Increased lead availability and enzyme activities in root-adhering soil of Lantana camara during phytoextraction in the presence of earthworms

Ecologically different earthworm species are the driving force of microbial hotspots influencing Pb uptake by the leafy vegetable Brassica campestris

Food chain contamination by soil lead (Pb), beginning with Pb uptake by leafy vegetables, is a threat to food safety and poses a potential risk to human health. This study highlights the importance of two ecologically different earthworm species (the anecic species Amynthas aspergillum and the epigeic species Eisenia fetida) as the driving force of microbial hotspots to enhance Pb accumulation in the leafy vegetable Brassica campestris at different Pb contamination levels (0, 100, 500, and 1,000 mg·kg-1). The fingerprints of phospholipid fatty acids (PLFAs) were employed to reveal the microbial mechanism of Pb accumulation involving earthworm-plant interaction, as PLFAs provide a general profile of soil microbial biomass and community structure. The results showed that Gram-positive (G+) bacteria dominated the microbial community. At 0 mg·kg-1 Pb, the presence of earthworms significantly reduced the total PLFAs. The maximum total of PLFAs was found at 100 mg·kg-1 Pb with E. fetida inoculation. A significant shift in the bacterial community was observed in the treatments with E. fetida inoculation at 500 and 1,000 mg·kg-1 Pb, where the G+/G- bacteria ratio was significantly decreased compared to no earthworm inoculation. Principal component analysis (PCA) showed that E. fetida had a greater effect on soil microbial hotspots than A. aspergillum, thus having a greater effect on the Pb uptake by B. campestris. Redundancy analysis (RDA) showed that soil microbial biomass and structure explained 43.0% (R2 = 0.53) of the total variation in Pb uptake by B. campestris, compared to 9.51% of microbial activity. G- bacteria explained 23.2% of the total variation in the Pb uptake by B. campestris, significantly higher than the other microbes. The Mantel test showed that microbial properties significantly influenced Pb uptake by B. campestris under the driving force of earthworms. E. fetida inoculation was favorable for the G- bacterial community, whereas A. aspergillum inoculation was favorable for the fungal community. Both microbial communities facilitated the entry of Pb into the vegetable food chain system. This study delivers novel evidence and meaningful insights into how earthworms prime the microbial mechanism of Pb uptake by leafy vegetables by influencing soil microbial biomass and community composition. Comprehensive metagenomics analysis can be employed in future studies to identify the microbial strains promoting Pb migration and develop effective strategies to mitigate Pb contamination in food chains.

Recent Advances in Microbial-Assisted Remediation of Cadmium-Contaminated Soil

Soil contamination with cadmium (Cd) is a severe concern for the developing world due to its non-biodegradability and significant potential to damage the ecosystem and associated services. Industries such as mining, manufacturing, building, etc., rapidly produce a substantial amount of Cd, posing environmental risks. Cd toxicity in crop plants decreases nutrient and water uptake and translocation, increases oxidative damage, interferes with plant metabolism and inhibits plant morphology and physiology. However, various conventional physicochemical approaches are available to remove Cd from the soil, including chemical reduction, immobilization, stabilization and electro-remediation. Nevertheless, these processes are costly and unfriendly to the environment because they require much energy, skilled labor and hazardous chemicals. In contrasting, contaminated soils can be restored by using bioremediation techniques, which use plants alone and in association with different beneficial microbes as cutting-edge approaches. This review covers the bioremediation of soils contaminated with Cd in various new ways. The bioremediation capability of bacteria and fungi alone and in combination with plants are studied and analyzed. Microbes, including bacteria, fungi and algae, are reported to have a high tolerance for metals, having a 98% bioremediation capability. The internal structure of microorganisms, their cell surface characteristics and the surrounding environmental circumstances are all discussed concerning how microbes detoxify metals. Moreover, issues affecting the effectiveness of bioremediation are explored, along with potential difficulties, solutions and prospects.

Earthworm-mediated nitrification and gut digestive processes facilitate the remobilization of biochar-immobilized heavy metals

Earthworms and biochar tend to have opposite effects on heavy metal bioavailability in soil. However, the influence and controlling process of earthworms on the immobilisation effect of biochar remain poorly understood. Through the co-cultivation of earthworms with rice-husk biochar and sludge biochar in heavy metal-contaminated soil and desorption experiments involving simulated earthworm gut, we explored the factors that earthworms influence the heavy metal immobilisation ability of biochar. Our results showed that rice-husk biochar and sludge biochar effectively immobilized heavy metals in soil, whereas earthworm activity mobilised heavy metals in biochar-treated soil, which weakens the immobilisation of biochar. The soil pH reduction effect of earthworms by increasing the abundance of soil ammonia-oxidising bacteria to promote soil nitrification is an important mechanism through which earthworms mobilise heavy metals; however, this process did not occur within 10 days of incubation. Nitrification inhibitors effectively inhibit the mobilisation of heavy metals in soil by earthworms. In addition, the bioavailability of heavy metals in earthworm casts was significantly higher than those in the surrounding soil and earthworm-free soil. Moreover, simulated earthworm gut fluid promoted the re-release of heavy metals from the soil and biochar particles. These results suggest that the gut digestion of earthworms is another important mechanism by which earthworms mobilise soil heavy metals and weaken the immobilisation of biochar. Therefore, earthworms weakened the immobilisation effect of biochar mainly by promoting nitrification to reduce soil pH and through gut digestion.

How a functional soil animal-earthworm affect arbuscular mycorrhizae-assisted phytoremediation in metals contaminated soil?

