Evaluation of the phytostabilisation efficiency in a trace elements contaminated soil using soil health indicators

Enhancing the Resilience of Agroecosystems Through Improved Rhizosphere Processes: A Strategic Review

As farming practices evolve and climate conditions shift, achieving sustainable food production for a growing global population requires innovative strategies to optimize environmentally friendly practices and minimize ecological impacts. Agroecosystems, which integrate agricultural practices with the surrounding environment, play a vital role in maintaining ecological balance and ensuring food security. Rhizosphere management has emerged as a pivotal approach to enhancing crop yields, reducing reliance on synthetic fertilizers, and supporting sustainable agriculture. The rhizosphere, a dynamic zone surrounding plant roots, hosts intense microbial activity fueled by root exudates. These exudates, along with practices such as green manure application and intercropping, significantly influence the soil's microbial community structure. Beneficial plant-associated microbes, including Trichoderma spp., Penicillium spp., Aspergillus spp., and Bacillus spp., play a crucial role in improving nutrient cycling and promoting plant health, yet their interactions within the rhizosphere remain inadequately understood. This review explores how integrating beneficial microbes, green manures, and intercropping enhances rhizosphere processes to rebuild microbial communities, sequester carbon, and reduce greenhouse gas emissions. These practices not only contribute to maintaining soil health but also foster positive plant-microbe-rhizosphere interactions that benefit entire ecosystems. By implementing such strategies alongside sound policy measures, sustainable cropping systems can be developed to address predicted climate challenges. Strengthening agroecosystem resilience through improved rhizosphere processes is essential for ensuring food security and environmental sustainability in the future. In conclusion, using these rhizosphere-driven processes, we could develop more sustainable and resilient agricultural systems that ensure food security and environmental preservation amidst changing climate situations.

The scientific landscape of phytoremediation of tailings: a bibliometric and scientometric analysis

This article seeks to evaluate the scientific landscape of the phytoremediation of mine tailings through a series of bibliometric and scientometric techniques. Phytoremediation has emerged as a sustainable approach to remediate metal-contaminated mine waste areas. A scientometric analysis of 913 publications indexed in Web of Science from 1999 to 2023 was conducted using CiteSpace. The results reveal an expanding, interdisciplinary field with environmental sciences as the core category. Keyword analysis of 561 nodes and 2,825 links shows a focus on plant-metal interactions, microbial partnerships, bioavailability, and field validation. Co-citation analysis of 1,032 nodes and 2,944 links identifies seminal works on native species, plant-microbe interactions, and amendments. Temporal mapping of 15 co-citation clusters indicates a progression from early risk assessments and native plant inquiries to integrated biological systems, economic feasibility, and sustainability considerations. Recent trends emphasize multidimensional factors influencing adoption, such as plant-soil-microbe interactions, organic amendments, and field-scale performance evaluation. The findings demonstrate an intensifying translation of phytoremediation from scientific novelty to engineering practice. This quantitative and qualitative analysis of research trends aids in understanding the development of phytoremediation for mine tailings. The results provide valuable insights for researchers and practitioners in this evolving field.

Sewage sludge fertilization affects microbial community structure and its resistome in agricultural soils

Global sewage sludge production is rapidly increasing, and its safe disposal is becoming an increasingly serious issue. One of the main methods of municipal sewage sludge management is based on its agricultural use. The wastewater and sewage sludge contain numerous antibiotic resistance genes (ARGs), and its microbiome differs significantly from the soil microbial community. The aim of the study was to assess the changes occurring in the soil microbial community and resistome after the addition of sewage sludge from municipal wastewater treatment plant (WWTP) in central Poland, from which the sludge is used for fertilizing agricultural soils on a regular basis. This study used a high-throughput shotgun metagenomics approach to compare the microbial communities and ARGs present in two soils fertilized with sewage sludge. The two soils represented different land uses and different physicochemical and granulometric properties. Both soils were characterized by a similar taxonomic composition of the bacterial community, despite dissimilarities between soils properties. Five phyla predominated, viz. Planctomycetes, Actinobacteria, Proteobacteria, Chloroflexi and Firmicutes, and they were present in comparable proportions in both soils. Network analysis revealed that the application of sewage sludge resulted in substantial qualitative and quantitative changes in bacterial taxonomic profile, with most abundant phyla being considerably depleted and replaced by Proteobacteria and Spirochaetes. In addition, the ratio of oligotrophic to copiotrophic bacteria substantially decreased in both amended soils. Furthermore, fertilized soils demonstrated greater diversity and richness of ARGs compared to control soils. The increased abundance concerned mainly genes of resistance to antibiotics most commonly used in human and animal medicine. The level of heavy metals in sewage sludge was low and did not exceed the standards permitted in Poland for sludge used in agriculture, and their level in fertilized soils was still inconsiderable.

Construction of an effective method combining in situ capping with electric field-enhanced biodegradation for treating PAH-contaminated soil at abandoned coking sites

The simultaneous application of in situ capping and electro-enhanced biodegradation may be a suitable method for ensuring the feasibility and safety of reusing abandoned coking sites. However, the capping layer type and applied electric field pattern may affect the efficiency of sequestering and removing pollutants. This study investigated changes in electric current, soil moisture content and pH, polycyclic aromatic hydrocarbon (PAH) concentration, bacterial number, and microbial community structure and metabolic function during soil remediation at abandoned coking plant sites under different applied electric field patterns and barrier types. The results indicated that polarity-reversal electric field was more conducive to maintaining electric current, soil properties, resulting in higher microbial number, community diversity, and functional gene abundance. At 21d, the mean PAH concentrations in contaminated soil, the capping layer's clean soil and barrier were 78.79, 7.56, and 1.57 mg kg-1 lower than those with a unidirectional electric field, respectively. The mean degradation rate of PAHs in the bio-barrier was 10.12 % higher than that in the C-Fe barrier. In the experiment combining a polarity-reversal electric field and a bio-barrier, the mean PAH concentrations in contaminated soil and the capping layer were 706.68 and 27.15 mg kg-1 lower than those in other experiments, respectively, and no PAHs were detected in the clean soil, demonstrating that the combination of the polarity-reversal electric field and the bio-barrier was effective in treating soil at abandoned coking plant sites. The established method of combining in situ capping with electro-enhanced biodegradation will provide technical support for the treatment and reuse of heavily PAH-contaminated soil at abandoned coking plant sites.

Influence of conditioner and straw on the herbaceous plant-based phytoremediation copper tailings: a field trial at Liujiagou tailings pond, China

A field trial was performed to carry out an enhanced phytoremediation technique for multi-metal contaminated copper tailings by Sudan grass (Sorghum Sudanese), ryegrass (Lolium perenne L.), and Bermuda grass (Cynodon dactylon), using conditioner (TH-LZ01) and straw combination into composite amendments as soil amendments, aimed to obtain the maximum of phytoremediation effect. The results showed that compared with untreated herbaceous plants, the application of conditioner and straw planted with herbaceous plants reduced the pH and conductivity and increased the organic matter and water content of the copper tailings to different degrees. With the addition of conditioner and straw, the DTPA-Cd, DTPA-Cu, DTPA-Pb, and DTPA-Zn contents in the copper tailings showed a decreasing trend compared with the untreated group. The herbaceous plants were promoted to reduce the percentage contents of acid soluble fractions Cd, Cu, Pb, and Zn and to increase the percentage contents of reducible, oxidizable, and residual fractions heavy metals (Cd, Cu, Pb, and Zn) in the copper tailings to different degrees. The contents of Cd, Cu, Pb, and Zn in the underground part of herbaceous plants were higher than those in the aboveground part, and the contents of Cd, Cu, Pb, and Zn in the aboveground part and underground part decreased after adding conditioner and straw, which indicated that the conditioner and straw inhibited the transport of heavy metals in the plant. Furthermore, the principal component analysis showed that the application of conditioner and straw with planting ryegrass had more potential for improving the physicochemical properties of copper tailings and reducing heavy metal toxicity, followed by Bermuda grass and Sudan grass.

