Earthworms from each population were introduced to the microcosm treatments with some microcosms serving as controls without earthworms. One series of microcosms was destructively sampled after 16 weeks incubation in a climate chamber. Survival, growth, reproduction and decomposition by earthworms in each treatment were measured. An additional microcosm series was sampled for soil and earthworm measurements at four weeks intervals to determine temporal changes in the availability of metals in the soils and their accumulation into earthworms. Cu and Zn were sequentially extracted from the soil samples of both microcosm series to estimate mobility and availability of the metals in the soil. Earthworms with long-term exposure history to metal-contaminated soil seemed to tolerate higher soil metal concentrations than earthworms without earlier exposure. Both earthworms and metals affected soil respiration (CO₂ production) and nitrogen mineralization. In addition, earthworms seemed to decrease the mobility and bioavailability of metals in the soil through their burrowing activity.     

Oral sealing using glue: a new method to distinguish between intestinal and dermal uptake of metals in earthworms

Earthworms may take up chemicals from soil and pore water, both through their skin (dermal) and by ingestion (oral). It remains unclear, however, what the relative importance of these pathways is. To assess bioavailability of pollutants in soil to earthworms, it is necessary that the contribution of each pathway is known. Lumbricus rubellus were sealed by means of medical histoacryl glue, to block ingestion of soil particles and pore water. For 6 d, these earthworms showed good survival and vitality and no soil ingestion was found. Equal metal uptake was found by sealed and unsealed earthworms exposed to an inert sand matrix continuously flushed with contaminated water. Therefore, pore water uptake via ingestion contributes little to metal accumulation. Uptake rates of Cd, Cu and Pb in sealed and unsealed earthworms exposed to two contaminated field soils were similar. Uptake and elimination kinetics of Zn were significantly lower in sealed earthworms exposed to one of the two field soils. Body concentrations of Cu and Pb could be completely attributed to the dermal route. For internal Cd and Zn concentrations, however, 0-17 and 21-30%, respectively, were derived from ingestion. It is concluded that for metals the dermal route is the uptake route of importance. The sealing method described here may be useful in a variety of earthworm nutrition and contamination-effect studies.     

Beneficial effects of earthworms and arbuscular mycorrhizal fungi on establishment of leguminous trees on Pb/Zn mine tailings

Planting trees to stabilize metalliferous mine tailings is a widely used form of land reclamation although substantial soil amendment is invariably required, both to improve the physico-chemical status of the tailings and to ameliorate toxicity prior to planting. Here, we report a glasshouse study of the combined effects of burrowing earthworms (Pheretima guillelmi) and arbuscular mycorrhizal fungi (Glomus spp., AMF) on establishment of a naturally invasive, woody, nitrogen-fixing legume, Leucaena leucocephala, on topsoil-amended Pb/Zn mine tailings. AMF provided the most effective preliminary inoculant, improving N, P and K uptake, but earthworms had more influence improving N nutrition. In most cases, the combined effects of AMF and earthworms were additive and proved to be beneficial to plant growth, plant nutrition and for protection against uptake of toxic metals. AMF influenced metal uptake more than earthworms, but together they reduced mobility of Pb and Zn in soil by as much as 25%. Some minor but significant negative interactions were also evident; for example, earthworms enhanced soil microbial activity but inhibited the beneficial effects of AMF on N₂-fixation. We argue that increased attention to ecological interactions in soil could reduce costs and improve the efficacy of restoring a vegetation cover to land impacted by contaminated spoils.     

Regenwürmer als Bioindikatoren für Schwermetallbelastungen von Böden unter Freilandbedingungen