Phytoremediation is a promising and sustainable technology to remediate the risk of heavy metals (HMs) contaminated soils, however, this way is limited to some factors contributing to slow plant growth and low remediation efficiency. As soil beneficial microbe, arbuscular mycorrhizal fungi (AMF) assisted phytoremediation is an environment-friendly and high-efficiency bioremediation technology. However, AMF-symbiotic formation and their functional expression responsible for HMs-polluted remediation are significantly influenced by edaphic fauna. Earthworms as common soil fauna, may have various effects on formation of AMF symbiosis, and exhibit synergy with AMF for the combined remediation of HMs-contaminated soils. For now, AMF-assisted phytoremediation incorporating earthworm coexistence is scarcely reported. Therefore, the main focus of this review is to discuss the AMF effects under earthworm coexistence. Effects of AMF-symbiotic formation influenced by earthworms are fully reviewed. Moreover, underlying mechanisms and synergy of the two in HMs remediation, soil improvement, and plant growth were comprehensively elucidated. Phenomenon of "functional synergism" between earthworms and AMF may be a significant mechanism for HMs phytoremediation. Finally, this review analyses shortcomings and prescriptions in the practical application of the technology and provides new insights into AMF- earthworms synergistic remediation of HMs-contaminated soils.

Diversity and composition of soil bacteria between abandoned and selective-farming farmlands in an antimony mining area

Background and aims

Land abandonment and selective farming are two common management methods to restore the soil conditions of low-pollution farmland in mining areas. The soil bacterial community plays an important role in farmland soil restoration; however, few studies have compared the composition and diversity of soil bacteria between the abandoned farmlands (AFS) and selective-farming farmlands (FFS). Here, the effects of AFS and FFS on soil properties and bacterial diversity were evaluated in an antimony (Sb) mining area in southern China. This study aimed to identify effective land management methods in terms of positive or negative changes in soil environment and bacterial diversity.

Methods

16S rRNA high-throughput sequencing was used to compare the diversity and composition of soil bacteria between AFS and FFS in the Xikuangshan (the largest Sb mine in the world).

Results

Compared to AFS, FFS had higher Sb concentration and nutritional properties (e.g., available N, P, and K) and lower Zn concentration (p < 0.05). The bacterial alpha diversity including Chao1 index, Simpson index, Shannon index and Pielou₋e index in FFS was higher than AFS (p < 0.05). At the phylum level, FFS had higher relative abundances of Chloroflexi, Acidobacteria, Gemmatimonadetes, and Rokubacteria, and lower relative abundances of Firmicutes, Actinobacteria, and Bacteroidetes. At the genus level, FFS had higher relative abundances of Acidothermus, Haliangium, and Rokubacteriales, and lower relative abundances of Bacillus, Rhodococcus, Sphingomonas, and 67-14. Redundancy analysis indicated that soil heavy metal content and soil fertility were closely correlated with the soil bacterial community. Altogether, selective farming of low-pollution farmland in the mining area can improve soil properties and soil bacterial diversity.

Effects of earthworms and arbuscular mycorrhizal fungi on improvement of fertility and microbial communities of soils heavily polluted by cadmium

Soil bacterial community (SBC) and fertility are pivotal for the evaluation of phytoremediation performance. Although affected by earthworms (E) and arbuscular mycorrhizal fungi (AMF), little is known about the impacts of the E-AMF interaction on the variation of SBC and fertility in cadmium (Cd)-spiked soil. We elucidated these impacts in rhizosphere soil of Solanum nigrum L. Loss of nutrient availability, and SBC diversity was observed in Cd-polluted soil. AMF increased available phosphorous (AP), whereas E increased available potassium (AK). In soils with 60 and 120 mg/kg Cd, the contents of AK, AP, and soil organic matter (SOM) increased by 7.0-19.7%, 23.7-25.5%, and 11.5-17.4%, respectively; and the residual Cd after remediation decreased by 7.9-8.5% in soils treated with EAM compared to untreated soil. EAM-treated soil had higher alpha diversity estimators compared to uninoculated soil. The predominant bacterial phyla were Proteobacteria and Bacteroidetes, accounting for 72.5-84.0%. Redundancy analysis showed that total carbon (TC), SOM, pH, and C/N ratio were key factors determining SBC at the phylum level, explaining 26.9, 24.1, 15.1, and 14.8% of the total variance, respectively. These results suggested that EAM affected SBC composition by altering SOM, TC, and C/N ratio. The E-AMF cooperation ameliorates soil nutrients, SBC diversity, and composition, facilitating phytoextraction processes.

Earthworm and arbuscular mycorrhiza interactions: Strategies to motivate antioxidant responses and improve soil functionality

Earthworms and arbuscular mycorrhizal fungi (AMF) act synergistically in the rhizosphere and may increase host plant tolerance to Cd. However, mechanisms by which earthworm-AMF-plant partnerships counteract Cd phytotoxicity are unknown. Thus, we evaluated individual and interactive effects of these soil organisms on photosynthesis, antioxidant capacity, and essential nutrient uptake by Solanum nigrum, as well as on soil quality following Cd exposure (0-120 mg kg^-1). Decreases in biomass and photosynthetic activity, as well as nutrient imbalances were observed in Cd-stressed plants; however, the addition of AMF and earthworms reversed these effects. Cd exposure increased superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) activities, whereas inoculation with Rhizophagus intraradices decreased those. Soil enzymatic activity decreased by 15-60% with increasing Cd concentrations. However, Cd-mediated toxicity was partially reversed by soil organisms. Earthworms and AMF ameliorated soil quality based on soil enzyme activity. At 120 mg kg^-1 Cd, the urease, catalase, and acid phosphatase activities were 1.6-, 1.4-, and 1.2-fold higher, respectively, in soils co-incubated with earthworms and AMF than in uninoculated soil. Cd inhibited shoot Fe and Ca phytoaccumulation, whereas AMF and earthworms normalized the status of essential elements in plants. Cd detoxification by earthworm-AMF-S. nigrum symbiosis was manifested by increases in plant biomass accumulation (22-117%), chlorophyll content (17-63%), antioxidant levels (SOD 10-18%, POD 9-25%, total polyphenols 17-22%, flavonoids 15-29%, and glutathione 7-61%). It also ameliorated the photosynthetic capacity, and macro- and micronutrient statuses of plants; markedly reduced the levels of malondialdehyde (20-27%), superoxide anion (29-36%), and hydrogen peroxide (19-30%); and upregulated the transcription level of FeSOD. Thus, the combined action of earthworms and AMF feasibly enhances metal tolerance of hyperaccumulating plants and improves the quality of polluted soil.