Heavy fuel oil-contaminated soil remediation by individual and bioaugmentation-assisted phytoremediation with Medicago sativa and with cold plasma-treated M. sativa

Developing an optimal environmentally friendly bioremediation strategy for petroleum products is of high interest. This study investigated heavy fuel oil (HFO)-contaminated soil (4 and 6 g kg-1) remediation by individual and combined bioaugmentation-assisted phytoremediation with alfalfa (Medicago sativa L.) and with cold plasma (CP)-treated M. sativa. After 14 weeks of remediation, HFO removal efficiency was in the range between 61 and 80% depending on HFO concentration and remediation technique. Natural attenuation had the lowest HFO removal rate. As demonstrated by growth rate and biomass acquisition, M. sativa showed good tolerance to HFO contamination. Cultivation of M. sativa enhanced HFO degradation and soil quality improvement. Bioaugmentation-assisted phytoremediation was up to 18% more efficient in HFO removal through alleviated HFO stress to plants, stimulated plant growth, and biomass acquisition. Cold plasma seed treatment enhanced HFO removal by M. sativa at low HFO contamination and in combination with bioaugmentation it resulted in up to 14% better HFO removal compared to remediation with CP non-treated and non-bioaugmented M. sativa. Our results show that the combination of different remediation techniques is an effective soil rehabilitation strategy to remove HFO and improve soil quality. CP plant seed treatment could be a promising option in soil clean-up and valorization.

Mixing Compost and Biochar Can Enhance the Chemical and Biological Recovery of Soils Contaminated by Potentially Toxic Elements

Biochar and compost are able to influence the mobility of potentially toxic elements (PTEs) in soil. As such, they can be useful in restoring the functionality of contaminated soils, albeit their effectiveness can vary substantially depending on the chemical and/or the (micro)biological endpoint that is targeted. To better explore the potential of the two amendments in the restoration of PTE-contaminated soils, biochar, compost (separately added at 3% w/w), and their mixtures (1:1, 3:1, and 1:3 biochar-to-compost ratios) were added to contaminated soil (i.e., 2362 mg kg-1 of Sb and 2801 mg kg-1 of Zn). Compost and its mixtures promoted an increase in soil fertility (e.g., total N; extractable P; and exchangeable K, Ca, and Mg), which was not found in the soil treated with biochar alone. All the tested amendments substantially reduced labile Zn in soil, while biochar alone was the most effective in reducing labile Sb in the treated soils (-11% vs. control), followed by compost (-4%) and biochar-compost mixtures (-8%). Compost (especially alone) increased soil biochemical activities (e.g., dehydrogenase, urease, and β-glucosidase), as well as soil respiration and the potential catabolic activity of soil microbial communities, while biochar alone (probably due to its high adsorptive capacity towards nutrients) mostly exhibited an inhibitory effect, which was partially mitigated in soils treated with both amendments. Overall, the biochar-compost combinations had a synergistic effect on both amendments, i.e., reducing PTE mobility and restoring soil biological functionality at the same time. This finding was supported by plant growth trials which showed increased Sb and Zn mineralomass values for rigid ryegrass (Lolium rigidum Gaud.) grown on biochar-compost mixtures, suggesting a potential use of rigid ryegrass in the compost-biochar-assisted phytoremediation of PTE-contaminated soils.

Techno-economic analysis of phytoremediation: A strategic rethinking

Phytoremediation is a cost-effective and environmentally sound approach, which uses plants to immobilize/stabilize, extract, decay, or lessen toxicity and contaminants. Despite successful evidence of field application, such as natural attenuations, and self-purification, the main barriers remain from a "promising" to a "commercial" approach. Therefore, the ultimate goal of this paper is to examine factors that contribute to phytoremediation's underutilization and discuss the real costs of phytoremediation when the time and land values are considered. We revisit mechanisms and processes of phytoremediation. We synthesize existing information and understanding based on previous works done on phytoremediation and its applications to provide the technical assessment and perspective views in the commercial acceptance of phytoremediation. The results show that phytoremediation is the most suitable for remote regions with low land values. Since these regions allow a longer period to be restored, land vegetation covers can be established in more or less time like natural attenuation. Since the length of phytoremediation is an inherent limitation, this inherent disadvantage limits its adoption in developed business regions, such as growing urban areas. Because high land values could not be recovered in the short term, phytoremediation is not cost-effective in those regions. We examine the potential measures that can enhance the performance of phytoremediation, such as soil amendments, and agricultural practices. The results obtained through review can clarify where/what conditions phytoremediation can provide the most suitable solutions at a large scale. Finally, we identify the main barriers and knowledge gaps to establishing a vegetation cover in large-scale applications and highlight the research priorities for increased acceptance of phytoremediation.

Vertical distribution of nutrients, enzyme activities, microbial properties, and heavy metals in zinc smelting slag site revegetated with two herb species: Implications for direct revegetation

Direct revegetation is an important measure to immobilize heavy metals and improve the microecological properties of metal smelting slag sites. However, the vertical distribution of nutrients, microecological properties, and heavy metals at a directly revegetated metal smelting slag site remains unclear. Here, the distribution characteristics of nutrients, enzyme activities, microbial properties, and heavy metals in the vertical profile at a zinc smelting slag site directly revegetated with two herb species (Lolium perenne and Trifolium repens) for 5 years were investigated. The results showed that the nutrient contents, enzyme activities, and microbial properties decreased with increasing slag depth after revegetation with the two herb species. The nutrient contents, enzyme activities, and microbial properties of the surface slag revegetated with Trifolium repens were better than those in the surface slag revegetated with Lolium perenne_. The higher root activity in the surface slag (0-30 cm) resulted in relatively higher contents of pseudo-total and available heavy metals in the surface slag. Moreover, the contents of pseudo-total heavy metals (except for Zn) and available heavy metals in the slag revegetated with Trifolium repens were lower than those in the slag revegetated with Lolium perenne at most slag depths. Overall, the greater phytoremediation efficiency of the two herb species occurred mainly in the surface slag (0-30 cm), and the phytoremediation efficiency of Trifolium repens was higher than that of Lolium perenne. The findings are beneficial for understanding the phytoremediation efficiency of direct revegetation strategies for metal smelting slag sites.

Past, present and future trends in the remediation of heavy-metal contaminated soil - Remediation techniques applied in real soil-contamination events

Most worldwide policy frameworks, including the United Nations Sustainable Development Goals, highlight soil as a key non-renewable natural resource which should be rigorously preserved to achieve long-term global sustainability. Although some soil is naturally enriched with heavy metals (HMs), a series of anthropogenic activities are known to contribute to their redistribution, which may entail potentially harmful environmental and/or human health effects if certain concentrations are exceeded. If this occurs, the implementation of rehabilitation strategies is highly recommended. Although there are many publications dealing with the elimination of HMs using different methodologies, most of those works have been done in laboratories and there are not many comprehensive reviews about the results obtained under field conditions. Throughout this review, we examine the different methodologies that have been used in real scenarios and, based on representative case studies, we present the evolution and outcomes of the remediation strategies applied in real soil-contamination events where legacies of past metal mining activities or mine spills have posed a serious threat for soil conservation. So far, the best efficiencies at field-scale have been reported when using combined strategies such as physical containment and assisted-phytoremediation. We have also introduced the emerging problem of the heavy metal contamination of agricultural soils and the different strategies implemented to tackle this problem. Although remediation techniques used in real scenarios have not changed much in the last decades, there are also encouraging facts for the advances in this field. Thus, a growing number of mining companies publicise in their webpages their soil remediation strategies and efforts; moreover, the number of scientific publications about innovative highly-efficient and environmental-friendly methods is also increasing. In any case, better cooperation between scientists and other soil-related stakeholders is still required to improve remediation performance.

Biorecovery of olive mill wastewater sludge from evaporation ponds

Olive mill wastewater (OMW) resulting from the olive oil extraction process is usually disposed of in evaporation ponds where it concentrates generating a sludge that pollutes the ponds nearby area. In this study, four bio-treatments were applied for the in-situ bioremediation and valorization of OMW sludge: Landfarming, phytoremediation, composting and vermicomposting. In all cases, the OMW sludge was added with organic residues (mushroom compost, rabbit manure, and chicken manure). The bio-treatments were carried out in duplicate, inoculated and non-inoculated, to determine the effect of a specialized fungal consortium (Aspergillus ochraceus H2 and Scedosporium apiospermum H16) on the efficacy of the bio-treatments. The evaluation of chemical parameters, toxicity, and functional microbial biodiversity revealed that the four techniques depleted the toxicity and favored the stimulation of functional microbiota. Landfarming and phytoremediation allowed the decontamination and improvement of soils. Composting and vermicomposting also offered high-quality products of agronomic interest. Inoculation improved the bioremediation effectiveness. Biological treatments are effective for the safe recovery of contaminated OMW sludge into high-quality services and products.

