Influence of earthworm mucus and amino acids on tomato seedling growth and cadmium accumulation

The potential of earthworms and arbuscular mycorrhizal fungi to enhance phytoremediation in heavy metal-contaminated soils: a review

Earthworms and arbuscular mycorrhizal fungi (AMF) are two different organisms playing crucial role in soil mechanisms. The integration of earthworms and AMF in phytoremediation strategies leverages their combined ability to improve soil structure, nutrient availability, and microbial activity while modulating metal bioavailability. These entities promote soil-plant interactions and enhance the phytoremediation process of heavy metals-contaminated soil. This review explores the mechanisms by which earthworms and AMF function individually and in combination in the phytoremediation of heavy metal-contaminated soil. The main objectives of this were determine earthworms heavy metals tolerance, absorption and transformation, as well as the synergistic effect between earthworms and plants. Further, the effects of AMF on heavy metals phytoremedoation process was also analyzed as well as the potential interactions between earthworms and AMF on heavy metals removal. This partnership can optimize plant health and remediation efficiency, making it a promising approach for restoring heavy metal-contaminated soils. Thus an integrated empirical study was conducted to summarize the effects earthworms and AMF interactions on heavy metals phytoremediation and to highlight the impact of their individual and combined actions on the phytoremediation paramters. Avenue for further studies towards improved phytoremediation process we discussed. This review emphasize that earthworms and AMF can be employed as biological method to enhance the phytoextraction by hyperaccumulator plants on severely heavy metal-contaminated soil. Alternatively, in moderately and lowly contaminated farmland, the transfer of heavy metals to the above-ground parts of crops can be reduced to promote safe production.

Effects of naturally aged microplastics on arsenic and cadmium accumulation in lettuce: Insights into rhizosphere microecology

Naturally aged microplastics (NAMPs) are commonly found in farmland soils contaminated with heavy metals (HMs), such as arsenic (As) and cadmium (Cd); yet their combined effects on soil-plant ecosystems remain poorly understood. In this study, we investigated the toxic effects of NAMPs and As-Cd on lettuce, considering the influence of earthworm activity, and examined changes in As-Cd bioavailability in the rhizosphere. Four experimental systems were established: soil-only, soil-lettuce, soil-earthworms, and soil-lettuce-earthworms systems, with four NAMPs concentrations (0, 0.1, 0.5, 1 %). Our results showed that exposure to 0.1 % NAMPs reduced As accumulation in lettuce shoots (0.17-0.25 mg kg-1) and roots (1.13-1.72 mg kg-1), while increasing biomass and enhancing root growth by alleviating toxicity. In contrast, the combined stress of higher NAMPs concentration (0.5 %/1 %) and As-Cd caused a 28.4-58.4 % reduction in root activity, which stimulated low-molecular-weight organic acid (LMWOA) secretion in the rhizosphere, increasing the bioavailability of As and Cd and enhancing their absorption by lettuce. Partial least squares path modeling (PLS-PM) revealed that co-exposure altered LMWOA content, soil enzyme activity, and microbial community stability in the rhizosphere, ultimately influencing the bioavailability and uptake of As and Cd by lettuce.

Unveiling resilience: coelomic fluid bacteria's impact on plant metabolism and abiotic stress tolerance

Earthworms' coelomic fluid (CF) has been discovered to possess properties that promote plant development. In particular, the earthworm's coelomic fluid-associated bacteria (CFB) are the primary factor influencing the plants' response. To investigate this, we used bacteria isolated from the CF and selected based on different plant growth-promoting traits, in a mesocosm ecosystem that includes plants. This experiment aimed to assess their impact on the metabolism of plants growing under abiotic stress environments (alkaline soil and nitrogen (N), phosphate (P), and potassium (K) deficit) and compare the lipid profiles of plants under the various treatments. We used seven different bacterial species isolated from the CF of Aporrectodea molleri and as a plant model Zea mays L. For the metabolomic analysis method, we used gas chromatography-mass spectrometry lipidomic. After observing the metabolomic profiles, we found that a few molecular pathways are involved in how plants react to bacterial biostimulants. The bacterial isolates belonging to Pantoea vagans, Pseudomonas aeruginosa, Bacillus paramycoides, and Bacillus thuringiensis have led to a significant increase in synthesizing several metabolites belonging to various chemical categories. Contrary to predictions, abiotic stress did not cause a drop in the composition and concentration of lipids in plants treated with the CFB, demonstrating the rigidity of the protective mechanisms. The statistical analysis based on the Pearson method revealed a positive significant correlation between plant growth parameters (length of the aerial part, surface of the leaves, and biomass) and some metabolites belonging to fatty acids, carboxylic acids, benzene derivatives, and alkanes. Moreover, the standard metabolic components of all treatments in much higher concentrations during bacterial treatments than the control treatment suggests that the bacteria have stimulated the overexpression of these metabolic components. According to these results, we could assume that plants treated with CFB exhibit an adaptability of abiotic stress defense mechanisms, which may be attributed to the upregulation of genes involved in lipid biosynthesis pathways.