The dual beneficial effects of vermiremediation: Reducing soil bioavailability of cadmium (Cd) and improving soil fertility by earthworm (Eisenia fetida) modified by seasonality

This study aimed to assess earthworm's capability of reducing the bioavailability of cadmium (Cd) in soil and increasing soil fertility with the modification of seasonal variations of ambient temperatures on the efficacy of vermiremediation. Earthworms were exposed in soil fortified with 0, 5, 10, and 20 mg Cd kg^-1, for 7, 14 and 21 days in winter and spring. The bioavailability of Cd in soil, which is represented in the form of diethylenetriaminepentaacetic acid-extractable fraction (DTPA-Cd), were significantly reduced, ranging from 7.9 to 18.3% in winter and 8.8 to 20.8% in spring. Meanwhile, we found earthworm activities could significantly improve the soil fertility as the results of increasing the availability of nitrogen, phosphorous, and potassium in soil, a prominent advantage of vermiremediation in heavy metal-contaminated soil. Although seasonality could increase Cd toxicity in earthworms, higher ambient temperature in spring season also promoted the reduction of Cd bioavailability and the increase of soil fertility, due to significant increase of microbial populations. In conclusion, we reported the dual beneficial effects of vermiremediation in reducing bioavailability of Cd in soil and simultaneously improving soil fertility in which both outcomes were modified by seasonality.

Efficiency of cow dung based vermi-compost on seed germination and plant growth parameters of Tagetes erectus (Marigold)

Vermi-composting is an environmental friendly and economic process to decompose organic waste. The objective of this study was to produce vermi-compost using E isenia fetida and to investigate the impact of vermi-compost (VC) and organic manure (cow dung) on seed germination, seedlings, and growth parameters of Tagetes erecta. Physio-chemical parameters of vermi-compost and organic manure were recorded. A potting experiment was designed, germination medium containing soil, sand, and various concentrations of vermi-composts. The composition of germinating media was: TO (Sand + Soil), TCC (Sand + Soil + Cow dung), 10% VC (Sand + Soil + 0.1 kg VC), 15% VC (Sand + Soil + 0.15 kg VC), 20% VC (Sand + Soil + 0.2 kg VC), 25% VC (Sand + Soil + 0.25 kg VC), 30% VC (Sand + Soil + 0.3 kg VC), and 35% VC (Sand + Soil + 0.35 kg VC). Seed germination, seedling, vegetative plant growth, and flowering parameters were evaluated in different germinating media. Pre and post-physio-chemical parameters of germination media were also recorded to check their stability and quality. Results showed that 20% VC was effective for the early initiation of seed germination (2.0 ± 0.0 days) and all growth parameters of marigold seedlings. The germination percentage at 20% VC was recorded as 87.5 ± 1.40 %. The best vegetative plant growth and flowering parameters of marigold plants were observed with 35% VC after transplantation. Findings showed that vermi-compost is the best-suited germination and growing media, which not only improved the soil health but also promoted seed germination and plant growth. Our study undoubtedly indicates that vermi-compost is a more encouraging and advantageous bio-fertilizer and can be used as a powerful and effective for immediate marigold production.

Role of biochar and Eisenia fetida on metal bioavailability and biochar effects on earthworm fitness

Biochar has gained extensive attention due to its remediation role in soil pollution. However, its hazardous effects on the soil fauna in contaminated soil and its remediation efficiency affected by soil organisms are still obscure. The individual and combined effects of biochar and earthworms (Eisenia fetida) on soil properties, metal bioavailability, and earthworm fitness were investigated in historically heavy metal (HM)-contaminated soil. The results showed that biochar increased the soil pH by 0.31, decreased DTPA-extractable Cd, Cu, Zn and Pb contents by 11.9%, 14.3%, 5.27% and 23.8%, respectively, and immobilized the HMs from a bioavailable fraction to a residual fraction. The co-incubation of biochar and E. fetida decreased soil pH by 0.11 and increased DTPA-extractable Cu, Zn, and Pb contents by 3.75%, 20.9% and 4.43%, respectively. The results of the correlation analysis showed that soil pH was significantly negatively correlated with HM bioavailability, and it was a potential factor contributed to this opposite effect. Furthermore, biochar decreased the biomass growth of E. fetida and inhibited the activities of SOD, CAT and GSH in E. fetida by 31.1%, 51.3% and 29.6% after 28 days of incubation. Overall, biochar and E. fetida showed the opposite effects on the soil remediation, and biochar also led to a negative effect on earthworms. These findings provided insights on verifying the actual remediation effects of biochar and its ecological risk in situ soil remediation.