Metal Lability and Mass Transfer Response to Direct-Planting Phytostabilization of Pyritic Mine Tailings

Understanding the temporal effects of organic matter input and water influx on metal lability and translocation is critical to evaluate the success of the phytostabilization of metalliferous mine tailings. Trends of metal lability, e.g., V, Cr, Mn, Co, Ni, Cu, Zn, and Pb, were investigated for three years following a direct-planting phytostabilization trial at a Superfund mine tailings site in semi-arid central Arizona, USA. Unamended tailings were characterized by high concentrations (mmol kg−1) of Fe (2100), S (3100), As (41), Zn (39), and Pb (11), where As and Pb greatly exceeded non-residential soil remediation levels established by Arizona. Phytostabilization treatments included a no-compost control, 100 g kg−1 compost with seed, and 200 g kg−1 compost with and without seed to the top 20 cm of the tailings profile. All plots received supplemental irrigation, effectively doubling the mean annual precipitation. Tailings cores up to 90 cm were collected at the time of planting and every summer for 3 years. The cores were sub-sectioned at 20 cm increments and analyzed via total digestion and an operationally defined sequential extraction for elemental analysis and the calculation of a mass transfer coefficient normalized to Ti as an assigned immobile element. The results indicate that Pb was recalcitrant and relatively immobile in the tailings environment for both the uncomposted control and composted treatments with a maximum variation in the total concentration of 9–14 mmol kg−1 among all samples. Metal lability and translocation above the redox boundary (ca. 30 cm depth) was governed by acid generation, where surficial pH was measured as low as 2.7 ± 0.1 in year three and strongly correlated with the increased lability of Mn, Co, Ni, Cu, and Zn. There was no significant pH effect on the lability of V, Cr, or Pb. Translocation to depths was greatest for Mn and Co; however, Zn, Ni, Cr, and Cu were also mobilized. The addition of organic matter enhanced the mobilization of Cr from the near surface to 40–60 cm depth (pH > 6) over the three-year phytostabilization study compared to the control. The increased enrichment of some metals at 60–90 cm indicates that the long-term monitoring of elemental translocation is necessary to assess the efficacy of phytostabilization to contain subsurface metal contaminants and thereby protect the surrounding community from exposure.

Intra-soil waste recycling provides safety of environment

Amelioration and remediation technology was developed for phosphogypsum utilization in Haplic Chernozem of South-European facies (Rostov Region). The technology comprises phosphogypsum dispersed application into the soil layer of 20-45 cm during intra-soil milling. In the model experiment, the phosphogypsum doses 0 (control), 10, 20, and 40 t ha^-1 were studied. The Cd thermodynamic forms in soil solution were calculated via the developed mathematical chemical-thermodynamic model and program ION-3. The form of ion in soil solution (or water extract) was considered accounting the calcium-carbonate equilibrium (CCE) and association of ion pairs CaCO₃⁰; CaSO₄⁰, MgCO₃⁰, MgSO₄⁰, CaHCO₃⁺, MgHCO₃⁺, NaCO₃^-, NaSO₄^-, CaOH⁺, MgOH⁺. For calculation of the equilibrium of microelements concentration in soil solution ion including heavy metals (HMs), the coefficient of microelement association k_as was proposed. According to calculations, Cd²⁺ ion in soil solution was mostly bounded to associates CdOH⁺, partly to associates CdCO₃⁰ and CdHCO₃⁺. The calculated k_as of Cd was 1.24 units in the control option of experiment and decreased to 0.95 units at phosphogypsum dose 40 t ha^-1. The ratio of "active [Cd²⁺] to total Cd" reduced from 33.5% in control option to 28.0% in the option of phosphogypsum dose 40 t ha^-1. The biogeochemical barrier for penetration of HMs from soil to plant roots was high after application of phosphogypsum. According to calculation by ION-3, the standard soil environmental limitations overestimate the toxicity of Cd in soil solution. New decision for intra-soil milling and simultaneous application of phosphogypsum was developed to provide the environmentally safe waste recycling.

Anaerobic digestate management, environmental impacts, and techno-economic challenges

Digestate is a nutrient-rich by-product from organic waste anaerobic digestion but can contribute to nutrient pollution without comprehensive management strategies. Some nutrient pollution impacts include harmful algal blooms, hypoxia, and eutrophication. This contribution explores current productive uses of digestate by analyzing its feedstocks, processing technologies, economics, product quality, impurities, incentive policies, and regulations. The analyzed studies found that feedstock, processing technology, and process operating conditions highly influence the digestate product characteristics. Also, incentive policies and regulations for managing organic waste by anaerobic digestion and producing digestate as a valuable product promote economic benefits. However, there are not many governmental and industry-led quality assurance certification systems for supporting commercializing digestate products. The sustainable and safe use of digestate in different applications needs further development of technologies and processes. Also, incentives for digestate use, quality regulation, and social awareness are essential to promote digestate product commercialization as part of the organic waste circular economy paradigm. Therefore, future studies about circular business models and standardized international regulations for digestate products are needed.

Impacts of corn stover management and fertilizer application on soil nutrient availability and enzymatic activity

Corn stover is a global resource used in many industrial sectors including bioenergy, fuel, and livestock operations. However, stover removal can negatively impact soil nutrient availability, especially nitrogen (N) and phosphorus (P), biological activity, and soil health. We evaluated the effects of corn stover management combined with N and P fertilization on soil quality, using soil chemical (nitrate, ammonium and Bray-1 P) and biological parameters (β-glucosidase, alkaline phosphatase, arylsulfatase activities and fluorescein diacetate hydrolysis—FDA). The experiment was performed on a Mollisol (Typic Endoaquoll) in a continuous corn system from 2013 to 2015 in Minnesota, USA. The treatments tested included six N rates (0 to 200 kg N ha⁻¹), five P rates (0 to 100 kg P₂O₅ ha⁻¹), and two residue management strategies (residue removed or incorporated) totalling 60 treatments. Corn stover management significantly impacted soil mineral-N forms and enzyme activity. In general, plots where residue was incorporated were found to have high NH₄⁺ and enzyme activity compared to plots where residue was removed. In contrast, fields where residue was removed showed higher NO₃⁻ than plots where residue was incorporated. Residue management had little effect on soil available P. Soil enzyme activity was affected by both nutrient and residue management. In most cases, activity of the enzymes measured in plots where residue was removed frequently showed a positive response to added N and P. In contrast, soil enzyme responses to applied N and P in plots where residue was incorporated were less evident. Soil available nutrients tended to decrease in plots where residue was removed compared with plots where residue was incorporated. In conclusion, stover removal was found to have significant potential to change soil chemical and biological properties and caution should be taken when significant amounts of stover are removed from continuous corn fields. The residue removal could decrease different enzymes related to C-cycle (β-glucosidase) and soil microbial activity (FDA) over continuous cropping seasons, impairing soil health.

Potentially Toxic Elements’ Contamination of Soils Affected by Mining Activities in the Portuguese Sector of the Iberian Pyrite Belt and Optional Remediation Actions: A Review

Both sectors of the Iberian Pyrite Belt, Portuguese and Spanish, have been exploited since ancient times, but more intensively during and after the second half of the 19th century. Large volumes of polymetallic sulfide ore were extracted in open pits or in underground works, processed without environmental concerns, and the generated waste rocks and tailings were simply deposited in the area. Many of these mining sites were abandoned for years under the action of erosive agents, leading to the spread of trace elements and the contamination of soils, waters and sediments. Some of these mine sites have been submitted to rehabilitation actions, mostly using constructive techniques to dig and contain the contaminated tailings and other waste materials, but the remaining soil still needs to be treated with the best available techniques to recover its ecosystem functions. Besides the degraded physical structure and poor nutritional status of these soils, they have common characteristics, as a consequence of the pyrite oxidation and acid drainage produced, such as a high concentration of trace elements and low pH, which must be considered in the remediation plans. This manuscript aims to review the results from studies which have already covered these topics in the Iberian Pyrite Belt, especially in its Portuguese sector, considering: (i) soils’ physicochemical characteristics; (ii) potentially toxic trace elements’ concentration; and (iii) sustainable remediation technologies to cope with this type of soil contamination. Phytostabilization, after the amelioration of the soil’s properties with organic and inorganic amendments, was investigated at the lab and field scale by several authors, and their results were also considered.