Earthworm mucus contributes significantly to the accumulation of soil cadmium in tomato seedlings

Whether earthworm mucus affects Cd transport behavior in soil-plant systems remains uncertain. Consequently, this study thoroughly assessed the impacts of earthworm mucus on plant growth and physiological responses, plant Cd accumulation, translocation, and distribution, as well as soil characteristics and Cd fractionation in a soil-plant (tomato seedling) system. Results demonstrated that the earthworm inoculation considerably enhanced plant Cd uptake and decreased plant Cd translocation, the effects of which were appreciably less significant than those of the earthworm mucus. This suggested that earthworm mucus may play a crucial role in the way earthworms influence plant Cd uptake and translocation. Moreover, the artificial mucus, which contained identical inorganic nitrogen contents to those in earthworm mucus, had no significant effect on plant Cd accumulation or translocation, implying that components other than inorganic nitrogen in the earthworm mucus may have contributed significantly to the overall effects of the mucus. Compared with the control, the earthworm mucus most substantially increased the root Cd content, the Cd accumulation amount of root and whole plant, and root Cd BCF by 93.7 %, 221.3 %, 72.2 %, and 93.7 %, respectively, while notably reducing the Cd TF by 48.2 %, which may be ascribed to the earthworm mucus's significant impacts on tomato seedling growth and physiological indicators, its considerable influences on the subcellular components and chemical species of root Cd, and its substantial effects on the soil characteristics and soil Cd fractionation, as revealed by correlation analysis. Redundancy analysis further suggested that the most prominent impacts of earthworm mucus may have been due to its considerable reduction of soil pH, improvement of soil DOC content, and enhancement of the exchangeable Cd fraction in soil. This work may help better understand how earthworm mucus influences the transport behavior of metals in soil-plant systems.

Responses of biomarkers, joint effect and drilosphere bacterial communities to antimony (III and/or V) contamination

Contamination of soils with antimony (Sb) is becoming increasingly severe and widespread, and the associated ecological risks cannot be ignored. To evaluate how different Sb forms affected the earthworm Eisenia fetida in soil, the biomarker response index (BRI), effect addition index (EAI), and microbial diversity were characterized after single and joint application of Sb(III) and Sb(V). The results showed that Sb(III) was better enriched by earthworms than Sb(V). The metallothionein (MT) content in earthworms increased under Sb stress, and the superoxide dismutase (SOD), catalase (CAT), and glutathione S-transferase (GST) activities also showed an increasing trend, suggesting waken-up antioxidant capacity. Severe alterations for health status were observed under combined treatment. Additionally, the EAI indicated that Sb(III) and Sb(V) had synergistic and antagonistic effects at low and high concentrations, respectively. The bacterial populations in the drilosphere (gut and burrow lining) appeared to be more susceptible to Sb contamination than in the non-drilosphere, their specific microecology may be an important factor in soil Sb migration and transformation. The abundance of Actinobacteria exhibited a significant decrease with increasing concentrations of single Sb(III) and Sb(V), while the abundance of Bacteroidia increased. The correlation heatmap showed that Sphingobacterium faecium was highly tolerant to Sb. These results provide not only an important basis for the ecological risk assessment of Sb in the soil environment but also new insights into the altered drilosphere bacterial communities under Sb stress.