The use of earthworms in monitoring soil pollution by heavy metals Total heavy metal contents (HNO₃-soluble) and exchangeable fractions (Ca(NO₃)₂-soluble) of Pb, Zn and Cd were measured in soils, litter layers and earthworms (dry masses) from forest, arable and pasture sites in a transect of the main wind direction and varying distances (1.5, 5.4, 11.4, 15.6 km) to a lead smelter near Bad Ems, Germany. Additionally, cast materials of Lumbricus terrestris-individuals were collected from the surface of the pasture sites. In the observed area total soil contents of Pb and Cd exceeded the C-level and total contents of Zn the B-level of the “Netherland-list”. Heavy metal contents in soils and earthworms decreased with increasing distance to the smelter. Pb showed the best correlation. Correlation between total contents of Pb and Cd in soils and earthworms were significant (rs = 0.66; p < 0.05 and rs = 0.67; p < 0.01, respectively). The uptake of heavy metals by Lumbricus rubellus, L. terrestris and Aporrectodea caliginosa was metal specific rather than species specific with factors of accumulation being <1 (Pb), 2.7–7.6 (Zn) and 19.5–85.5 (Cd). The heavy metal contents of the observed cast materials signified the different transfer of elements from soil material via earthworm individuals to the faeces. In the cast materials the amounts of Pb were high and the amounts of Zn and Cd were low. This indicates a high accumulation rate for Zn and especially for Cd in the tissues of the observed earthworm individuals. The present data support the necessity of ecotoxicological threshold levels.     

Relationship Between Extractable Metals in Acid Soils and Metals Taken Up by Tea Plants

Abstract

The accumulation of heavy metals in plants is related to concentrations andchemical fractions of the metals in soils. Understanding chemical fractions and availabilities of the metals in soils is necessary for management of the soils. In this study, the concentrations of copper (Cu), cadmium (Cd), lead (Pb), and zinc (Zn) in tea leaves were compared with the total and extractable contents of these heavy metals in 32 surface soil samples collected from different tea plantations in Zhejiang province, China. The five chemical fractions (exchangeable, carbonate‐bound, organic matter‐bound, oxides‐bound, and residual forms) of the metals in the soils were characterized. Five different extraction methods were also used to extract soil labile metals. Total heavy metal contents of the soils ranged from 17.0 to 84.0 mgCukg^?1, 0.03 to 1.09 mg Cd kg^?1, 3.43 to 31.2 mg Pb kg^?1, and 31.0 to 132.0 mg Zn kg^?1. The concentrations of exchangeable and carbonate‐bound fractions of the metals depended mainly on the pH, and those of organic matter‐bound, oxides‐bound, and residual forms of the metals were clearly controlled by their total concentrations in the soils. Extractable fractions may be preferable to total metal content as a predictor of bioconcentrations of the metals in both old and mature tea leaves. The metals in the tea leaves appeared to be mostly from the exchangeable fractions. The amount of available metals extracted by 0.01 mol L^?1 CaCl₂, NH₄OAc, and DTPA‐TEA is appropriate extractants for the prediction of metals uptake into tea plants. The results indicate that long‐term plantation of tea can cause sol acidification and elevated concentrations of bioavailable heavy metals in the soil and, hence, aggravate the risk of heavy metals to tea plants.     

Lethal critical body residues as measures of Cd,Pb, and Zn bioavailability and toxicity in the earthworm<Emphasis Type="Italic">Eisenia fetida</Emphasis>