Effect of different direct revegetation strategies on the mobility of heavy metals in artificial zinc smelting waste slag: Implications for phytoremediation

The establishment of vegetation cover is an important strategy to reduce wind and water erosion at metal smelting waste slag sites. However, the mobility of heavy metals in waste slag-vegetation-leachate systems after the application of revegetation strategies is still unclear. Large microcosm experiments were conducted for revegetation of waste slag for 98 d using combined amendments, i.e., phosphate rock and an organic waste coming from the anaerobic digestion of pig manure (named as biogas residue), and by single- and co-planted perennial ryegrass (Lolium perenne L.) and Trifolium repens (T. repens). The results showed that the application of biogas residue slightly increased the concentrations of Zn and Cd in the leachates; however, the establishment of plants could avoid the excessive leaching of heavy metals coming from the biogas residue. The bioavailability of Cu, Zn, and Cd slightly increased, but Pb bioavailability significantly decreased regardless of single- or co-planting patterns. Additionally, the bioavailability of Cu, Zn, and Cd in the waste slag revegetated with perennial ryegrass was lower than that in T. repens under the single-planting pattern. The change in the heavy metals bioavailability under different revegetation strategies was mainly due to the root-induced change in the pH and speciation of heavy metals in the waste slag. The application of biogas residue and phosphate rock tends to the immobilization of Pb. Heavy metals mainly accumulated in the underground parts of the two herbs, and the heavy metal contents in the underground parts of perennial ryegrass were higher than those in T. repens regardless of single- or co-planting patterns. The heavy metals accumulated in T. repens were lower than those in perennial ryegrass in the single-planting pattern. The bioaccumulation and transportation factors of the two herbs were extremely low. Thus, the two herbs are potential candidates for phytostabilization of zinc smelting waste slag sites.

Interactions among heavy metal bioaccessibility, soil properties and microbial community in phyto-remediated soils nearby an abandoned realgar mine

Soil samples were collected from a representative arsenic (As) contaminated region under phytoremediation of hyperaccumulation plants. Relative abundance and diversity of microbial communities in the soil samples were characterized via 16S rRNA genes sequencing. At the phylum level, Proteobacteria, Actinobacteria, Acidobacteria, Bacteroidetes, Gemmatimonadetes and Firmicutes shows the highest abundance, accounting for more than 90 % of the classified sequences in the soil samples. Physicochemical parameters including pH, total organic carbon (TOC), cation exchange capacity (CEC), and electrical conductivity (EC), and heavy metal concentrations including total and bioaccessible contents in the soil samples were determined to investigate potential relationships between the microbial communities and the environmental factors. Principal component analysis (PCA) based on the operational taxonomic units (OTUs) matrix revealed distinct separation among the samples. The soil pH was confirmed as the dominant force to discriminate the soil samples with similar land use type and heavy metal contamination. There was little relevance between the total concentrations of heavy metals and the microbial communities. However, the bioaccessible concentrations of heavy metals were associated with the physicochemical parameters and relative abundances of bacterial genera according to correlation analyses. Although the soil samples were considerably contaminated by As, the abundances of bacterial phyla linked with As were lower than 1.0 % in most of the soil samples. The results indicated that the abundances of microbial communities in the soils were the consequence of concerted effects from all the environmental factors.

Effects of Herbaspirillum sp. p5-19 assisted with alien soil improvement on the phytoremediation of copper tailings by Vetiveria zizanioides L

Microbial assisted phytoremediation and reclamation are both potential contaminated soil remediation technologies, but little is known about the combined application of the two technologies on real contaminated soils. This study investigated the potential of Herbaspirillum sp. p5-19 (p5-19) assisted with alien soil improvement on improving stress tolerance and enhancing the accumulation of Mn, Cu, Zn, and Cd by Vetiveria zizanioides L. in copper tailings. Phytoremediation potential was evaluated by plant biomass and the ability of plants to absorb and transfer heavy metals. Results showed that the biomass was increased by 19.64-173.81% in p5-19 inoculation treatments with and without alien soil improvement compared with control. Meanwhile, photosynthetic pigment contents were enhanced in co-inoculation treatment (p5-19 with alien soil improvement). In addition, the malondialdehyde (MDA) content was decreased, and the activities of antioxidant enzymes such as ascorbate peroxidase (APX), superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) were increased in p5-19 treatment, thereby alleviating the oxidative stress. Moreover, co-inoculation significantly (p < 0.05) increased the concentrations of Mn, Cu, Zn, and Cd in the roots and shoots of V. zizanioides. In particular, the highest concentrations of Mn, Zn, and Cd in the shoots (roots) were obtained in covering 10 cm combined with p5-19 inoculation treatment, which were 4.44- (2.71-), 4.73- (3.87-), and 5.93- (4.35-) fold as that of the controls, respectively. These results provided basis for the change of phytoremediation ability of V. zizanioides after inoculation. We concluded that p5-19 assisted with alien soil improvement was a potential strategy for enhancing phytoremediation ability in tailings.

Benefits of Corn-Cob Biochar to the Microbial and Enzymatic Activity of Soybean Plants Grown in Soils Contaminated with Heavy Metals

Synchronous effects of biochar on heavy metals stress, microbial activity and nodulation process in the soil are rarely addressed. This work studied the effects, under greenhouse conditions, of selected heavy metals Cd2+, Pb2+ and Ni2+ on soybean plants grown in two different soils amended with biochar, and studied their effect on the microbial and enzymatic activity. As a result of the interference between heavy metals and biochar, biochar overcame heavy metal problems and maintained a microbial population of major groups (bacteria–fungi). There was an increase in the degree of resistance (RS) of the major microbial groups to heavy metals when biochar was added to the soil under study. Numbers of bacterial nodules significantly increased, particularly by using the higher rate of biochar compared to the control, either by adding biochar alone or by mixing it with the selected heavy metals. The arginase activity was increased by 25.5% and 37.1% in clay and sandy soil, respectively, compared to the control. For urease (UR), the activity was increased by 105% and 83.8% in clay and sandy soil, respectively, compared to the control. As a result, considerations of using biochar as a soil amendment should be first priority.

The rhizosphere of Salix viminalis plants after a phytostabilization process assisted by biochar, compost, and iron grit: chemical and (micro)-biological analyses

Amendments, such as biochar, compost, and iron grit, used in phytostabilization studies, showed positive effects on soil physico-chemical properties, plant growth, and the microbial community. However, assisted phytostabilization studies do not always focus on the rhizosphere area where soil, plants, and microorganisms are affected by the amendments and plants and microorganisms can also interact with each other. The aims of this study were to evaluate the effects of amendment application on the exudation of organic acids by Salix viminalis plant roots, as well as the effects of amendments and plant development on the soil CHNS contents and the microbial community activity and diversity, assessed by measuring enzyme activities and using Biolog EcoPlates^TM tests and next-generation sequencing analyses. The results of the mesocosm experiment showed that soil C, H, and N contents were increased by amendment application, especially biochar and compost, while the one of S decreased. Enzyme activities, microbial activity, and diversity were also increased by the addition of amendments, except iron grit alone. Finally, the quantity of organic acids exuded by roots were little affected by amendments, which could in part explain the reduced effect of plant development on soil chemical and microbiological parameters. In conclusion, this study showed in particular that biochar and compost were beneficial for the soil CHN contents and the microbial community while affecting poorly Salix viminalis root exudates.

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.

Enhanced Carbon Sequestration in Marginal Land Upon Shift towards Perennial C4Miscanthus × giganteus: A Case Study in North-Western Czechia

Bioenergy crops such as Miscanthus × giganteus are foreseeable as an alternative source to replace fossil fuel and reduce greenhouse gas emissions. They are also assessed as an environment-friendly solution for polluted, marginal and low-quality agricultural soils. Several studies had been launched on soil organic carbon sequestration potentials of miscanthus culture along with its impacts on restoring soil functionality, most of which focus on the long-term basis of the plant’s cultivation. Nevertheless, information concerning the short term impacts as well as the situation in Czechia is still scarce. In this context, a field experiment was launched in 2017 in a poor-quality agricultural land in the city of Chomutov (North-Western Czechia) to compare the impacts of the perennial C4 miscanthus with an annual C3 forage crop (wheat) on the soil carbon stocks as well as enhancing its functionality. Results through the 0–30 cm soil profile examination showed that miscanthus plants played a role in improving the studied soil physico-chemical (bulk density and soil organic carbon concentrations) and biological (Phospholipid fatty acids stress indicator, basal respiration and fluorescein diacetate hydrolytic activity) parameters. The naturally occurring δ13C concentrations were used to evaluate the direct plant contribution to the total soil organic carbon (SOC) stocks and revealed considerable miscanthus contribution all over the detected soil layers (1.98 ± 0.21 Mg C. ha−1 yr−1) after only 3 growing seasons. It is thus suggested that the C4 perennial miscanthus possess remarkable prospects for SOC sequestration and restoring degraded lands.