Integration of earthworms and arbuscular mycorrhizal fungi into phytoremediation of cadmium-contaminated soil by Solanum nigrum L

Arbuscular mycorrhizal fungi (AMF) and earthworms independently enhance plant growth, heavy metal (HM) tolerance, and HM uptake, thus they are potential key factors in phytoremediation. However, few studies have investigated their interactions in HM phytoextraction by hyperaccumulators. This study highlights the independent and interactive effects of earthworms and AMF on Solanum nigrum. Plants inoculated with either AMF or earthworms exhibited ameliorated growth via enhancement of productivity, metal tolerance, and phosphorus (P) acquisition. Co-inoculation with both had more pronounced effects on plant biomass and P acquisition in shoots, but not in roots, and in Cd-polluted soils it significantly promoted (P < 0.05) shoot biomass (20.7-134.6 %) and P content (20.4-112.0 %). AMF and earthworms increased Cd accumulation in plant tissues, but only AMF affected Cd partitioning between shoots and roots. Although AMF decreased root-to-shoot translocation of Cd at high Cd levels, this was counterbalanced by earthworms. Both AMF and its combination with earthworms enhanced Cd phytoavailability by altering Cd chemical fractions and decreasing pH. Co-inoculation increased Cd removal amounts up to 149.3 % in 120 mg kg^-1 Cd-spiked soils. Interactions between the two organisms were synergistic in Cd phytoextraction. Thus, earthworm-AMF-plant symbiosis potentially plays an essential role in phytoremediation of HM-polluted soils.

Toxicity comparison of three imidazolium bromide ionic liquids to soil microorganisms

Ionic liquids (ILs) are extensively used in several chemistry fields. And research about the effects of ILs on soil microbes is needed. In this study, brown soil was exposed to 1-butyl-3-methylimidazolium bromide ([C₄mim]Br), 1-hexyl-3-methylimidazolium bromide ([C₆mim]Br) and 1-decyl-3-methylimidazolium bromide ([C₁₀mim]Br). The toxicities of the three ILs are evaluated by measuring the soil culturable microbial number, enzyme activity, microbial diversity and, abundance of the ammonia monooxygenase (amoA) genes of ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA). Results showed that all tested ILs caused a decrease in culturable microbial abundance. Tested ILs exposure inhibit urease activity and promote acid phosphatase and β-glucosidase activities. Tested ILs reduced soil microbial diversity and the abundances of AOB-amoA and AOA-amoA genes significantly. After a comparison of the integrated biomarker response (IBR) index, the toxicities of tested ILs to soil microorganisms were as follows: [C₁₀mim]Br > [C₆mim]Br > [C₄mim]Br. Among all collected biomarkers, the abundance of the AOA-amoA gene was the most sensitive one and was easily affected after ILs exposure.

Effects of trichloroethylene stress on the microbiological characteristics of Mollisol

Trichloroethylene (TCE), one of 129 kinds of priority pollutants, is the most common halogenated organic pollutant in the environment. To explore the changes in soil physicochemical properties and biological activities then clarify the effects of these factors on bacterial, fungal and actinomycetes communities in Mollisol under TCE stress is the significance of our research. The results indicated that when TCE concentration was greater than 10 mg kg^-1, soil quality declined and soil decomposition of organic matter and cycling of mineral nutrients were inhibited through an effect on soil microbial biomass. Operational taxonomic units (OTUs) richness of the bacteria in Mollisol was altered by TCE contamination. The S_Chao1 and H_Shannon indices of bacterial communities in Mollisol decreased when 40 mg kg^-1 TCE was applied. Meanwhile, the OTU richness of fungi in Mollisol was altered by TCE contamination. The H_Shannon indices of the fungal communities in Mollisol were inhibited by higher TCE concentrations (20 and 40 mg kg^-1 TCE). TCE altered the content of some bacteria, fungi and actinomycetes involved in soil carbon and nitrogen cycling and metabolism, such as Acidobacteria, Proteobacteria, Planctomycetes, Chytridiomycota, Streptomycetales, Pseudonocardiales, Propionibacteriales and Rhizobiales, and thus influenced nutrient cycling and the process of energy metabolism in Mollisol. In addition, redundancy analysis (RDA) results indicated that physicochemical properties and biological activities under TCE contamination significantly affected soil microbial community composition thus confirming that TCE interfered with the carbon and nitrogen cycling and metabolism of soil microorganisms. The results of this study are of great importance for revealing the effects of TCE stress on the microbiological characteristics of Mollisol, and also provide more useful information for determining the potential ecological risk of organic pollutants in Mollisol.

Enhancement of phytoextraction of Pb by compounded activation agent derived from fruit residue

Chelate-assisted phytoextraction is an attractive strategy to remove toxic metals from soil. However, there is lack of an effective and sustainable chelating agent. In this study, 11 kinds of fruit residue were extracted and selected to combine with N, N-bis (carboxymethyl) glutamic acid (GLDA) (0.7%) and tea saponin (4%) for the compounded activation agent (CAA), and its enhancement on Pb phytoextraction by Sedum alfredii was further evaluated by pot experiment. Among 11 fruit residue extracts, lemon residue showed the highest ability (34.7%) to extract Pb from soil. Through combining with GLDA (0.7%) and tea saponin (4%) at the optimal volume ratio of 15:2.5:2.5, the CAA removed Pb most effectively (57.1%) from soil and increased the solubility of three Pb mineral (PbS, PbCO₃ and PbSO₄) by 8.7-56.4 times. In pot experiment, the addition of high dosage (15 mL) CAA increased the biomass of S. alfredii by 52% and doubled the Pb accumulation. In addition, CAA-assisted phytoextraction also increased both water-soluble and acid-soluble Pb in soil, while reduced the proportion of the immobile Pb (oxidizable and residual). Generally, the compounded activation agent derived from lemon residue could be considered as-a promising enhancer for Pb phytoextraction.

