Are Grasses Really Useful for the Phytoremediation of Potentially Toxic Trace Elements? A Review

Establishing grassland mixtures on mine wastes - a two-year mesocosm study

Plant growth on mine wastes is restricted by the lack of water, nutrients, phytotoxic responses and the absence of a seedbank. In a mesocosm study, we addressed the establishment of vegetation on metalliferous mine wastes from two seed mixtures. Besides the composition of the vegetation and the increase in plant cover and biomass over time, we studied concentrations of heavy metals in the shoot and analyzed the quantity of throughflow, its pH and EC to follow pollutant discharge. We hypothesized that the types of mine wastes and sown grasslands will affect species composition and the formation of a protective plant cover. Our platform was well-suited to study build-up and succession of a vegetation layer and its potential to stabilize mine wastes. However, the establishing community was less diverse than expected. The dilution of wastes increased species number and biomass, and we found a reduction of material discharge with increasing vegetation cover. Over time, drainage was reduced, while pH of the throughflow did not change. However, it was higher under the addition of greywater. Interestingly, the use of greywater led to a higher biomass in one mixture and slight changes in the chemistry of the throughflow and the plant matter.

Unraveling the impact of nanopollution on plant metabolism and ecosystem dynamics

Nanopollution (NPOs), a burgeoning consequence of the widespread use of nanoparticles (NPs) across diverse industrial and consumer domains, has emerged as a critical environmental issue. While extensive research has scrutinized the repercussions of NPs pollution on ecosystems and human health, scant attention has been directed towards unraveling its implications for plant life. This comprehensive review aims to bridge this gap by delving into the nuanced interplay between NPOs and plant metabolism, encompassing both primary and secondary processes. Our exploration encompasses an in-depth analysis of the intricate mechanisms governing the interaction between plants and NPs. This involves a thorough examination of how physicochemical properties such as size, shape, and surface characteristics influence the uptake and translocation of NPs within plant tissues. The impact of NPOs on primary metabolic processes, including photosynthesis, respiration, nutrient uptake, and water transport. Additionally, this study explored the multifaceted alterations in secondary metabolism, shedding light on the synthesis and modulation of secondary metabolites in response to NPs exposure. In assessing the consequences of NPOs for plant life, we scrutinize the potential implications for plant growth, development, and environmental interactions. The intricate relationships revealed in this review underscore the need for a holistic understanding of the plant-NPs dynamics. As NPs become increasingly prevalent in ecosystems, this investigation establishes a fundamental guide that underscores the importance of additional research to shape sustainable environmental management strategies and address the extensive effects of NPs on the development of plant life and environmental interactions.

Inventing hyperaccumulator plants: improving practice in phytoextraction research and terminology

Toxic metals and metalloids, especially from anthropogenic sources, now pollute substantial areas of our planet. Phytoextraction is a proven technology with the potential to reduce metal/metalloid pollution, and where financially viable, recover valuable metals ('phytomining'). Toward these aims, there has been a surge of publications over the last two decades. While important progress is being made, ongoing propagation of poor practice, and the resultant drain from funding sources, is hindering this promising research area. This includes mis-ascribing hyperaccumulator species, hydroponics with extremely high dose levels, misuse of Bioconcentration Factors, use of food or biomass crops with low accumulation for phytoextraction, the phenomenon of 'template papers' in which a known hyperaccumulator for element X is dosed with element Y, or a common weed species dosed with any variety of elements to make it 'hyperaccumulate'. Here we highlight these misconceptions with the hope that this will help to: (i) disseminate accurate definitions for in planta metal accumulation; (ii) quash the propagation of poor practice by limiting the inflation of unnecessary publications via the practice of 'template paper' writing; (iii) be used by journal editors and reviewers to validate their reasoning to authors; and (iv) contribute to faster progress in delivering this technology to in-the-field practitioners.