Effects of hydroxyl, carboxyl, and amino functionalized carbon nanotubes on the functional diversity of microbial community in riverine sediment

Nowadays, more and more attention is focused on the environmental harm brought by the wide production and use of carbon nanotubes. In this study, the metabolic function of sediment microbial community was investigated after unfunctionalized or functionalized multi-walled carbon nanotubes (MWCNTs) were incorporated. The surface functional groups on the studied functionalized MWCNTs in this work were hydroxyl, carboxyl, and amino, respectively. The metabolic functional diversity was determined by Biolog EcoPlates after one-month exposure to MWCNTs. Incorporating 0.5 wt% amino functionalized MWCNTs significantly decreased the microbial activity and diversity, and all types of MWCNTs caused great inhibition on the microbial metabolism at the dosage of 2.0 wt%. The sediment microbes preferred polymers and amino acids. Principal component and similarity analysis indicated that the microbial carbon metabolism was more affected by the MWCNT dosage compared with the functionalization, and 2.0 wt% amino functionalized MWCNTs made the greatest difference in metabolic function of sediment microbial community. These consequences may help to assess the environmental risks of MWCNTs from the aspect of ecological relevance of sediment microbial community.

Compost and Sewage Sludge for the Improvement of Soil Chemical and Biological Quality of Mediterranean Agroecosystems

Conventional fertilization practices in agroecosystems concern the supply of bioavailable nutrients, such as mineral fertilizers. A consolidated alternative to restoring the long-term fertility of agricultural soils is their amendment with organic fertilizers. Soil amendment with biowaste compost or sewage sludge represents a sustainable strategy to avoid the landfilling of organic matter derived from urban waste and sewage sludge. This study aims at validating the use of quality biowaste compost and sewage sludge from secondary sedimentation (alone or in combination with mineral fertilizers) in a Mediterranean agroecosystem and their effects on soil chemical and biological quality, with a view to verifying the maintenance of soil fertility and functionality. In particular, the dynamics of soil organic matter, pH, potentially toxic elements and microbial community functionality were assessed, in experimental mesocosms, during 6 months of incubation. The research showed that, while soil amendment with biowaste compost induces positive effects on soil organic matter and phosphorous concentrations, as well as on microbial community functionality, the amendment with the selected sewage sludge does not determine any benefit to the microbial community or any danger in relation to soil potentially toxic element concentrations and toxicity. The quantity of sewage sludge employed, chosen according to regional directives, was thus not enough to stimulate the edaphic microflora activity.

Use of Brassica juncea and Dactylis glomerata for the phytostabilization of mine soils amended with compost or biochar

Phytostabilization of mine soils contaminated by potentially toxic elements (PTEs) requires plants tolerant to PTE toxicity and to the poor soil physico-chemical characteristics of these areas. A pot experiment was carried out to assess the phytostabilization potential of Brassica juncea and Dactylis glomerata in mine soils amended with compost and biochar. Furthermore, the Environmental Risk of the soils and the effects of the phytostabilization process on the microbiological population size and activity in the soils were also determined. According to the Ecological Risk Index (ERI) the soils studied presented "very high risk" and As, Cd and Pb were the target elements for phytostabilization. Both amendments improved soil conditions (e.g., increasing total-N and total organic-C concentrations) and contributed to PTE (Cd, Pb and Zn) immobilization in the soil. Compost showed a more marked effect on soil microbial biomass and nutrients release in soil, which led to higher B. juncea and D. glomerata biomass in compost treated soils. Biochar treatment showed a positive effect only on D. glomerata growth, despite it provoked strong PTE immobilization in both soils. The addition of both amendments resulted in an overall reduction of PTE concentration in the plants compared to the control treatment. In addition, both plant species showed higher accumulation of PTE in the roots than in the shoots (transfer factor<1) independently of the treatment received. Therefore, they can be considered as good candidates for the phytostabilization of PTE contaminated mine soils in combination with organic amendments like biochar and compost.

Keep and promote biodiversity at polluted sites under phytomanagement

The phytomanagement concept combines a sustainable reduction of pollutant linkages at risk-assessed contaminated sites with the generation of both valuable biomass for the (bio)economy and ecosystem services. One of the potential benefits of phytomanagement is the possibility to increase biodiversity in polluted sites. However, the unique biodiversity present in some polluted sites can be severely impacted by the implementation of phytomanagement practices, even resulting in the local extinction of endemic ecotypes or species of great conservation value. Here, we highlight the importance of promoting measures to minimise the potential adverse impact of phytomanagement on biodiversity at polluted sites, as well as recommend practices to increase biodiversity at phytomanaged sites without compromising its effectiveness in terms of reduction of pollutant linkages and the generation of valuable biomass and ecosystem services.

The responses of soil enzyme activities, microbial biomass and microbial community structure to nine years of varied zinc application rates

Zinc (Zn) fertilizer application can certainly improve the production and nutritional quality of cereal crops. However, Zn accumulation in the soil may lead to some deleterious environmental impacts in agroecosystems. The effects of long-term Zn application on soil microbial properties remain unclear, but it is imperative to understand such effects. In this study, we collected soil samples from a nine-year field experiment in a wheat-maize system that continuously received Zn applied at various rates (0, 2.3, 5.7, 11.4, 22.7 and 34.1 kg ha^-1) to evaluate the soil enzymes, microbial biomass and microbial community structure. The results showed that Zn application at the rate of 5.7 kg ha^-1 significantly increased the activities of urease, invertase, alkaline phosphatase and catalase in the soil, while the rate of 34.1 kg ha^-1 significantly decreased the evaluated enzyme activities. The microbial biomass carbon (C) and nitrogen (N) were not affected by Zn application rates, although an increase in the microbial biomass C was observed in the 11.4 kg ha^-1 treatment. Moreover, the alpha diversity of the bacterial and fungal communities did not vary among the nil Zn, optimal Zn (5.7 kg ha^-1) and excess Zn (34.1 kg ha^-1) treatments. However, the bacterial communities in the soil receiving the optimal and excess Zn application rates were slightly changed. Compared to the nil Zn treatment, the other Zn application rates increased the relative abundances of the Rhodospirillales, Gaiellales and Frankiales orders and decreased the abundance of the Latescibacteria phylum. The redundancy analysis further indicated that the soil bacterial community composition significantly correlated with the concentrations of soil DTPA-Zn and total Zn. These results highlight the importance of optimal Zn application in achieving high production and high grain quality while concurrently promoting soil microbial activity, improving the bacterial community and further maintaining the sustainability of the agroecological environment.

Chemical Soil-Biological Engineering Theoretical Foundations, Technical Means, and Technology for Safe Intrasoil Waste Recycling and Long-Term Higher Soil Productivity

The amelioration and remediation technology was developed on the basis of research of phosphogypsum and utilization in the Haplic Chernozem of South-European facies (Rostov Region). Phosphogypsum was utilized via dispersed application into a soil layer of 20-45 cm with intrasoil milling of this layer. The phosphogypsum utilization doses were 0, 10, 20, and 40 t ha^-1. The Pb analytical content in soil solution was studied in the model experiment. The soil solution Pb thermodynamic forms were calculated. The mathematical chemical-thermodynamic model ION-2 was developed to calculate the real soil solution (water extract) calcium-carbonate equilibrium (CCE) ion forms, considering the ion pair association. The associated ion pairs CaCO₃ ⁰, CaSO₄ ⁰, MgCO₃ ⁰, MgSO₄ ⁰, CaHCO₃ ⁺, MgHCO₃ ⁺, NaCO₃ ^-, NaSO₄ ^-, CaOH⁺, and MgOH⁺ were accounted for in soil solution equilibrium macroion form calculation. The procedure for the microelement ion [including heavy metals (HMs)] equilibrium concentration in the soil solution coefficient k _as calculation was proposed to account for the real soil solution CCE, macroions, and HM (including Pb) association. The Pb²⁺ ion in soil solution was mostly bound to associates PbOH⁺, Pb(OH)₂ ⁰, PbCO₃ ⁰, Pb(CO₃)₂ ^2-, and PbHCO₃ ⁺. The calculation of CCE and ion association in soil solution revealed 14.5-21.5 times HM passivation compared to HM water-soluble values. The calculated HM activity in the soil solution in the example of the Pb²⁺ ion was less than 4% after phosphogypsum application in the target amelioration layer of 20-45 cm. The studied phosphogypsum doses were substantiated as environmentally safe. This was because the real soil solution CCE provided HM ion form association and consequent passivation. The dry steppe soil remediation after phosphogypsum application was justified as highly probable. The intrasoil milling chemical soil-biological engineering technology was developed for simultaneous soil amelioration and remediation on the basis of the biogeosystem technique (BGT*) transcendental methodology. The BGT*-based technology was tested in the long-term field experiments and is capable of ensuring the priority geophysical micro- and macroaggregate structure via intrasoil milling and mixing of soil illuvial and transitional horizons. This helps synthesize soil multilevel architecture, providing intrasoil-dispersed environmentally safe recycling of wastes of different origin. Addressing the environment safety concerns, a new decision of the intrasoil milling device was proposed for phosphogypsum and other substance application to soil.