Influence of earthworm bioturbation on metals phytoavailability and human gastric bioaccessibility

At the global scale, urban agriculture is increasingly developing in cities due to demographic growth and sustainable food concerns. But, urban soils are frequently polluted with metals. In urban gardens, organic matter is also commonly added both to valorize organic household waste and to promote biophysicochemical fertility. As earthworms promote the decomposition and the recycling of soil organic matter, they can also influence the biogeochemical cycle of metals in urban polluted soils. In order to produce safe vegetables in urban areas, it is crucial to highlight the mechanisms involved in complex soil-earthworm-plant ecosystems. An experiment was set up to examine these relationships using lettuce cultivated in controlled conditions with RHIZOtest® devices. Thanks to the RHIZOtest® devices, metal transfer and bioaccessibility were for the first time compared for urban polluted soil without (1-urban soil polluted with Pb, Cd, Cu, and Zn: essential or toxic metals currently found in environment, SNB) and with bioturbation (2-this metal-polluted soil subjected to earthworm bioturbation, SB) and earthworm casts (3-earthworm casts produced in this polluted soil and naturally enriched in organic matter and microorganisms, T). Metal concentration, phytoavailability, and human gastric bioaccessibility were determined in the different samples. Results showed that earthworm bioturbation increased the phytoavailability of all the metals. For the experimental condition SB, the phytoavailability of metals was increased up to 75% compared to SNB. In addition, surprisingly, metal phytoavailability was always superior in SB compared to earthworm casts (T). Moreover, earthworms led to an increase in Zn gastric bioaccessibility up to 10% in the soils in the same way as for phytoavailability, meaning Zn bioaccessibility in SB > T > SNB, whereas it remained unchanged in the lettuces. These data are important to promote sustainable agriculture activities in urban areas; actually, databases concerning different experimental conditions are needed to develop decision support tools.

A newly discovered Cd-hyperaccumulator Lantana camara L

The identification of hyperaccumulators is a key step for the phytoextraction of contaminated soils. However, few cadmium (Cd) hyperaccumulators have been identified in the plant kingdom. In our previous field investigations, Lantana camara L. plants exhibited some traits of hyperaccumulators. To confirm whether this species is a Cd hyperaccumulator, laboratory dose-gradient experiments and field sample analysis experiments were first designed and implemented in an integrated manner. The results showed that lantana plants did not exhibit any visible damage or marked reduction in shoot biomass when grown in Cd-contaminated soil with less than 100 mg kg^-1 Cd. Moreover, the lantana plants exhibited high Cd tolerance with effective coordination of photosynthesis and rapid reactive oxygen species scavenging. Most importantly, the bioaccumulation factors (BFs) and translocation factors (TFs) were greater than 1.0 in all the Cd treatments, while the Cd concentrations in the shoots were all greater than those in the roots and were also greater than 100 mg kg^-1, the threshold value for a Cd hyperaccumulator. Our data provide comprehensive evidence that lantana plants have the typical characteristics of a Cd hyperaccumulator and thus can be regarded as potential Cd-hyperaccumulating plants for the restoration of Cd-polluted soils.

In situ phytoremediation of copper and cadmium in a co-contaminated soil and its biological and physical effects

Phytoremediation is a potential cost-effective technology for remediating heavy metal-contaminated soils.

Cadmium phytoextraction potential of king grass (Pennisetum sinese Roxb.) and responses of rhizosphere bacterial communities to a cadmium pollution gradient

Screening for tolerant and high biomass producing plants is important for phytoextraction efforts in remediating agricultural soils contaminated by heavy metals. We carried out a greenhouse experiment involving a soil cadmium (Cd) concentration gradient (0.1, 0.5, 1, 2, 4, and 8 mg kg^-1) to assess growth and phytoextraction capacity of king grass (Pennisetum sinese Roxb.) in soils contaminated by Cd and to explore changes in diversity and structure of rhizosphere soil bacterial communities in response to long-term Cd pollution. A significant positive relationship was observed between Cd concentrations in P. sinese stems, leaves, and roots and soil Cd concentration. The highest Cd concentrations in shoots and roots were 28.87 and 34.01 mg kg^-1, respectively, at 8 mg kg^-1of soil Cd supply. Total extraction amounts of Cd in P. sinese were 0.22-1.86 mg plant^-1 corresponding to treatment with 0.5-8 mg kg^-1 Cd. Most of the Cd was stored in shoots, and the largest accumulation was 1.56 mg plant^-1 with 54.02 g dry shoot weight. After phytoextraction, changes in rhizobacterial community composition were found with different levels of Cd application, whereas there were no clear trends in diversity and richness. Results of this study show the feasibility of P. sinese in accumulating Cd and provide support for its application in remediation of soil moderately contaminated by Cd.

Arsenic characteristics in the terrestrial environment in the vicinity of the Shimen realgar mine, China

In this study, multiple types of samples, including soils, plants, litter and soil invertebrates, were collected from a former arsenic (As) mine in China. The total As concentrations in the soils, earthworms, litter and the aboveground portions of grass from the contaminated area followed the decreasing order of 83-2224 mg/kg, 31-430 mg/kg, 1-62 mg/kg and 2-23 mg/kg, respectively. X-ray absorption near-edge structure (XANES) analysis revealed that the predominant form of As in the soils was arsenate (As(V)), while no arsenite (As(III)) was detected. In the grass and litter of the native plant community, inorganic As species (As(V) and As(III)) were the main species, while minor amounts of DMA, MMA, AsC, and an unknown As species were also detected in the extracts analyzed with high-performance liquid chromatography inductively coupled plasma mass spectrometry (HPLC-ICP-MS). The As speciation and As concentrations varied with the plant species, and very high As levels (197-584 mg/kg) and proportions of inorganic As (>99%) were found in two As-hyperaccumulating ferns, Pteris vittata and Pteris cretica. The major As species extracted from earthworms were inorganic, with proportions of 51-53% As(III) and 38-48% As(V). AsB was the only organic species present in the earthworm samples, although at low proportions (<8.99%). The internal bioconversion of other As species is hypothesized to contribute greatly to the formation and accumulation of AsB in earthworms, although the direct external absorption of organic As from soils might be another source. This study sheds light on the potential sources of complex organoarsenicals, such as AsB, in terrestrial organisms.