Background. Earthworm heavy metal concentrations (critical body residues, CBRs) may be the most relevant measures of heavy metal bioavailability in soils and may be linkable to toxic effects in order to better assess soil ecotoxicity. However, as earthworms possess physiological mechanisms to secrete and/or sequester absorbed metals as toxicologically inactive forms, total earthworm metal concentrations may not relate well with toxicity. Objective  The objectives of this research were to: i) develop LD₅₀s (total earthworm metal concentration associated with 50% mortality) for Cd, Pb, and Zn; ii) evaluate the LD₅₀ for Zn in a lethal Zn-smelter soil; iii) evaluate the lethal mixture toxicity of Cd, Pb, and Zn using earthworm metal concentrations and the toxic unit (TU) approach; and iv) evaluate total and fractionated earthworm concentrations as indicators of sublethal exposure. Methods  Earthworms (Eisenia fetida (Savigny)) were exposed to artificial soils spiked with Cd, Pb, Zn, and a Cd-Pb-Zn equitoxic mixture to estimate lethal CBRs and mixture toxicity. To evaluate the CBR developed for Zn, earthworms were also exposed to Zn-contaminated field soils receiving three different remediation treatments. Earthworm metal concentrations were measured using a procedure devised to isolate toxicologically active metal burdens via separation into cytosolic and pellet fractions. Results and Discussion  Lethal CBRs inducing 50% mortality (LD₅₀, 95% CI) were calculated to be 5.72 (3.54-7.31), 3.33 (2.97-3.69), and 8.19 (4.78-11.6) mmol/kg for Cd, Pb, and Zn, respectively. Zn concentrations of dead earthworms exposed to a lethal remediated Zn-smelter soil were 3-fold above the LD₅₀ for Zn and comparable to earthworm concentrations in lethal Zn-spiked artificial soils, despite a 14-fold difference in total soil Zn concentration between lethal field and artificial soils. An evaluation of the acute mixture toxicity of Cd, Pb, and Zn in artificial soils using the Toxic Unit (TU) approach revealed an LD₅₀ (95% CI) of 0.99 (0.57-1.41) TU, indicating additive toxicity. Conclusions  Total Cd, Pb, and Zn concentrations in earthworms were good indicators of lethal metal exposure, and enabled the calculation at LD₅₀s for lethality. The Zn-LD₅₀ developed in artificial soil was applicable to earthworms exposed to remediated Zn-smelter soil, despite a 14-fold difference in total soil Zn concentrations. Mixture toxicity evaluated using LD₅₀s from each single metal test indicated additive mixture toxicity among Cd, Pb, and Zn. Fractionation of earth worm tissues into cytosolic and pellet digests yielded mixed results for detecting differences in exposure at the sublethal level Recommendation and Outlook  CBRs are useful in describing acute Cd, Pb, and Zn toxicity in earthworms, but linking sublethal exposure to total and/or fractionated residues may be more difficult. More research on detoxification, regulation, and tissue and subcellular partitioning of heavy metals in earthworms and other invertebrates is needed to establish the link between body residue and sublethal exposure and toxicity. Keywords: Bioavailability; Cd; critical body residues; earthworms; metals; Pb; soil; Zn An erratum to this article is available at .     

Comparison of biological collectors of airborne heavy metals near ferrochrome and steel works

The value of several biological monitors of the distribution of airborne Cr and Ni dust was studied in the vicinity of ferrochrome and stainless steel works. The following indigenous biomonitors were compared: the forest mosses Pleurozium schreberi and Hylocomium splendens, the epiphytic lichen Hypogymnia physodes, bark of Scots pine (Pinus silvestris), needle litter, earthworms (Dendrobaena octaedra) and moths (mainly Xylena vetusta). Results were compared and related to the heavy metal deposition estimated by the vertical snow sampling method. The representative sampling size at different pollution levels was investigated. In Cr it varied from 1 to 300 mg m^?2 a^?1. Of the biomonitors studied, the mosses showed most effective accumulation of heavy metals, their results having a clear relationship to the deposition and good replicability. The lichen gave almost as good results as the mosses. Both mosses and lichens collected heavy metals more effectively at low to moderate pollution loads than near the pollution source. The accumulation of heavy metals in bark was lower than in the mosses or lichen. The heavy metal content of earthworms indicated the degree of pollution well, but the replicability of the method was poor. The accumulation of heavy metals in moths was low.     

Tea Plantation–Induced Activation of Soil Heavy Metals

Abstract

The accumulation of heavy metals in tea leaves is of concern because of its impact on tea quality. This study characterized long‐term changes of soil properties and heavy‐metal fractions in tea gardens and their effect on the uptake of metals from soils by the plants. Soil and tea leaf samples were collected from five plantations with a history of 2–70 years in Jinghua, Zhejiang Province, southeast China. The six chemical fractions (water‐soluble, exchangeable, carbonate‐bound, organic‐matterbound, oxide‐bound, and residual forms) of cadmium (Cd), chromium (Cr), copper (Cu), iron (Fe), nickel (Ni), manganese (Mn), lead (Pb), and zinc (Zn) in the soils were characterized. Dissolved organic‐matter accumulation in the soils and effects of low‐molecular‐weight organic acids on solubility of soil heavy metals were also tested. Long‐term tea plantation use resulted in accumulation of dissolved organic matter, decrease of soil pH, and elevation of water‐soluble and exchangeable metal fractions, thereby increasing metal contents in leaves. The influence was more significant when soil pH was less than 4.4. The results indicated that both acidification and accumulation of dissolved organic matter induced by tea plantations were also important causes of increased accumulation of the metals in the tea leaves. This was particularly true for the soils polluted with low concentration of heavy metals, because availability of the metals in these soils was mainly controlled by pH and dissolved organic matter.     