Serine Rejuvenated Degenerated Volvariella volvacea by Enhancing ROS Scavenging Ability and Mitochondrial Function

Serine is a functional amino acid that effectively regulates the physiological functions of an organism. This study investigates the effects of adding exogenous serine to a culture medium to explore a feasible method for the rejuvenation of V. volvacea degenerated strains. The tissue isolation subcultured strains T6, T12, and T19 of V. volvacea were used as test strains, and the commercially cultivated strain V844 (T0) was used as a control. The results revealed that the addition of serine had no significant effect on non-degenerated strains T0 and T6, but could effectively restore the production characteristics of degenerated strains T12 and T19. Serine increased the biological efficiency of T12 and even helped the severely degenerated T19 to regrow its fruiting body. Moreover, exogenous serine up-regulated the expression of some antioxidant enzyme genes, improved antioxidase activity, reduced the accumulation of reactive oxygen species (ROS), lowered malondialdehyde (MDA) content, and restored mitochondrial membrane potential (MMP) and mitochondrial morphology. Meanwhile, serine treatment increased lignocellulase and mycelial energy levels. These findings form a theoretical basis and technical support for the rejuvenation of V. volvacea degenerated strains and other edible fungi.

Physiological and Proteomic Analyses of mtn1 Mutant Reveal Key Players in Centipedegrass Tiller Development

Tillering directly determines the seed production and propagation capacity of clonal plants. However, the molecular mechanisms involved in the tiller development of clonal plants are still not fully understood. In this study, we conducted a proteome comparison between the tiller buds and stem node of a multiple-tiller mutant mtn1 (more tillering number 1) and a wild type of centipedegrass. The results showed significant increases of 29.03% and 27.89% in the first and secondary tiller numbers, respectively, in the mtn1 mutant compared to the wild type. The photosynthetic rate increased by 31.44%, while the starch, soluble sugar, and sucrose contents in the tiller buds and stem node showed increases of 13.79%, 39.10%, 97.64%, 37.97%, 55.64%, and 7.68%, respectively, compared to the wild type. Two groups comprising 438 and 589 protein species, respectively, were differentially accumulated in the tiller buds and stem node in the mtn1 mutant. Consistent with the physiological characteristics, sucrose and starch metabolism as well as plant hormone signaling were found to be enriched with differentially abundant proteins (DAPs) in the mtn1 mutant. These results revealed that sugars and plant hormones may play important regulatory roles in the tiller development in centipedegrass. These results expanded our understanding of tiller development in clonal plants.

The effects of Eisenia fetida and Metaphire guillelmi on the soil micro-food web in a microcosm experiment

Numerous studies have shown that the function of earthworms may depend on their ecotype and density, which affects how they impact soil microbial and nematode communities. However, it is unclear how different earthworm species and densities alter the composition of soil microbial and nematode communities and how these modifications impact the soil micro-food web. The structural equation model (SEM) is a more accurate tool for identifying the intricate relationships between various trophic levels in the soil micro-food webs than the widely used bivariate data analysis. In order to ascertain the effects of earthworm species, including epigeic earthworm Eisenia fetida and anecic earthworm Metaphire guillelmi, as well as varying densities on the composition of main microbial groups, soil nematodes and their relationships, a microcosm experiment was conducted in a greenhouse. After nine weeks of observation, compared with the control treatments, Eisenia fetida increased the biomasses of total microorganism and bacteria, whereas Metaphire guillelmi decreased the biomasses of total microorganism, bacteria, and gram-positive bacteria, but showed an increase in AMF biomass. Additionally, both two earthworm species decreased the abundance of total soil nematode, bacterivores, and omnivore-predators, which is in contrast with the control treatments. The SEM results indicated that the addition of Eisenia fetida at different densities had indirect negative effects on the abundance of omnivore-predators, as it significantly increased the content of soil organic carbon, ammonium nitrogen, and nitrate nitrogen. The bottom-up effects were found to be the dominant forces, which promoted bacterial-dominated decomposition channels. The addition of Metaphire guillelmi with different density had direct negative impact on bacterivores and fungivores. Moreover, it had indirect negative effects on omnivore-predators by altering the soil properties. The dominant forces were still the bottom-up effects. Our study suggests that different earthworm species have distinct mechanisms that affect the soil micro-food web in different ways.