Effects of Elevated CO2 Concentration and Nitrogen Addition on Soil Respiration in a Cd-Contaminated Experimental Forest Microcosm

Forests near rapidly industrialized and urbanized regions are often exposed to elevated CO2, increased N deposition, and heavy metal pollution. To date, the effects of elevated CO2 and/or increased N deposition on soil respiration (Rs) under heavy metal contamination are unclear. In this study, we firstly investigated Rs in Cd-contaminated model forests with CO2 enrichment and N addition in subtropical China. Results showed that Rs in all treatments exhibited similar clear seasonal patterns, with soil temperature being a dominant control. Cadmium addition significantly decreased cumulative soil CO2 efflux by 19% compared to the control. The inhibition of Rs caused by Cd addition was increased by N addition (decreased by 34%) was partially offset by elevated CO2 (decreased by 15%), and was not significantly altered by the combined N addition and rising CO2. Soil pH, microbial biomass carbon, carbon-degrading hydrolytic enzymes, and fine root biomass were also significantly altered by the treatments. A structural equation model revealed that the responses of Rs to Cd stress, elevated CO2, and N addition were mainly mediated by soil carbon-degrading hydrolytic enzymes and fine root biomass. Overall, our findings indicate that N deposition may exacerbate the negative effect of Cd on Rs in Cd-contaminated forests and benefit soil carbon sequestration in the future at increasing atmospheric CO2 levels.

Variations of soil microbial communities accompanied by different vegetation restoration in an open-cut iron mining area

Overexploitation of iron mining in China has caused serious environmental pollution. Therefore, establishing a stable ecological restoration with vegetation in mining areas has gradually aroused people's awareness and obtained extensive concerns. This study aimed to evaluate vegetation restoration with Robinia pseudoacacia (RP), Acer mono (AM) and Pinus koraiensis (PK) in iron mining compared with unrestored area, to investigate the soil environment factors and microbial communities, and to better understand the correlations between soil environment factors and soil microbial communities. Vegetation restoration could reduce soil pH and alleviate soil alkaline, and remarkably increase soil nutrients, especially in RP site. Analysis of 16S rRNA and ITS rRNA gene sequences provided a total of 645,004 and 906, 276 valid sequences clustered into 7091 OTUs and 1689 OTUs at a 0.03 genetic distance for bacteria and fungi, respectively. The predominant bacterial and fungal phyla were Actinobacteria and Ascomycota in studied sites, respectively. Additionally, revegetation significantly increased the relative abundances of Proteobacteria, Gemmatimonadetes, Bacteroidetes and Patescibacteria, and decreased the relative abundance of Actinobacteria. Robinia pseudoacacia harbored the highest soil fungal community diversity, and bacterial Simpson index and Shannon index. Vegetation restoration with RP could clearly shifted soil communities compared to AM and PK. Along with the restoration of vegetation, the remarkable abiotic changes were the accumulation of total C, total N, total P, available P, available N and available K and the decreasing of soil pH, which were the most important factors affecting soil microbial communities. Our results addressed that Robinia pseudoacacia was the best preferable species than AM and PK in improving soil nutrients, soil community diversity and structure in Fe mining, providing a helpful guideline for selection of tree species.

Copper stress in flooded soil: Impact on enzyme activities, microbial community composition and diversity in the rhizosphere of Salix integra

Climate change has increased flooding frequency, making the heavy metal polluted areas more vulnerable, and led to increased global land degradation. Information about the alteration of soil microbiota under heavy metal pollution and flooding is still rather limited. Fast-growing trees are candidates for phytoremediation of heavy metal polluted soils. Therefore, the impact of Cu pollution on microbiota in soil used for cultivating Salix integra Thunb. was investigated with and without flooding for 60 d. Bacterial and fungal communities were accessed via partial 16S rRNA (V₃-V₄) and internal transcribed spacer (ITS) genes. The activity of invertase, urease and cellulase were markedly decreased by 28.5-59%, 55.0-76.7% and 17.3-34.1%, respectively, with increasing Cu levels. Flooding significantly increased the activity of polyphenol oxidase and peroxidase by 56.3% and 41.4% at the highest Cu level compared to its respective non-flooded condition. High Cu concentration significantly decreased the richness and diversity of the bacterial community, and fungi were more sensitive than bacteria under flooding conditions. Redundancy analysis suggests that Cu, Fe and soil organic matter are the key determinants affecting the composition of microbial communities. Our findings provide new insight into the responses of soil microbes to Cu-contamination and contribute to our understanding of metal toxicity in soil-woody plant systems under flooded conditions.

An insight to municipal solid waste management of Varanasi city, India, and appraisal of vermicomposting as its efficient management approach

Varanasi, India's historic cultural capital, struggles with efficient waste management practices. This impacts environment and human well-being in terms of waste generation that is estimated around 550-650 TPD with a generation rate of 0.42 kg capita^-1 day^-1 (n = 117). The present study aims to explore and characterize wastes, current practices, ecological profiling, and phytotoxicity of an abandoned open dumping site, and vermicomposting of organic fraction of municipal solid waste (OFMSW) as sustainable waste management approach. Compositional analysis of waste indicates organic fraction (46.13%) as a major component along with a considerable amount of heavy metals. The calorific value and moisture content of municipal solid waste (MSW) was 2351.4 cal g^-1 and 34.72%, respectively. Ecological profiling of the dumping site revealed that floral diversity and ecological species/indicators were negatively affected. Likewise, phytotoxicity results displayed a negative impact on germination and physiology of maize (Zea mays L.) plants grown on dumping site soil. Vermistabilization of OFMSW showed a significant increase in N (56.10-89.48%), P (33.93-82.87%), and K (25.55-50.42%) and a decrease in total organic carbon (15.15-24.81%). Similarly, C/N and C/P ratios decreased by 1.89-2.51 and 1.72-2.18 folds, respectively. A survey of stakeholders suggested that open dumping was the main practice adopted by Varanasi Municipal Corporation (VMC) during 2013-2015. Recently (2017-2018), VMC adopted different methods, such as door-to-door collection and source segregation for effective waste management. Waste characteristics and nutrient profile of the vermicompost explains that vermicomposting could be used for efficient waste management in Varanasi, further reducing the collection, transportation, and disposal costs of waste, which enables to close the loop and move towards a circular economy. Moreover, implications of existing waste management practices and possible management options need to be addressed scientifically. Therefore, this research outcome will help in designing a successful waste management plan for Varanasi and other cities with similar waste characteristics.

Immobilization of Potentially Toxic Elements (PTE) by Mineral-Based Amendments: Remediation of Contaminated Soils in Post-Industrial Sites

In many post-industrial sites, the high contents and high mobility of different potentially toxic elements (PTEs) make the soils unsuitable for effective management and use. Therefore, immobilization of PTE seems to be the best remediation option for such areas. In the present study, soil samples were collected in post-industrial areas in Northeastern Poland. The analyzed soil was characterized by especially high contents of Cd (22 mg·kg−1), Pb (13 540 mg·kg−1), and Zn (8433 mg·kg−1). Yellow lupine (Lupinus luteus L.) and two types of mineral-based amendments were used to determine their combined remediation effect on PTE immobilization. A greenhouse pot experiment was conducted to evaluate the influence of chalcedonite and halloysite on plant growth, chlorophyll a fluorescence, the leaf greenness index (SPAD), PTE uptake, and the physicochemical properties and toxicity of soil. The application of chalcedonite resulted in the greatest increase in soil pH, whereas halloysite contributed to the greatest reduction in the contents of Ni, Pb, Zn, and Cr in soil, compared with the control treatment. The addition of halloysite significantly increased plant biomass. The application of mineral-based amendments increased the ratio of variable fluorescence to maximum chlorophyll fluorescence (Fv/Fm) in yellow lupine leaves. The leaf greenness index was highest in plants growing in soil amended with chalcedonite. The results of this study suggest that mineral-based amendments combined with yellow lupine could potentially be used for aided phytostabilization of multi-PTE contaminated soil in a post-industrial area.