Comparative environmental and economic life cycle assessment of phytoremediation of dredged sediment using Arundo Donax, integrated with biomass to bioenergy valorization chain

The economic and environmental life cycle assessment (LCA) was integrated into a laboratory-based experiment to evaluate the feasibility and sustainability of phytoremediation of chloride-rich marine dredged sediment, using perennial reed Arundo Donax along with biomass valorization. As a prerequisite for life cycle assessments, a baseline mathematical model was developed to estimate the yields of biomass to bioenergy valorization chain including the estimation of biomass yield per m3 sediment, bioenergy yields from valorization schemes, expected green electricity yield, and the phytoremediation time frame. This mathematical model was applied to develop a parametric life cycle inventory for two scenarios of sediment phytoremediation separately or integrated with biomass valorization, for LCA and further sensitivity and uncertainty analysis. Comparative LCA unveiled that the cost and environmental impacts of annual phytoremediation of 1m3 sediment alone or integrated with biomass valorization are much inferior to the corresponding sediment landfill as the inevitable alternative approach for sediment management. With the chloride bioaccumulation capacity of 9940 mg per kg dry biomass of A. donax, the phytoremediation of sediment with chloride concentration higher than 1650 mg/kg may not be achievable in a realistic time frame. Due to the importance of considering sediment depth and the effectiveness of the plant rooting system in estimating the performance of phytoremediation and the time frame, the volume of sediment (1m3) is a more appropriate functional unit than the surface area (ha) for LCA studies of phytoremediation. In addition, considering the volume of sediment as a functional unit retains comparability to other valorization scenarios such as sediment incorporation in cementitious matrices and management scenarios such as landfill, which are generally expressed on a volume or mass basis. Integrating biomass-derived bioenergy production into phytoremediation could offer local and global benefits in terms of economy and environment mainly due to carbon sequestration and avoiding fossil-based fuels.

Transcriptome Analysis Reveals the Stress Tolerance Mechanisms of Cadmium in Zoysia japonica

Cadmium (Cd) is a severe heavy metal pollutant globally. Zoysia japonica is an important perennial warm-season turf grass that potentially plays a role in phytoremediation in Cd-polluted soil areas; however, the molecular mechanisms underlying its Cd stress response are unknown. To further investigate the early gene response pattern in Z. japonica under Cd stress, plant leaves were harvested 0, 6, 12, and 24 h after Cd stress (400 μM CdCl2) treatment and used for a time-course RNA-sequencing analysis. Twelve cDNA libraries were constructed and sequenced, and high-quality data were obtained, whose mapped rates were all higher than 94%, and more than 601 million bp of sequence were generated. A total of 5321, 6526, and 4016 differentially expressed genes were identified 6, 12, and 24 h after Cd stress treatment, respectively. A total of 1660 genes were differentially expressed at the three time points, and their gene expression profiles over time were elucidated. Based on the analysis of these genes, the important mechanisms for the Cd stress response in Z. japonica were identified. Specific genes participating in glutathione metabolism, plant hormone signal and transduction, members of protein processing in the endoplasmic reticulum, transporter proteins, transcription factors, and carbohydrate metabolism pathways were further analyzed in detail. These genes may contribute to the improvement of Cd tolerance in Z. japonica. In addition, some candidate genes were highlighted for future studies on Cd stress resistance in Z. japonica and other plants. Our results illustrate the early gene expression response of Z. japonica leaves to Cd and provide some new understanding of the molecular mechanisms of Cd stress in Zosia and Gramineae species.

Oil-Contaminated Soil Remediation with Biodegradation by Autochthonous Microorganisms and Phytoremediation by Maize (Zea mays)