The characteristic difference between non-drilosphere and drilosphere-aged biochar: Revealing that earthworms accelerate the aging of biochar

Numerous researches have been conducted on the effects of biotic and abiotic-induced aging on the physicochemical characteristics and functions of biochar; however, the impacts of earthworm-induced aging on biochar have not been reported. Hence, we conducted a microscopic experiment simulating a 'drilosphere' to explore the influence of earthworm activity on the natural aging of rice husk biochar (RHBC) through the difference in biochar characteristics after aging in drilosphere and non-drilosphere. The earthworm activity increases the available nitrogen (AN) and dissolved organic matter (DOM) contents of aged RHBC and changes its composition. The increase of DOM and AN content may recruit more microorganisms to colonize biochar and accelerate the biological oxidation of biochar. Furthermore, earthworm activity significantly increased the contents of oxygen (O) and O-containing functional groups in the aged RHBC and decreased the stability (aromaticity) of the aged RHBC, suggesting that the earthworm activity accelerates the natural aging of biochar. Earthworm feeding promotes physical damage to biochar. Besides, the earthworm activity decreased the pH, hydrophilicity and specific surface area (SSA) of aged RHBC but enhanced the adsorption capacity of aged RHBC for heavy metals. The higher content of O-containing functional groups on the surface of drilosphere-aged RHBC was the main reason for its higher adsorption performance. Earthworm feeding promotes physical damage to biochar. These results indicate that earthworm activity can accelerate the natural aging of biochar and alter its physicochemical characteristics and functions. This study illustrates how biochar characteristics change in earthworm-soil systems, which will help scientifically evaluate the long-term effectiveness of biochar.

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.

Compositional changes and ecological characteristics of earthworm mucus under different electrical stimuli

Earthworm mucus is rich in nutrients that can initiate the mineralization and humification of organic matter and is of great importance for contaminated soil remediation and sludge reutilization. In this study, six voltage and current combinations were utilized to promote earthworm mucus production (5 V and 6 V at 10, 20 and 30 mA, respectively), to explore the compositional changes of the mucus produced under different electrical stimuli, and to propose the best electrical stimulation group and mucus fraction applicable to soil heavy metal pollution remediation and sludge reutilization. The results showed that the mucus produced by the six electrical stimuli was mainly composed of proteins, amino acids, carbohydrates, fatty acids, and polysaccharides, with small amounts of alcohol, phenol, and ester organic substances. Under different electrical stimuli, each component changed significantly (P < 0.05). pH and conductivity were higher at 6 V 20 mA, total nitrogen and phosphorus contents reached their maximum at 5 V 30 mA, and total potassium at 6 V 10 mA. Protein, amino acids, and carbohydrates were most abundant in the mucus produced at 5 V 10 mA, while trace metal elements reached their lowest values at 5 V 10 mA. Finally, based on principal component analysis and combined with previous studies, it was concluded that the mucus produced at 5 V 10 mA was weakly alkaline, high in amino acids and nutrients and low in trace metal elements, and most suitable for sludge and straw composting experiments, soil remediation and amendment experiments.

Earthworms as candidates for remediation of potentially toxic elements contaminated soils and mitigating the environmental and human health risks: A review

Global concerns towards potentially toxic elements (PTEs) are steadily increasing due to the significant threats that PTEs pose to human health and environmental quality. This calls for immediate, effective and efficient remediation solutions. Earthworms, the 'ecosystem engineers', can modify and improve soil health and enhance plant productivity. Recently, considerable attention has been paid to the potential of earthworms, alone or combined with other soil organisms and/or soil amendments, to remediate PTEs contaminated soils. However, the use of earthworms in the remediation of PTEs contaminated soil (i.e., vermiremediation) has not been thoroughly reviewed to date. Therefore, this review discusses and provides comprehensive insights into the suitability of earthworms as potential candidates for bioremediation of PTEs contaminated soils and mitigating environmental and human health risks. Specifically, we reviewed and discussed: i) the occurrence and abundance of earthworms in PTEs contaminated soils; ii) the influence of PTEs on earthworm communities in contaminated soils; iii) factors affecting earthworm PTEs accumulation and elimination, and iv) the dynamics and fate of PTEs in earthworm amended soils. The technical feasibility, knowledge gaps, and practical challenges have been worked out and critically discussed. Therefore, this review could provide a reference and guidance for bio-restoration of PTEs contaminated soils and shall also help developing innovative and applicable solutions for controlling PTEs bioavailability for the remediation of contaminated soils and the mitigation of the environment and human risks.