Earthworms influence metal transfer from soil to snails

Most of the studies focusing on metal transfer from soil to biota ignore the possible non-trophic influence of an organism on pollutant transfer to other species. We hypothesised that an earthworm (Aporrectodea tuberculata) might modify the bioavailability of metals in soil and thus, their transfer to the snail Helix aspersa. Snails were exposed for 2 weeks to a multicontaminated field soil with or without earthworms, under controlled conditions. When exposed with earthworms, snails had higher concentrations of Cd, Cu and Zn than when they were exposed alone, while no difference was detected for Pb. For Cd only, the difference in snail bioaccumulation corresponded to an increase in its water-soluble fraction. Internal concentrations of metals in earthworms remained similar in the presence or absence of snails. Two non-exclusive possible mechanisms, including variations in bioavailable fractions and/or total accessible pools of metals, are proposed to explain how earthworms could modulate the transfer of metals from soil to snails. This work demonstrated that metal transfer from soil to one invertebrate species was influenced by another invertebrate. We conclude that the concept of intermediary species, usually used to describe interactions among species, should be extended to the interactions between biota and pollutants in non-biotic compartments.     

Interactions between earthworms, trees, soil nutrition and metal mobility in amended Pb/Zn mine tailings from Guangdong, China

This paper reports research that attempts to rehabilitate toxic Pb/Zn mine tailings, in Guangdong, China, to achieve a healthy functional soil that supports sustainable vegetation. We studied the effects of the earthworm Pheretima guillelmi on the growth of a woody legume Leucaena leucocephala on Pb/Zn mine tailings diluted with varying amounts of mineral soil in pot experiments. L. leucocephala grew successfully on tailings with a 25% (w/w) soil amendment, but P. guillelmi only survived and actively burrowed with a 50% soil amendment. The presence of earthworms improved the yield of plants by 10-30%. Whilst earthworms marginally increased available N and P in soil, they increased uptake of phosphorus (by about 10%) to above-ground plant tissues. Six-month-old plants were more sensitive than 10-month plants to metal stress. P. guillelmi increased bioavailable metal concentrations in the amended spoils, accompanied by a direct increase of metal uptake by the plants. Increased metal uptake by plants was largely due to the higher dry matter production stimulated by earthworm activity, but this increased the rate of metal uptake into plants from spoil by at least 16% and as much as 53%. These results demonstrate that we should broaden the ecological context of phytoremediation by considering the plant-soil-animal interactions that influence metal mobility.     

Short-term changes of metal availability in soil. II: The influence of earthworm activity

This study aimed to evaluate short-term earthworm-induced changes in the availability of metals applied to soil directly (metal-spiked) or via an organic matrix (sludge-amended). A laboratory experiment was performed using destructive sampling of microcosms filled with agricultural soil. A concentration gradient of industrial sludge contaminated predominantly with Cr, Cu, Ni, and Zn, and a soil freshly spiked with the same metal concentrations were applied on top of the soil columns. Individuals of Dendrobaena veneta (mimicking a realistic density of 500 earthworms per m²) were introduced in half of the replicates of each treatment. Total and 0.01 M CaCl₂ extractable metal concentrations were measured in soil after 0, 3, 6 and 12 weeks and metal concentrations in earthworms and percolates were measured after 3, 6, and 12 weeks. Earthworm activity did not affect metal availability of any treatment over time, but Ni and Cu concentrations in D. veneta were higher at the highest treatment levels. Earthworm Zn concentrations were similar in all treatments while Cr concentrations increased with increasing soil total metal content only for sludge treatments. Existing relationships of earthworm metal concentrations with total metal content in soil, taken from the literature, were not able to predict the metal levels measured in D. veneta. Results demonstrated that although over 12 weeks earthworm activity did not affect metal availability in soil, their burrowing activities did influence the metal concentrations of percolates over time.     