Improvement of the phytoremediation efficiency of Neyraudia reynaudiana for lead-zinc mine-contaminated soil under the interactive effect of earthworms and EDTA

Slow plant growth, low biomass, and low bioavailability of heavy metals in soil are important factors that limit remediation efficiencies. This study adopted a pot cultivation method to evaluate the phytoremediation efficiency of Neyraudia reynaudiana, planted in contaminated soil from a lead-zinc mining area. The soil was inoculated with earthworms (Eisenia fetida), and mixed with the chelating agent ethylenediaminetetraacetic acid (EDTA) one month after planting. The addition of earthworms significantly increased the aboveground biomass of N. reynaudiana and activated heavy metals in the soil, thus facilitating heavy metal uptake by N. reynaudiana. The addition of EDTA significantly increased the incorporation and transport of heavy metals, reduced the uptake of heavy metals by the plant cell wall, and increased the proportions of cellular soluble constituents. Especially with regard to lead, inoculation with earthworms and EDTA application significantly promoted the accumulation efficiency of N. reynaudiana, increasing it 7.1-16.9-fold compared to the control treatment without earthworms and EDTA, and 1.5-2.3-fold compared to a treatment that only used EDTA.

Effects of Short-Term Set-Aside Management Practices on Soil Microorganism and Enzyme Activity in China

Set-aside farmland can effectively improve the self-rehabilitation of arable soil. Long-term set-asides however cannot satisfy provisionment, therefore the use of short-term set-asides to restore cultivated soil is a better option. Few studies have compared short-term set-aside patterns, and the effects of set-asides on soil microbial community and enzyme enzymes. We analyzed the bacterial structure, microbial biomass carbon/nitrogen and enzyme activity of farmland soil under different set-aside regimes in the Yellow River Delta of China. Bacterial alpha diversity was relatively lower under only irrigation, and farmyard manure applications showed clear advantages. Set-asides should consider their influence on soil organic carbon and nitrogen, which were correlated with microbial community structure. Nitrospira (0.47–1.67%), Acidobacteria Gp6 (8.26–15.91%) and unclassified Burkholderiales (1.50–2.81%) were significantly altered (p < 0.01). Based on functions of these genera, some set-aside patterns led to a relative balance in nitrogen and carbon turnover. Partial treatments showed a deficiency in organic matter. In addition, farmyard manure may lead to the increased consumption of organic matter, with the exception of native plants set-asides. Conventional farming (control group) displayed a significant enzyme activity advantage. Set-aside management practices guided soil microbial communities to different states. Integrated soil microbiota and the content of carbon and nitrogen, native plants with farmyard manure showed an equilibrium state relatively, which would be helpful to improve land quality in the short-term.

Important Issues in Ecotoxicological Investigations Using Earthworms

The importance and beneficial effects of earthworms on soil structure and quality is well-established. In addition, earthworms have proved to be important model organisms for investigation of pollutant effects on soil ecosystems. In ecotoxicological investigations effects of various pollutants on earthworms were assessed. But some important issues regarding the effects of pollutants on earthworms still need to be comprehensively addressed. In this review several issues relevant to soil ecotoxicological investigations using earthworms are emphasized and guidelines that should be adopted in ecotoxicological investigations using earthworms are given. The inclusion of these guidelines in ecotoxicological studies will contribute to the better quantification of impacts of pollutants and will allow more accurate prediction of the real field effects of pollutants to earthworms.

Improvement of the soil nitrogen content and maize growth by earthworms and arbuscular mycorrhizal fungi in soils polluted by oxytetracycline

Interactions between earthworms (Eisenia fetida) and arbuscular mycorrhizal fungi (Rhizophagus intraradices, AM fungi) have been suggested to improve the maize nitrogen (N) content and biomass and were studied in soils polluted by oxytetracycline (OTC). Maize was planted and amended with AMF and/or earthworms (E) in the soil with low (1mgkg(-1) soil DM) or high (100mgkg(-1) soil DM) amounts of OTC pollution in comparison to soil without OTC. The root colonization, shoot and root biomass, shoot and root N contents, soil nitrogen forms, ammonia-oxidizing bacteria (AOB) and archaea (AOA) were measured at harvest. The results indicated that OTC decreased maize shoot and root biomass (p<0.05) by mediating the soil urease activity and AOB and AOA abundance, which resulted in a lower N availability for maize roots and shoots. There was a significant interaction between earthworms and AM fungi on the urease activity in soil polluted by OTC (p<0.05). Adding earthworms or AM fungi could increase the maize biomass and N content (p<0.05) in OTC polluted soil by increasing the urease activity and relieving the stress from OTC on the soil N cycle. AM fungi and earthworms interactively increased maize shoot and root biomass (p<0.05) in the OTC polluted soils through their regulation of the urease activity and the abundance of ammonia oxidizers, resulting in different soil NH4(+)-N and NO3(-)-N contents, which may contribute to the N content of maize shoots and roots. Earthworms and AM fungi could be used as an efficient method to relieve the OTC stress in agro-ecosystems.

In situ phytoextraction of copper and cadmium and its biological impacts in acidic soil

Phytoremediation is a potential cost-effective technology for remediating heavy metal-contaminated soils. In this study, we evaluated the biomass and accumulation of copper (Cu) and cadmium (Cd) of plant species grown in a contaminated acidic soil treated with limestone. Five species produced biomass in the order: Pennisetum sinese > Elsholtzia splendens > Vetiveria zizanioides > Setaria pumila > Sedum plumbizincicola. Over one growing season, the best accumulators for Cu and Cd were Pennisetum sinese and Sedum plumbizincicola, respectively. Overall, Pennisetum sinese was the best species for Cu and Cd removal when biomass was considered. However, Elsholtzia splendens soil had the highest enzyme activities and microbial populations, while the biological properties in Pennisetum sinese soil were moderately enhanced. Results would provide valuable insights for phytoremediation of metal-contaminated soils.