Insights into the mechanisms underlying the remediation potential of earthworms in contaminated soil: A critical review of research progress and prospects

In recent years, soil pollution is a major global concern drawing worldwide attention. Earthworms can resist high concentrations of soil pollutants and play a vital role in removing them effectively. Vermiremediation, using earthworms to remove contaminants from soil or help to degrade non-recyclable chemicals, is proved to be an alternative, low-cost technology for treating contaminated soil. However, knowledge about the mechanisms and framework of the vermiremediation various organic and inorganic contaminants is still limited. Therefore, we reviewed the research progress of effects of soil contaminants on earthworms and potential of earthworm used for remediation soil contaminated with heavy metals, polychlorinated biphenyls (PCBs), polybrominated diphenyl ethers (PBDEs), polycyclic aromatic hydrocarbons (PAHs), pesticides, as well as crude oil. Especially, the possible processes, mechanisms, advantages and limitations, and how to boost the efficiency of vermiremediation are well addressed in this review. Finally, future prospects of vermiremediation soil contamination are listed to promote further studies and application of vermiremediation in contaminated soils.

Biological removal of cadmium from biogas residues during vermicomposting, and the effect of earthworm hydrolysates on Trichoderma guizhouense sporulation

The removal efficiency of Cd from biogas residues (BR) by earthworms (Eisenia fetida) during vermicomposting and the optimum addition of earthworm hydrolysates for production of Trichoderma guizhouense NJAU 4742 spores were determined. The results showed that vermicomposting could effectively remove Cd (up to 18.9%) from the BR. Synchrotron radiation based FTIR spectromicroscopy demonstrated a weakened correlation between functional groups after vermicomposting, suggesting that the activity of earthworms affects the binding sites and bioavailability of heavy metals. Under optimum conditions, the hydrolysis rate of earthworms was ~97% and the removal efficiency of Cd was up to 93%. Furthermore, addition of 20% of earthworm hydrolysate promoted the largest production of Trichoderma sporulation (~2.95 × 10⁸ cfu/g straw), indicating the possibility of earthworm hydrolysates promoting the growth of Trichoderma guizhouense is a suitable way to recycle earthworms after vermicomposting.

Earthworm Eisenia andrei modulates oxidative stress in bean plants Vicia faba irrigated with treated wastewater

With respect to reducing the pressure on freshwater resources, treated wastewater (TWW) irrigation represents a sustainable alternative in agriculture. Due to their low quality and variable composition, TWW could entail harmful consequences for living organisms in terrestrial ecosystems. This study aims to evaluate how earthworm (Eisenia andrei) can modulate oxidative stress in bean plants (Vicia faba) that are irrigated over a course of 60 days with two doses of TWW (50 and 100%) in addition to a control condition (0%) irrigated with distilled water. This is achieved by measuring glutathione-S-transferase (GST) activity and malondialdehyde accumulation (MDA) in plants. Furthermore, catalase (CAT), GST, MDA, and acetylcholinesterase (AChE) activities of the earthworms are also assessed. Our results show that growth and physiological parameters are modified when applying TWW irrigation. Moreover, oxidative stress apprehended by GST activity and MDA accumulation is exacerbated in V. faba plants after exposure to increased TWW doses. Similarly, TWW irrigation enhances oxidative stress parameters in earthworms with a crucial decrease in AChE activity. In addition, the presence of earthworms increases growth and physiological parameters; it also results in a significant reduction in GST activity and MDA rate in V. faba plants. Our results provide new insights into the impact of TWW irrigation on soil organisms and the importance of earthworms in the reduction of oxidative stress in plants.

Assessment of earthworm activity on Cu, Cd, Pb and Zn bioavailability in contaminated soils using biota to soil accumulation factor and DTPA extraction