The role of earthworms (<Emphasis Type="Italic">Eisenia fetida</Emphasis>) in influencing bioavailability of heavy metals in soils

The effects of earthworm (Eisenia fetida) activity on soil pH, dissolved organic carbon (DOC), microbial populations, fraction distribution and bioavailability of heavy metals (Zn, Cu, Cr, Cd, Co, Ni, and Pb) in five Chinese soils were investigated using pot experiments. A three-step extraction procedure recommended by the European Community Bureau of Reference (BCR; now Standards, Measurements and Testing Programme of the European Community) was used to fractionate the metals in soils into water soluble, exchangeable and carbonate bound (B1), Fe-oxides and Mn-oxides bound (B2) and organic matter and sulfide bound (B3). After the soils were treated with earthworms, the soil pH, water-soluble metal fraction and DOC increased. A significant correlation was obtained between the increased DOC and the increased metals in the water-soluble fraction. The heavy metals in fraction B1 increased after earthworm treatments, while those in fraction B3 decreased. No significant differences were observed for heavy metals in fraction B2. The microbial populations in soil were enumerated with the dilution plate method using several media in the presence of earthworms. The microbial populations increased due to earthworm activity. The biomass of wheat shoots and roots, and the heavy metal concentrations in wheat roots and shoots, were also increased due to the earthworm activity. The present results demonstrated that earthworm activity increases the mobility and bioavailability of heavy metals in soils.     

Trace elements bioavailability to <Emphasis Type="Italic">Triticum aestivum</Emphasis> and <Emphasis Type="Italic">Dendrobaena veneta</Emphasis> in a multielement-contaminated agricultural soil amended with drinking water treatment residues

Purpose

The in situ stabilization of multielement-contaminated agricultural soils has limited effectiveness when using common single amendments. This study examined the use of drinking water treatment residues (WTR), based on (hydr)oxides of Fe, Al, or Mn, as a cost-effective solution to optimize the immobilization of metals (Cd, Pb, Zn) and As.

Materials and methods

Trace elements (TE) bioavailability was assessed under semi-controlled conditions in a pot study cultivating winter wheat (Triticum aestivum L. cv. Tiger) until maturity. An Fe-based WTR and a Mn-based WTR, applied at rates of 0.5 and 1% (m/m), were related to effects of lime marl (LM) application. Additionally, a bioassay with earthworms (Dendrobaena veneta) was conducted. Both bioassays were compared with measurements of NH₄NO₃-soluble, diffusive gradients in thin film (DGT)-available and soil solution TE concentrations, representing well-established surrogates for mimicking the bioavailable element fractions in soil.

Results and discussion

The application of the Fe-based WTR reduced As accumulation in vegetative wheat tissues (by up to 75%) and earthworms (by up to 41%), which corresponded with the findings from soil chemical analyses and improved plant growth and earthworm body weight. However, As concentrations in cereal grains were not affected, Cd or Pb accumulation by wheat was not mitigated, and Zn uptake was enhanced. By contrast, the Mn-based WTR effected the greatest reduction in Pb uptake, and lowered Cd transfer to wheat grain (by up to 25%). Neither the NH₄NO₃-soluble nor DGT-available concentrations matched with Cd and Zn accumulation in plants or earthworms, indicating interferences due to competition for binding sites according to the biotic ligand model.

Conclusions

The results obtained in this study suggest that a bioassay with key species prior to field application should be mandatory when designing in situ stabilization options. The application of WTR to an agricultural soil strongly affected TE bioavailability to plants and earthworms. Low application rates tended to improve biomass production of biota. Higher application rates involved risks (e.g., P fixation, TE inputs), and none of the amendments tested could immobilize all targeted elements.