A proposal of "core enzyme" bioindicator in long-term Pb-Zn ore pollution areas based on topsoil property analysis

To study the effects of long-term mining activities on the agricultural soil quality of Mengnuo town in Yunnan province, China, the heavy metal and soil enzyme activities of soil samples from 47 sites were examined. The results showed that long-term mining processes led to point source heavy metal pollution and Pb, Cd, Zn and As were the primary metal pollutants. Polyphenoloxidase was found the most sensitive soil enzyme activity and significantly correlated with almost all the metals (P < 0.05). Amylase (for C cycling), acid phosphatase (for P cycling) and catalase (for redox reaction) activities showed significantly positive correlations (P < 0.05) with Pb, Cd, Zn and As contents. The correlations between soil enzymes activities and Cd, Pb and Zn contents were verified in microcosm experiments, it was found that catalase activity had significant correlations (P < 0.05) with these three metals in short-term experiments using different soils under different conditions. Based on both field investigation and microcosm simulation analysis, oxidoreductases activities (rather than a specific enzyme activity) were suggested to be used as "core enzyme", which could simply and universally indicate the heavy metal pollution degrees of different environments. And hydrolases (for C, N, P and S recycling) could be used as a supplement to improve correlation accuracy for heavy metal indication in various polluted environments.

Increased plant growth and copper uptake of host and non-host plants by metal-resistant and plant growth-promoting endophytic bacteria

The effects of inoculation with two metal-resistant and plant growth-promoting endophytic bacteria (Burkholderia sp. GL12 and Bacillus megaterium JL35) were evaluated on the plant growth and Cu uptake in their host Elsholtzia splendens and non-host Brassica napus plants grown in natural Cu-contaminated soil. The two strains showed a high level of ACC deaminase activities. In pot experiments, inoculation with strain GL12 significantly increased root and above-ground tissue dry weights of both plants, consequently increasing the total Cu uptake of E. splendens and Brassica napus by 132% and 48.2% respectively. Inoculation with strain JL35 was found to significantly increase not only the biomass of B. napus, consequently increasing the total Cu uptake of B. napus by 31.3%, but Cu concentration of E. splendens for above-ground tissues by 318% and roots by 69.7%, consequently increasing the total Cu uptake of E. splendens by 223%. The two strains could colonize the rhizosphere soils and root interiors of both plants. Notably, strain JL35 could colonize the shoot tissues and significantly increase the translocation factors and bioaccumulation factors of E. splendens. These results suggested that Burkholderia sp. GL12 and B. megaterium JL35 were valuable bacterial resource which had the potential in improving the efficiency of Cu phytoextraction by E. splendens and B. napus in a natural Cu-contaminated soil.

Determining soil enzyme activities for the assessment of fungi and citric acid-assisted phytoextraction under cadmium and lead contamination

Microorganism or chelate-assisted phytoextraction is an effective remediation tool for heavy metal polluted soil, but investigations into its impact on soil microbial activity are rarely reported. Consequently, cadmium (Cd)- and lead (Pb)-resistant fungi and citric acid (CA) were introduced to enhance phytoextraction by Solanum nigrum L. under varied Cd and Pb pollution levels in a greenhouse pot experiment. We then determined accumulation of Cd and Pb in S. nigrum and the soil enzyme activities of dehydrogenase, phosphatase, urease, catalase, sucrase, and amylase. Detrended canonical correspondence analysis (DCCA) was applied to assess the interactions between remediation strategies and soil enzyme activities. Results indicated that the addition of fungi, CA, or their combination enhanced the root biomass of S. nigrum, especially at the high-pollution level. The combined treatment of CA and fungi enhanced accumulation of Cd about 22-47 % and of Pb about 13-105 % in S. nigrum compared with the phytoextraction alone. However, S. nigrum was not shown to be a hyperaccumulator for Pb. Most enzyme activities were enhanced after remediation. The DCCA ordination graph showed increasing enzyme activity improvement by remediation in the order of phosphatase, amylase, catalase, dehydrogenase, and urease. Responses of soil enzyme activities were similar for both the addition of fungi and that of CA. In summary, results suggest that fungi and CA-assisted phytoextraction is a promising approach to restoring heavy metal polluted soil.

Earthworm-induced carboxylesterase activity in soil: Assessing the potential for detoxification and monitoring organophosphorus pesticides

Soil enzyme activities are attracting widespread interest due to its potential use in contaminant breakdown, and as indicators of soil deterioration. However, given the multiple environmental and methodological factors affecting their activity levels, assessment of soil pollution using these biochemical endpoints is still complex. Taking advantage of the well-known stimulatory effect of earthworms on soil microbes, and their associated enzyme activities, we explored some toxicological features of carboxylesterases (CbEs) in soils inoculated with Lumbricus terrestris. A microplate-scale spectrophotometric assay using soil-water suspensions was first optimized, in which kinetic assay parameters (Km, Vmax, dilution of soil homogenate, and duration of soil homogenization) were established for further CbE determinations. Optimal conditions included a soil-to-water ratio of 1:50 (w/v), 30-min of shaking, and 2.5mM of substrate concentration. As expected, CbE activity increased significantly in soils treated with L. terrestris. This bioturbed soil was used for exploring the role of CbE activity as a bioscavenger for organophosphorus (OP) pesticides. Soil treated with two formulations of chlorpyrifos revealed that CbE activity was a significant molecular sink for this pesticide, reducing its impact on soil microbial activity as shown by the unchanged dehydrogenase activity. Dose-dependent curves were adjusted to an exponential kinetic model, and the median ecological dose (ED50) for both pesticide formulations was calculated. ED50 values decreased as the time of pesticide exposure increased (14 d-ED50s=20.4-26.7 mg kg(-1), and 28 d-ED50s=1.8-2.3 mg kg(-1)), which suggested that chlorpyrifos was progressively transformed into its highly toxic metabolite chlorpyrifos-oxon, but simultaneously was inactivated by CbEs. These results were confirmed by in vitro assays that showed chlorpyrifos-oxon was a more potent CbE inhibitor (IC50=35.5-4.67 nM) than chlorpyrifos (0.41-0.84 μM). The results showed that earthworm-induced CbE activity is an efficient bioscavengers for OP pesticides, acting as a soil safeguarding system. Moreover, the simple dose-response curves against OP exposure suggest that this enzyme--combined with other enzyme activities (e.g., dehydrogenase)--may be a suitable biomarker of pesticide exposure.