The study aims to investigate effect of earthworm activity on metal bioavailability in soils using their BSAF-metals. Based on a microcosmic laboratory experiment, epigeic species Amynthas corticis (A. corticis) and endogeic species Amynthas robustus (A. robustus) were cultured in two types of soils contaminated by Cd, Zn, Pb and Cu for 120 days. Earthworm characteristics (i.e. numbers, biomass and BSAF), soil properties (i.e. pH, organic C and N contents along with their components such as mineralization and microbial masses) and DTPA extracted metals in soil were determined. After the incubation, the biomass and survival numbers of both earthworm species decreased significantly (P < 0.05). The accumulation of Cd, Zn and Pb in earthworm tissues and BSAF-metals were earthworm species dependent. According to two-way ANOVA, BSAF-Pb clearly showed the effect of different species of earthworms while BSAF-Cu indicated an interactive effect of earthworms and soil type. Earthworms changed soil properties significantly, especially for mineralized C (C_min), dissolved N (N_dis) and pH (P < 0.05). Earthworm activity increase DTPA extracted Zn and Cu, and the effect of A. robustus were stronger than for A. corticis. Redundancy analysis (RDA) showed that BSAF-Cu and BSAF-Pb contributed for respectively 51.9% and 51.7% of soil properties and DTPA metal changes, indicating that the effects of BSAF-Cu and BSAF-Pb on soil properties and on metal bioavailability in soil were similar. BSAF-Cu, indicating the interactive effect of earthworms and soil, accounted for 38.5% and 45.1% of soil properties and soil metal bioavailability changes. BSAF-Pb, representing the effect of earthworm species, accounted for 13.3% and 6.6% of soil property and soil metal bioavailability variations. Stepwise regression indicated that earthworm might change soil properties through their activities and interactions with soil, and hence increase heavy metal bioavailability. It suggested that BSAF is an important indicator for evaluating the effect of earthworm activity on soil metal bioavailability and designing remediation strategies.

Cd induced generation of free radical species in Brassica juncea is regulated by supplementation of earthworms in the drilosphere

The antioxidant defense system of Brassica juncea under Cd stress was examined on supplementation of earthworms in the rhizosphere at different concentrations of Cd (0.50, 0.75, 1.00 and 1.25 mM i.e. 56, 84, 112 and 140 mg kg^-1 respectively). Seedlings were raised in small pots containing soil spiked with Cd and earthworms under controlled conditions for 15 days. Improved Cd accumulation, as well as enhanced plant dry weight and metal tolerance were observed following the addition of earthworms. Earthworm supplementation reduced reactive oxygen species (ROS) generation by 7.3% for hydrogen peroxide (H₂O₂), 7.1% for superoxide anion (O₂^-), and 8.4% for malondialdehyde (MDA) in plants treated with 1.25 mM (140 mg kg^-1) Cd. Confocal microscopy revealed improved cell viability and reduced H₂O₂ content due to enhanced antioxidative activity. Activity and expression levels of genes coding for antioxidative enzymes (superoxide dismutase; SOD, catalase; CAT, guaicol peroxidase; POD, glutathione reductase; GR, and glutathione-S-transferase; GST) were higher in plants raised in soils inoculated with earthworms, with expression of SOD increasing by 58.8%, CAT by 75%, POD by 183%, GR by 106.6%, and GST by 11.8%. Moreover, plant pigment (chlorophyll a, chlorophyll b, total chlorophyll, and carotenoids) concentrations increased by 8%, 9.1%, 9.1%, and 7.7% respectively, in plants grown in soils supplemented with earthworms. The results of our study suggest that the addition of earthworms to soil increases antioxidative enzyme activities, gene expression in plants, and ROS inhibition, which enhances tolerance to Cd during the phytoextraction process.

Effect of phenanthrene on the physicochemical properties of earthworm casts in soil

Earthworms have been widely studied as bioindicators of soil health for their important role in sustaining soil structure and functions. Many soil contaminants such as phenanthrene have been confirmed to exert adverse effects on earthworms' growth, reproduction, behaviors and biochemical conditions. However, their effects on the properties of earthworm casts have been little studied. In the present study, the effect of different doses of phenanthrene (PHE) (0, 2, 5, 10, 20 mg/kg) on the six physicochemical properties and Fourier transform infrared spectroscopy (FTIR) spectra characteristics of earthworm casts was assessed in artificial soil in a laboratory. 1) Residual concentration of PHE in soils and casts increased with the increasing exposure concentrations and followed the order of casts > soil, concluding that K_ow values are the important factor affecting the distribution of hydrophobic organic contaminants (HOCs) in soil and casts; 2) Earthworms produced casts with improved total organic carbon (TOC) (15-19%), NH₄⁺-N (550-800^%), total available phosphorus (TAP) (300-450%), cation exchange capacity (CEC) (about 15%) and available potassium (AK) (7-12.6%) compared to that in unpolluted soil, indicating that earthworms still have the ability to play the role of ecological engineers even in polluted soil; 3) The sensitivity of different properties of casts to phenanthrene varies, the order of sensitivity being (most sensitive first) NH₄⁺-N ( triggered as 2 mg/kg of exposure concentrations) > AK (5 mg/kg) > Olsen-P (10 mg/kg) > TOC = pH= CEC (no response within the range of exposure concentrations). NH₄⁺-N content in casts shows a clear dose-response relationship when the exposure exceeds 2 mg/kg, indicating that the index might be a potential sensitive biomarker to provide early warning for soil pollution. 4) FTIR spectra showed that the constitution of casts from earthworms in PHE-spiked soil was not significantly alternated. However, FTIR spectra revealed that the concentrations of C-O of polysaccharide in casts increased with the elevated exposure concentrations, indicating that intensities of C-O of polysaccharide at 1032 cm^-1 of casts might be also a potential biomarker for the early-warning of soil pollution. This study advances the knowledge of earthworm ecology in polluted soil, and further extends the scope of earthworm casts as a potential biomarker in soil pollution assessment.