Implications of mycoremediated dry olive residue application and arbuscular mycorrhizal fungi inoculation on the microbial community composition and functionality in a metal-polluted soil

Metal-polluted soils represent hostile environments affecting the composition and functions of soil microbial communities. This study evaluated the implication of combining the mycoremediated dry olive residue (MDOR) amendment application with the inoculation of the arbuscular mycorrhizal fungi (AMF) Funneliformis mosseae in restoring the quality, composition, and functionality of soil microbial communities. To achieve this aim, a mesocosms experiment was set up that included three variations: i) with and without application of Penicillium chrysogenum-10-transformed MDOR (MDOR_Pc), and Chondrosterum purpureum-transformed MDOR (MDOR_Cp) amendments; ii) with and without F. mosseae inoculation; and iii) 30-day and 60-day soil treatment time. As a result of this combined treatment, changes in the soil labile organic C and N fractions were observed throughout the experiment. Increases in the abundance of phospholipid fatty acids (PLFAs) for bacteria, actinobacteria, and Gram- and Gram+ bacteria were also recorded at the end of the experiment. The addition of MDOR amendments boosted fungal and AM fungi communities. AM fungi root and soil colonization was also enhanced as the result of improvement nutrient turnover and spatial conditions caused by adding MDOR in combination with an inoculation of F. mosseae. The composition and functionality of microbial communities seemed to be an important ecological attribute indicating an apparently fully functional restoration of this metal-polluted soil and therefore suggesting the suitability of the combined MDOR and AM fungus treatment as a reclamation practice.

Potential Benefits and Risks for Soil Health Derived From the Use of Organic Amendments in Agriculture

The use of organic amendments in agriculture is a common practice due to their potential to increase crop productivity and enhance soil health. Indeed, organic amendments of different origin and composition (e.g., animal slurry, manure, compost, sewage sludge, etc.) can supply valuable nutrients to the soil, as well as increase its organic matter content, with concomitant benefits for soil health. However, the application of organic amendments to agricultural soil entails a variety of risks for environmental and human health. Organic amendments often contain a range of pollutants, including heavy metals, persistent organic pollutants, potential human pathogens, and emerging pollutants. Regarding emerging pollutants, the presence of antibiotic residues, antibiotic-resistant bacteria, and antibiotic-resistance genes in agricultural amendments is currently a matter of much concern, due to the concomitant risks for human health. Similarly, currently, the introduction of microplastics to agricultural soil, via the application of organic amendments (mainly, sewage sludge), is a topic of much relevance, owing to its magnitude and potential adverse effects for environmental health. There is, currently, much interest in the development of efficient strategies to mitigate the risks associated to the application of organic amendments to agricultural soil, while benefiting from their numerous advantages.

Municipal solid wastes as a resource for environmental recovery: Impact of water treatment residuals and compost on the microbial and biochemical features of As and trace metal-polluted soils

In this study we evaluated the microbiological and biochemical impact of iron-based water treatment residuals (Fe-WTRs) and municipal solid waste compost (MSWC), alone and combined, on three different soils co-contaminated with arsenic (As) and trace-metals (TM), i.e. Pb, Cu and Zn. Overall, all the amendments considered significantly increased the abundance of culturable heterotrophic bacteria, with MSWC showing the greatest impact across all soils (up to a 24% increase). In most of treated soils this was accompanied by a significant reduction of both the (culturable) fungal/bacterial ratio, and the proportion of culturable As(V)- and As(III)-resistant bacteria with respect to total bacterial population. The catabolic potential and versatility of the resident microbial communities (assessed by community level physiological profile) was highly soil-dependent and substantial increases of both parameters were observed in the amended soils with the higher total As concentration (from approx. 749 to 22,600 mg kg^-1). Moreover, both carbon source utilisation profile and 16S rRNA soil metagenome sequencing indicated a significant impact of MSWC and Fe-WTRs on the structure and diversity of soil microbial communities, with Proteobacteria, Actinobacteria and Firmicutes being the most affected taxa. The assessment of selected soil enzyme activities (dehydrogenase, urease and β-glucosidase) indicated an increase of metabolic functioning especially in soils treated with MSWC (e.g. dehydrogenase activity increased up to 19.5-fold in the most contaminated soil treated with MSWC). Finally, the microbial and biochemical features of treated (and untreated) contaminated soils (i.e. total bacterial counts, catabolic potential and versatility and soil enzyme activities) were highly correlated with the concentrations of labile As and TM in these latter soils and supported a clear role of the tested amendments (especially MSWC) as As- and TM-immobilising agents.

Combination of soil organic and inorganic amendments helps plants overcome trace element induced oxidative stress and allows phytostabilisation

Trace element (TE)-contaminated soils require the improvement of their physico-chemical properties in order to allow their restoration through phytostabilization technologies. This study aimed to determine the usefulness of oxidative stress related parameters to validate the suitability of two different combinations of organic (solid fraction of pig slurry) and inorganic (paper mill sludge or a commercial red mud derivative) amendments for the phytostabilization of an acidic (4.2) TE-contaminated mine soil from SE Spain. Two wild species (Silybum marianum and Piptatherum miliaceum) were greenhouse cultivated and the development of the plants, their ionome, and oxidative stress related parameters were determined. Both amendment combinations increased significantly soil pH (to 5-6) and soil/pore water total organic C and total N concentrations, allowing an adequate plant growth and development (plants did not grow in untreated soils). The combination of amendments significantly reduced metal availability and showed to be effective (specially the one including the red mud derivative) in limiting shoot TE concentrations, which were all within common ranges (exclusion based tolerance of these species). Both protein carbonylation and lipid peroxidation were significantly higher in S. marianum plants from phytostabilized soils than in those from non-contaminated soils, which confirms the oxidative stress these plants suffer despite their satisfactory growth in the treated soils. P. miliaceum plants showed no differences between phytostabilized and non-contaminated soils. Therefore, the combination of amendments and TE-tolerant autochthonous species would be a suitable option for the phytostabilisation of soils contaminated by mining activities, reducing TE solubility and allowing an adequate plant growth.

The impact of nanoscale zero-valent iron particles on soil microbial communities is soil dependent

The application of nanoscale zero-valent iron particles (nZVI) for the remediation of contaminated sites is very promising. However, information concerning the ecotoxicity of nZVI on soil microbial communities and, hence, soil quality, is still scarce. We carried out a three-month experiment to evaluate the impact of the application of different concentrations of nZVI (from 1 to 20 mg g DW soil^-1) on soil microbial properties in a clay-loam versus a sandy-loam soil. Data on microbial biomass (total bacteria and fungi by qPCR, microbial biomass carbon), activity (β-glucosidase, arylsulphatase and urease activities), and functional (Biolog Ecoplates^™) and structural (ARISA, 16S rRNA amplicon sequencing) diversity evidenced that the sandy-loam soil was more vulnerable to the presence of nZVI than the clay-loam soil. In the sandy-loam soil, arylsulphatase activity and bacterial abundance, richness and diversity were susceptible to the presence of nZVI. The high content of clay and organic matter present in the clay-loam soil may explain the observed negligible effects of nZVI on soil microbial properties. It was concluded that the impact of nZVI on soil microbial communities and, hence, soil quality, is soil dependent.

Application of sewage sludge to agricultural soil increases the abundance of antibiotic resistance genes without altering the composition of prokaryotic communities

The application of sewage sludge as soil amendment is a common agricultural practice. However, wastewater treatment plants, sewage sludge and sewage sludge-amended soils have been reported as hotspots for the appearance and dissemination of antibiotic resistance, driven, among other factors, by selection pressure exerted by co-exposure to antibiotics and heavy metals. To address this threat to environmental and human health, soil samples from a long-term (24 years) field experiment, carried out to study the impact of thermally dried and anaerobically digested sewage sludge (at different doses and frequencies of application) on agricultural soil quality, were investigated for the presence of genes encoding antibiotic resistance (ARGs) and mobile genetic elements (MGEs). Sewage sludge-induced changes in specific soil physicochemical and microbial properties, as indicators of soil quality, were also investigated. The application of sewage sludge increased the total concentration of copper and zinc in amended soils, but without affecting the bioavailability of these metals, possibly due to the high values of soil pH and organic matter content. Soil microbal quality, as reflected by the value of the Soil Quality Index, was higher in sewage sludge-amended soils. Similarly, the application of sewage sludge increased soil microbial activity and biomass, as well as the abundance of ARGs and MGE genes, posing a risk of dissemination of antibiotic resistance. In contrast, the composition of soil prokaryotic communities was not significantly altered by the application of sewage sludge. We found correlation between soil Cu and Zn concentrations and the abundance of ARGs and MGE genes. It was concluded that sewage sludge-derived amendments must be properly treated and managed if they are to be applied to agricultural soil.