Field isotopic study of lead fate and compartmentalization in earthworm-soil-metal particle systems for highly polluted soil near Pb recycling factory

Earthworms are important organisms in soil macrofauna and play a key role in soil functionality, and consequently in terrestrial ecotoxicological risk assessments. Because they are frequently observed in soils strongly polluted by metals, the influence of earthworm bioturbation on Pb fate could therefore be studied through the use of Pb isotopes. Total Pb concentrations and isotopic composition ((206)Pb, (207)Pb and (208)Pb) were then measured in earthworms, casts and bulk soils sampled at different distance from a lead recycling factory. Results showed decreasing Pb concentrations with the distance from the factory whatever the considered matrix (bulk soils, earthworm bodies or cast samples) with higher concentrations in bulk soils than in cast samples. The bivariate plot (208)Pb/(206)Pb ratios versus (206)Pb/(207)Pb ratios showed that all samples can be considered as a linear mixing between metallic process particulate matter (PM) and geochemical Pb background. Calculated anthropogenic fraction of Pb varied between approximately 84% and 100%. Based on Pb isotopic signatures, the comparison between casts, earthworms and bulk soils allowed to conclude that earthworms preferentially ingest the anthropogenic lead fraction associated with coarse soil organic matter. Actually, soil organic matter was better correlated with Pb isotopic ratios than with Pb content in soils. The proposed hypothesis is therefore a decrease of soil organic matter turnover due to Pb pollution with consequences on Pb distribution in soils and earthworm exposure. Finally, Pb isotopes analysis constitutes an efficient tool to study the influence of earthworm bioturbation on Pb cycle in polluted soils.

Impacts of dimethyl phthalate on the bacterial community and functions in black soils

Dimethyl phthalate (DMP), a known endocrine disruptor and one of the phthalate esters (PAEs), is a ubiquitous pollutant. Its impacts on living organisms have aroused great concern. In this study, the impacts of DMP contamination on bacterial communities and functions were tested by using microcosm model in black soils. The results showed that the operational taxonomic unit (OTUs) richness and bacterial diversity were reduced by DMP contamination. The relative percentages of some genera associated with nitrogen metabolism were increased by DMP contamination, while the relative percentages of some other genera that were extremely beneficial to soil health were decreased by DMP contamination. Further, the relative percentages of some genera that possessed the capability to degrade DMP were increased by the DMP treatment at low concentrations (5, 10, and 20 mg/kg), but were decreased by the high concentration DMP treatment (40 mg/kg). Clearly, DMP contamination changed the bacterial community structure and disturbed the metabolic activity and functional diversity of the microbes in black soils. Our results suggest that DMP pollution can alter the metabolism and biodiversity of black soil microorganisms, thereby directly impact fertility and ecosystem functions.

Effects of historic metal(loid) pollution on earthworm communities

The effects of metal(loid)s (Pb, Cd, Cu, Zn, As and Sb) from atmospheric fallout on earthworm communities were investigated in a fallow meadow located close to a 60-year-old lead recycling factory. We examined abundance and species diversity as well as the ratio of adult-to-juvenile earthworms, along five 140 m parallel transects. The influence of soil pollution on the earthworm community at the plot scale was put in context by measuring some physico-chemical soil characteristics (OM content, N content, pH), as well as total and bioavailable metal(loid) concentrations. Earthworms were absent in the highly polluted area (concentration from 30,000 to 5000 mg Pb·kg(-1) of dried soil), just near the factory (0-30 m area). A clear and almost linear relationship was observed between the proportion of juvenile versus mature earthworms and the pollution gradient, with a greater proportion of adults in the most polluted zones (only adult earthworms were observed from 30 to 50 m). Apporectodea longa was the main species present just near the smelter (80% of the earthworms were A. longa from 30 to 50 m). The earthworm density was found to increase progressively from five individuals·m(-2) at 30 m to 135 individuals·m(-2) at 140 m from the factory. On average, metal(loid) accumulation in earthworm tissues decreased linearly with distance from the factory. The concentration of exchangeable metal(loid)s in earthworm surface casts was higher than that of the overall soil. Finally, our field study clearly demonstrated that metal(loid) pollution has a direct impact on earthworm communities (abundance, diversity and proportion of juveniles) especially when Pb concentrations in soil were higher than 2050 mg·kg(-1).

Earthworm bioturbation influences the phytoavailability of metals released by particles in cultivated soils

The influence of earthworm activity on soil-to-plant metal transfer was studied by carrying out six weeks mesocosms experiments with or without lettuce and/or earthworms in soil with a gradient of metal concentrations due to particles fallouts. Soil characteristics, metal concentrations in lettuce and earthworms were measured and soil porosity in the mesocosms was determined. Earthworms increased the soil pH, macroporosity and soil organic matter content due to the burying of wheat straw provided as food. Earthworm activities increased the metals concentrations in lettuce leaves. Pb and Cd concentrations in lettuce leaves can increase up to 46% with earthworm activities … These results and the low correlation between estimated by CaCl2 and EDTA and measured pollutant phytoavailability suggest that earthworm bioturbation was the main cause of the increase. Bioturbation could affect the proximity of pollutants to the roots and soil organic matter.