     

Rhizosphere Influence and Seasonal Impact on Phytostabilisation of Metals—A Field Study

Field experiments were conducted to assess the influence of plant growth and amendment addition on phytostabilisation of copper (Cu), lead (Pb), manganese (Mn) and zinc (Zn) along highway soil in southwest British Columbia, Canada. The plant species tested were Lolium perenne L (perennial rye grass), Festuca rubra L. (creeping red fescue) and Poa pratensis L. (Kentucky blue grass) and the amendments, lime and phosphate. The treatment efficiencies were assessed during different seasons as a completely randomized factorial experiment in split plot design. The research tasks involved: (1) quantifying the seasonal extent of metal accumulation in soil and assessing the seasonal impact on metal speciation for different soil amendments and plant species; (2) determining seasonal accumulation differences between sampling periods in plant parts; and (3) assessing the influence of root–soil interactions on metal dynamics. The amendments decreased the exchangeable fraction and plant uptake of all four metals. The lowest mobile fractions (exchangeable and carbonate bound) were found in soils growing Festuca for Cu, Lolium for Mn and a Lolium/Poa/Festuca combination for Pb and Zn. Metal accumulation and metal dynamics in the rhizosphere soil are compared with those of the bulk soil. The final outcome was the development of a remediation strategy for all four metals involving suitable plants and amendments and incorporating seasonal and rhizosphere influences.     

Earthworms and collembola relationships: effects of predatory centipedes and humus forms

Relationships between anecic earthworms (Lumbricus terrestris and Aporrectodea giardi) and the collembolan species Heteromurus nitidus (Templeton, 1835), which is known to be attracted to earthworms, were investigated in an 8-week laboratory experiment. Our aims were (1) to assess whether earthworms influence the population dynamics of H. nitidus, and (2) to study pathways of influence and how earthworm effects are modified by humus forms and predators. Using microcosms with three defaunated humus forms, then provided with earthworms and predators, we intended to demonstrate that, amongst possible favourable effects of earthworms on springtail populations, earthworm activity may provide greater access and more pathways for springtails to explore soil and avoid predation. We expected that the effects of predators (centipedes) on the abundance of H. nitidus would increase from less (calcic mull) to more (moder) compact soil, and we hypothesized that earthworms would reduce predation pressure on H. nitidus by providing escape routes through increased macroporosity. Humus forms and earthworms only affected the population size of H. nitidus under high predation pressure, when collembolan numbers were higher in calcic mull than in moder, and were increased by the presence of earthworms. These results corroborate the hypothesis that earthworms, by increasing soil macroporosity, improve the escape routes for Collembola and thus evade predation. In moder humus earthworms increased the density of H. nitidus whether predators were present or not, so we cannot exclude that earthworms were also directly beneficial to H. nitidus. However, the hypothesis of a functional relationship mediated by soil macroporosity seems relevant since it was supported by differences observed when considering body size. When two size classes were distinguished within populations of H. nitidus (1) the positive effect of earthworms in moder was observed only on larger Collembola (>1 mm), (2) the density of the larger Collembola was decreased by predation only in moder and not in mull, (3) the effects of predators on the smaller individuals were not influenced by the presence of earthworms whatever the humus form, and was not decreased by the presence of earthworms. Nevertheless, factors other than macroporosity may operate as the presence of earthworms in acidic mull led to an unexplained decrease in the abundance of small-sized H. nitidus.     

Growth and Metal Accumulation of Mycorrhizal Sorghum Exposed to Elevated Copper and Zinc

Arbuscular mycorrhizal fungi (AMF) alter heavy metal acquisition by higher plants and may alter plant response to soil-contaminating heavy metals. Two communities comprised of Glomus intraradices and G. spurcum were investigated for their influence on copper (Cu) and zinc (Zn) resistance of Sorghum bicolor. One community was isolated from a Cu- and Zn-contaminated soil (AMF-C) and one consisted of isolates from non-contaminated soil (AMF-NC). Non-mycorrhizal (NM) sorghum plants were also included. The two community ecotypes differed in their capacity to protect sorghum from Cu and Zn toxicity and exhibited differential metal uptake into hyphae and altered heavy metal uptake by roots and translocation to plant shoots. AMF-C reduced Cu acquisition under elevated Cu conditions, but increased Cu uptake and translocation by sorghum under normal Cu conditions, patterns not exhibited by AMF-NC or NM plants. Hyphae of both fungal ecotypes accumulated high concentrations of Cu under Cu exposure. AMF-C exhibited elevated hyphal Zn accumulation and stimulated Zn uptake and translocation in sorghum plants compared to AMF-NC and NM plants. Differences in metal resistance between fungal treatments and between mycorrhizal and non-mycorrhizal plants were not related to differences in nutrient relations. The enhanced Cu resistance of sorghum and altered patterns of Cu and Zn translocation to shoots facilitated by AMF isolated from the metal-contaminated soil highlight the potential for metal-adapted AMF to increase the phytoremediation potential of mycotrophic plants on metal-contaminated environments.     