Earthworms, Rice Straw, and Plant Interactions Change the Organic Connections in Soil and Promote the Decontamination of Cadmium in Soil

The joint effects of earthworms and crop straw on toxic metal speciation are not clear, and very limited information is available regarding the effects of their interaction on Cd mobility in Cd contaminated soil or in remediation processes involving plants. This study evaluated their impacts on Cd mobile form changes in soil and their effects on Cd uptake by plants. Treatments included both planted and unplanted-Cd-contaminated soil with or without rice straw and/or earthworms. The results revealed that earthworms, rice straw, and plant interactions change the Cd mobile forms in soil. The order of Cd concentration of different chemical forms was as follows: exchangeable > residual > bound to Fe-Mn oxide > bound to organic matter for earthworms, and exchangeable > bound to organic matter > residual > bound to Fe-Mn oxide for rice straw treatment, with a recovery rate of 96 ± 3%. The accumulation of Cd in plants increased in the presence of earthworms and decreased in the presence of rice straw. FT-IR spectra indicated that the degradation of rice straw increases C⁻O, C⁻O⁻H, C⁻H, and O⁻H functional groups which could complex with Cd ions. These findings highlighted that earthworms' activities and crop straw can modify soil properties and structure and promote the remediation of heavy metal. This study suggests that the ecological context of remediation instead of being limiting on soil-earthworms-plant interaction, should integrate the natural resources forsaken which can provide a positive influence on both plant health and the remediation of heavy metal in contaminated soil.

Impact of osmoregulation on the differences in Cd accumulation between two contrasting edible amaranth cultivars grown on Cd-polluted saline soils

This study aimed to investigate the difference of osmoregulation between two edible amaranth cultivars, Liuye (high Cd accumulator) and Quanhong (low Cd accumulator), under salinity stress and determine the effects of such difference on Cd accumulation. A pot experiment was conducted to expose the plants to sewage-irrigated garden soil (mean 2.28 mg kg^-1 Cd) pretreated at three salinity levels. Under salinity stress, the concentrations of Cd in the two cultivars were significantly elevated compared with those in the controls, and the Cd concentration in Liuye was statistically higher than that in Quanhong (p < 0.05). Salinity-induced osmoregulation triggered different biogeochemical processes involved in Cd mobilization in the rhizosphere soil, Cd absorption, and translocation by the two cultivars. Rhizosphere acidification induced by an imbalance of cation over anion uptake was more serious in Liuye than in Quanhong, which obviously increased soil Cd bioavailability. Salinity-induced injuries in the cell wall pectin and membrane structure were worse in Liuye than in Quanhong, increasing the risk of Cd entering the protoplasts. The chelation of more cytoplasmic Cd²⁺ with Cl^- ions in the roots of Liuye promoted Cd translocation into the shoots. Furthermore, the less organic solutes in the root sap of Liuye than in that of Quanhong also favored Cd translocation into the shoots. Hence, osmoregulation processes can be regarded as important factors in reducing Cd accumulation in crop cultivars grown on saline soils.

The influence of the earthworm Lampito mauritii (Kinberg) on the activity of selected soil enzymes in cadmium-amended soil

The effects of cadmium (CdCl2·7H2O) on cellulase, urease, amylase, invertase and phosphatase were assessed for a period of 45 days in the presence and absence of earthworms [Lampito mauritii (Kinberg)] in alfisol soil. The activities of all enzymes significantly increased with longer incubation times (45 days) under laboratory conditions in both control and Cd-amended soils (both with and without earthworm incubation). However, the activities of all enzymes decreased with increasing Cd concentrations under laboratory conditions, both in the presence and absence of earthworms. In the presence of earthworms, cellulase, urease, invertase and amylase activities increased. However, phosphatase activity was lower in most of the Cd-amended soils in the presence of earthworms compared to its activity levels in soils lacking earthworms. These results show that earthworms modulated the stress imposed by Cd by providing suitable substrates, which in turn acted as stimulants for extracellular enzyme secretion by microbes, and by removing Cd through its accumulation in the tissues of the earthworms.