Effects of Cd, Cu, Zn and their combined action on microbial biomass and bacterial community structure

Heavy metal pollution can decrease the soil microbial biomass and significantly alter microbial community structure. In this study, a long-term field experiment (5 years) and short-term laboratory experiment (40 d) were employed to evaluate the effects of heavy metals (Cd, Cu, Zn), and their combinations at different concentrations, on the soil microbial biomass and the bacterial community. The ranges of heavy metal concentration in the long-term and short-term experiments were similar, with concentration ranges of Cd, Cu and Zn of about 0.3-1.5, 100-500, and 150-300 mg kg^-1, respectively. Microbial biomass decreased with increasing soil heavy metal concentrations in both the long-term and short-term experiments. The interaction between soil physicochemical factors (pH, TN, TC) and heavy metals (Cd, Cu, Zn) played a major role in change in the bacterial community in long-term polluted soil. In the laboratory experiment, although each heavy metal had an adverse effect on the microbial biomass and community structure, Cu appeared to have a greater role in the changes compared to Cd and Zn. However, the synergistic effects of the heavy metals were greater than those of the single metals and the synergistic effect between Cu and Cd was greater than that of Cu and Zn.

Immobilization of heavy metals in two contaminated soils using a modified magnesium silicate stabilizer

Heavy metal contamination is a severe environmental issue over the world. A lot of work has been done to develop effective stabilizers. In the present work, hydrothermal carbon-modified magnesium silicate (MS-C) was synthesized and used for the remediation of two heavy metal-polluted soils with different physicochemical properties. Soil samples were exposed to different doses of MS-C over 60 days (1, 3, and 5 wt%). The toxicity characteristic leaching procedure (TCLP) and the community bureau of reference sequential extraction procedure (BCR) were used to evaluate the remediation efficiency. The bioavailability of heavy metals in both soils was reduced by 20-86.7%, and the toxicity of heavy metals was reduced by 26.6-73.2% after MS-C added. Meanwhile, soil pH and water soluble organic carbon (WSOC) were increased. In addition, soil microbial biomass was increased, which indicated the improvement of soil condition. The immobilization of heavy metals was mainly caused by electrostatic attraction and cation exchange between MS-C and heavy metals. The significantly negative correlation between extractable heavy metals and pH/WSOC indicated the positive role of pH/WSOC in metal stabilization. Thus, this new stabilizer holds great application potentials for both single and multi-metal-contaminated soil remediation. ᅟ Graphical abstract.

Comparative assessment of autochthonous bacterial and fungal communities and microbial biomarkers of polluted agricultural soils of the Terra dei Fuochi

Organic and inorganic xenobiotic compounds can affect the potential ecological function of the soil, altering its biodiversity. Therefore, the response of microbial communities to environmental pollution is a critical issue in soil ecology. Here, a high-throughput sequencing approach was used to investigate the indigenous bacterial and fungal community structure as well as the impact of pollutants on their diversity and richness in contaminated and noncontaminated soils of a National Interest Priority Site of Campania Region (Italy) called "Terra dei Fuochi". The microbial populations shifted in the polluted soils via their mechanism of adaptation to contamination, establishing a new balance among prokaryotic and eukaryotic populations. Statistical analyses showed that the indigenous microbial communities were most strongly affected by contamination rather than by site of origin. Overabundant taxa and Actinobacteria were identified as sensitive biomarkers for assessing soil pollution and could provide general information on the health of the environment. This study has important implications for microbial ecology in contaminated environments, increasing our knowledge of the capacity of natural ecosystems to develop microbiota adapted to polluted soil in sites with high agricultural potential and providing a possible approach for modeling pollution indicators for bioremediation purposes.

Phytoremediation of mine tailings with Atriplex halimus and organic/inorganic amendments: A five-year field case study

Mine tailings have adverse chemical and physical conditions, including high concentrations of metals and salts, low organic matter content, and unbalanced rates of nutrients which limit the development of vegetation. A large scale field experiment was conducted to reclaim a tailing pond by triggering the growth of native species by spontaneous colonization by tilling (TL) the tailing pond surface and using marble waste (CaCO₃; MW), pig slurry (PS) and their combination (MW + PS) as soil amendments. Soil physicochemical properties and water and DTPA extractable metal concentrations of bulk and rhizosphere soils were analyzed after five year from the application of the treatments. In addition, plants of Atriplex halimus from each treatment were collected and metals in roots, leaves and stems analyzed. Before amendments application, the studied pond showed a neutral pH, high salinity and a moderate organic carbon content. After five years, the pH value was significantly increased only in MW plot. The results showed significant increases of DTPA-extractable Zn in MW and MW + PS plots, Pb in all treatments except MW plot, Cd only in PS plot, and Cu only in MW + PS plot. A. halimus was the most dominant species, growing spontaneously in all plots, with lower vegetation cover in CT and MW plots, 6% and 2% respectively. Application of MW increased leaf Pb accumulation by 2.5-fold and Cd by 55%, when compared to the CT. The high initial salinity and probable substitution of metals by Ca²⁺ on exchangeable surfaces of soil particles may be the reasons for higher uptake of metals in MW plot when compared to the other plots. Although this plant is widely utilized in contaminated sites for phytostabilization purposes, it may absorb and translocate high concentrations of metals to the aboveground tissues in saline contaminated sites.

Assessing the agromining potential of Mediterranean nickel-hyperaccumulating plant species at field-scale in ultramafic soils under humid-temperate climate

Nickel (Ni) agromining of ultramafic soils has been proposed as an eco-friendly option for metal recovery, which can also improve the fertility and quality of these low productive soils. The selection of adequate plant species and the analysis of their performance under the different climatic conditions are of interest for optimising the process and evaluating its full viability. A one-year field experiment was carried out to evaluate the viability of the two Ni-hyperaccumulating Mediterranean species, Alyssum murale and Leptoplax emarginata, for agromining purposes in ultramafic soils under a humid-temperate climate. Field plots of 50 m² were established and the soil was fertilised with gypsum and inorganic NPK fertilisers prior to cropping. Alyssum murale produced a slightly higher Ni yield than L. emarginata, but Ni bioaccumulation was dependent on the plant phenological stage for both species, being maximal at mid-flowering (4.2 and 3.0 kg Ni ha^-1, respectively). In both species, Ni was mainly stored in the leaves, especially in leaves of vegetative stems, but also in flowers and fruits in the case of L. emarginata. The main contributors to Ni yield of A. murale were flowering stems and their leaves, while for L. emarginata they were flowering stems and fruits. Implementing the agromining system increased soil nutrient availability, and modified microbial community structure and metabolic activity (due to fertilisation and plant root activity). The soil bacterial communities were dominated by Proteobacteria, Actinobacteria, Acidobacteria and Chloroflexi, and the agromining crops modified the relative abundance of some phyla (increasing Proteobacteria, Bacteroidetes and Nitrospirae and reducing Acidobacteria and Planctomycetes). Cultivating A. murale increased the densities of total culturable bacteria, while L. emarginata selected Ni-tolerant bacteria in its rhizosphere. In summary, both species showed great potential for their use in Ni agromining systems, although optimising soil and crop management practices could improve the phytoextraction efficiency.

Trace element transfer from two contaminated soil series to Medicago sativa and one of its herbivores, Spodoptera exigua

Alfalfa was cultivated in two potted soil series obtained from two sandy soils contaminated by Cu (SM) and metal(loids)/PAH (CD). Shoot production was monitored for 8 weeks. Then, larvae of Spodoptera exigua were reared on alfalfa of both soil series for eight days. A biotest (using Phaseolus vulgaris) was used to assess the soil phytotoxicity. Increasing soil contamination reduced P. vulgaris growth, but alfalfa growth was only reduced on the SM soil series. Exposure to the SM soil was mirrored by shoot Cu and Cr concentrations of alfalfa (respectively, in mg kg ^-1 DW, Cu and Cr ranged from 11.9 and 0.4 in the CTRL soil to 98.5 and 1.2 in the SM one). Exposure to the CD soil series was mirrored by shoot Zn concentrations (i.e., 48-91.6 mg kg^-1 DW). Internal metal(loid) concentrations of S. exigua remained generally steady across both soil series (respectively Cd 0.05-0.16, Cr 0.5-3.3, Cu 5.8-98.5, Ni 0.6-1.6, Pb 0.4-1.3, and Zn 57-337 mg kg^-1 DW), and most of the associated transfer factors were lower than 1. Here, due to the excluder phenotype of alfalfa across our TE contamination gradients, S. exigua could cope with high total metal(loid) concentration in both contaminated soils.