Toxicity of 2,3,7,8-TCDD and short-term bioaccumulation by earthworms (oligochaeta)

The earthworms Allolobophora catiginosa and Lumbricus rubellus were used to study the toxicity of 2,3,7,8-TCDD (dioxin) for earthworms. The earthworms were exposed to soil containing concentrations ranging from 0.05 to 5.0 μgg^?1. No worms were killed or showed any other observable toxicological effects when exposed to concentrations up to 5 μg g^?1 for 85 days in soil. The lethal threshold concentration for TCDD to earthworms falls between 5 and 10 μg g^?1 in this study. In soils containing 0.05 μg g^?1 earthworms accumulated TCDD up to 5 times the original soil concentration within 7 days. Worms were also exposed to TCDD on filter paper to study the behaviour of earthworms and the uptake of TCDD after surface contact. The earthworms did not avoid TCDD in their environment, indicating an indifference to it. No active penetration of TCDD into the body occurred where earthworms were exposed to surface concentrations. No indication was found of possible biological breakdown of TCDD on passing through the earthworm gut, although the search for metabolites was limited to the mono-, bi- and trichlorinated dioxins. There was a steady decrease (a T_{$\text{1}\text{2}$}-value of 80–400 days) in the amount of TCDD recovered from worm-worked soil compared to soil without worms.     

The significant contribution of mycorrhizal fungi and earthworms to maize protection and phytoremediation in Cd-polluted soils

Mycorrhizal fungi and earthworms can individually or interactively influence plant growth and heavy metal uptake. The influence of earthworms and arbuscular mycorrhizal (AM) fungi either alone or in combination on maize (Zea mays L.) growth and cadmium (Cd) uptake was investigated in a calcareous soil artificially spiked with Cd. Soils were contaminated with Cd (10 and 20 mg Cd kg⁻¹), inoculated or un-inoculated with the epigeic earthworm Lumbricus rubellus and two AM fungal species (Rhizophagus irregularis and Funneliformis mosseae) for two months of growth under greenhouse conditions. Generally, earthworms alone increased both shoot P uptake and biomass but decreased shoot Cd concentration and root Cd uptake. AM fungi individually often increased total maize P uptake, declined shoot Cd concentration, and consequently produced higher total biomass. However, R. irregularis enhanced shoot Cd uptake at low Cd level and root Cd uptake at high Cd level. In plants inoculated with F. mosseae species, earthworms increased shoot biomass and Cd uptake, decreased root biomass and Cd uptake at all Cd levels, and increased shoot Cd concentration at low Cd level. In plants colonized by R. irregularis species, however, earthworm addition decreased maize biomass only at high Cd level and root Cd concentration and total maize Cd uptake at both Cd levels. Earthworm activity decreased Cd transfer from the soil to maize roots at low Cd level, but this was counterbalanced in the presence of F. mosseae. Mycorrhizal symbiosis significantly reduced the transfer of Cd from roots to shoots, independence of earthworm effect. Overall, it is concluded that L. rubellus and AM fungi, in particular F. mosseae isolate, improved maize tolerance to Cd toxicity both individually and interactively by increasing plant growth and P nutrition, and restricting Cd transfer to the aboveground biomass. Consequently, the single and interactive effects of the two soil organisms might potentially be important not only in protecting maize plants against Cd toxicity, but also in Cd phytostabilization in soils polluted by this highly toxic metal.