Can commonly measurable traits explain differences in metal accumulation and toxicity in earthworm species?

There is no clear consensus in the literature on the metal accumulation pattern and sensitivity of different earthworm species. In the present study, accumulation and toxicity of Cu, Cd, Ni, and Zn in the earthworms Lumbricus rubellus (epigeic), Aporrectodea longa (anecic), and Eisenia fetida (ultra-epigeic) were determined after 28 days exposure in two soils. Metal accumulation and sensitivity were interpreted using the specific traits of different earthworm species. Results showed that for all four metals tested L. rubellus was the most sensitive species, followed by A. longa and E. fetida. At the same exposure concentration, internal concentrations followed the order: L. rubellus > E. fetida > A. longa for Cu and Ni, L. rubellus ≈ E. fetida ≈ A. longa for Cd, and L. rubellus > A. longa > E. fetida for Zn. Langmuir isotherms were used to model metal accumulation at both nontoxic and toxic exposure concentrations. The Cu, Cd, and Zn concentrations in E. fetida generally leveled off at high exposure concentrations but not for the other two species. A. longa showed a high capability of regulating internal Ni concentrations. The traits-based approaches suggested that most likely a group of earthworm traits together determined (differences in) metal accumulation and sensitivity. More research is needed in this respect to build up solid relationships between species-specific responses and traits, enabling cross-species extrapolation of accumulation and toxicity data.

The effect of humic and glutamic acids in nutrient solution on the N metabolism in lettuce

BACKGROUND

This paper reports for the first time the influence of partially replacing the nitrogen (N) source of a nutrient solution with amino acids and humic acid on the physiological and antioxidant activities and N metabolism in lettuce. A hydroponic experiment was designed to replace 25 and 50% of the N in nutrient solutions (NSs) with glutamic acid (GA) and humic acid (HA) and evaluate the effects on growth, nitrate (NO(3)) assimilation, protein content, nitrate reductase (NR) activity and antioxidant changes in lettuce.

RESULTS

The results showed that, when a portion of the N was replaced with GA and HA, the fresh and dry weights of lettuce shoots did not change significantly compared with the full NO(3) treatment. The titratable acidity was not affected by adding HA and/or GA to the NS. The nitrite concentration and NR activity decreased with reductions in the N concentration of the NS and improved with the addition of GA and HA. GA enhanced the NO(3) uptake and protein content more than HA. Changes in the superoxide dismutase (SOD) and peroxidase (POD) activities and malondialdehyde content were inconclusive: SOD and POD activities decreased with decreasing N content of the NS, and HA addition improved the SOD and POD activities.

CONCLUSION

When HA and GA were substituted for NO(3) in an NS, the acids effectively adjusted the N metabolism and growth in lettuce and decreased the N consumption of the NS.

Impact of gut passage and mucus secretion by the earthworm Lumbricus terrestris on mobility and speciation of arsenic in contaminated soil

Earthworms inhabiting arsenic contaminated soils may accelerate the leaching of As into surface and ground waters. We carried out three experiments to determine the impact of passage of As contaminated soil (1150 mg As kg(-1)) through the gut of the earthworm Lumbricus terrestris on the mobility and speciation of As and the effects of earthworm mucus on As mobility. The concentration of water soluble As in soil increased (from 1.6 to 18 mg kg(-1)) after passage through the earthworm gut. Casts that were aged for 56 days still contained more than nine times greater water soluble As than bulk earthworm inhabited soil. Changes were due to increases in As(V) mobility, with no change in As(III). Dilute mucus extracts reduced As mobility through the formation of As-amino acid-iron oxide ternary complexes. More concentrated mucus extracts increased As mobility. These changes, together with those due to the passage through the gut, were due to increases in pH, phosphate and soluble organic carbon. The mobilisation of As from contaminated soils in the environment by cast production and mucus secretion may allow for accelerated leaching or uptake into biota which is underestimated when bulk soil samples are analysed and the influence of soil biota ignored.