Biological methods are currently the most commonly used methods for removing hazardous substances from land. This research work focuses on the remediation of oil-contaminated land. The biodegradation of aliphatic hydrocarbons and PAHs as a result of inoculation with biopreparations B1 and B2 was investigated. Biopreparation B1 was developed on the basis of autochthonous bacteria, consisting of strains Dietzia sp. IN118, Gordonia sp. IN101, Mycolicibacterium frederiksbergense IN53, Rhodococcus erythropolis IN119, Rhodococcus globerulus IN113 and Raoultella sp. IN109, whereas biopreparation B2 was enriched with fungi, such as Aspergillus sydowii, Aspergillus versicolor, Candida sp., Cladosporium halotolerans, Penicillium chrysogenum. As a result of biodegradation tests conducted under ex situ conditions for soil inoculated with biopreparation B1, the concentrations of TPH and PAH were reduced by 31.85% and 27.41%, respectively. Soil inoculation with biopreparation B2 turned out to be more effective, as a result of which the concentration of TPH was reduced by 41.67% and PAH by 34.73%. Another issue was the phytoremediation of the pre-treated G6-3B2 soil with the use of Zea mays. The tests were carried out in three systems (system 1-soil G6-3B2 + Zea mays; system 2-soil G6-3B2 + biopreparation B2 + Zea mays; system 3-soil G6-3B2 + biopreparation B2 with γ-PGA + Zea mays) for 6 months. The highest degree of TPH and PAH reduction was obtained in system 3, amounting to 65.35% and 60.80%, respectively. The lowest phytoremediation efficiency was recorded in the non-inoculated system 1, where the concentration of TPH was reduced by 22.80% and PAH by 18.48%. Toxicological tests carried out using PhytotoxkitTM, OstracodtoxkitTM and Microtox® Solid Phase tests confirmed the effectiveness of remediation procedures and showed a correlation between the concentration of petroleum hydrocarbons in the soil and its toxicity. The results obtained during the research indicate the great potential of bioremediation practices with the use of microbial biopreparations and Zea mays in the treatment of soils contaminated with petroleum hydrocarbons.

Galactoglucomannan oligosaccharides alleviate cadmium toxicity by improving physiological processes in maize

Phosphate fertilisers and past mining activity are significant source of cadmium (Cd) pollution; thus, the concentration of Cd in agricultural soils has been substantially rising. Various substances have been tested for their potential to alleviate the toxicity of Cd and stimulate the accumulation of Cd in plant organs. This study brought new insight of the impact of galactoglucomannan oligosaccharides (GGMOs) on the maize plants grown under/in Cd stress. The application of GGMOs reduced concentration of Cd in the maize leaves and thus GGMOs increased their growth (by 24%), concentration of photosynthetic pigments (up to 39.4%), effective quantum yield of photosystem II (up to 29.6%), and net photosynthetic rate (up to 19.6%). The concentrations of stress markers increased in the Cd and Cd + GGMOs treatment; however, significantly lower concentration was detected in the Cd + GGMOs treatment (malondialdehyde by 21.7%, hydrogen peroxide by 13%). The concentration of auxin increased almost by two-fold in the Cd + GGMOs treatment compared to the Cd treatment. The recovered auxin level and enhanced nutrient uptake are proposed mechanisms of GGMOs' action during stress. GGMOs are molecules with biostimulant potential that could support vitality of maize plants in Cd stress.

Effect of selenium and soil pH on cadmium phytoextraction by Urochloa decumbens grown in Oxisol

It has been speculated that selenium (Se) supply can affect cadmium (Cd) 'availability' and increase the Cd tolerance of plants used for phytoextraction, in a pH-dependent process. Thus, we evaluated the interaction Cd-Se and the effects of soil pH in this interaction on plant availability of Cd and phytoextraction efficiency of Urochloa decumbens cv. Basilisk grown in Oxisol. Two soil concentrations of Cd (0.93 and 3.6 mg kg^-1) and Se (<0.2 and 1 mg kg^-1) and two soil pH (0.01 mol L^-1 CaCl₂) conditions (4.1 and 5.7) were considered. At both pH, Se supply increased the exchangeable fraction of Cd and decreased the residual Cd fraction. At pH 4.1, the growth of U. decumbens was impaired by Se addition, regardless of Cd exposure. The lower root growth and tillering of U. decumbens exposed to Cd disappeared at pH 5.7 due to uptake of low Se concentrations. Thus, the toxic or beneficial effects of Se on growth of U. decumbens used for Cd phytoextraction depend on the amount of Se assimilated. The Cd phytoextraction efficiency of U. decumbens was not improved by Se supply, regardless of soil pH. Therefore, we cannot recommend the application of Se to increase Cd phytoextraction by this grass.

Combining phytoremediation with bioenergy production: developing a multi-criteria decision matrix for plant species selection

The use of plants to extract metal contaminants from soils has been proposed as a cost-effective means of remediation, and utilizing energy crops for this phytoextraction process is a useful way of attaining added value from the process. To simultaneously attain both these objectives successfully, selection of an appropriate plant species is crucial to satisfy a number of imporTant criteria including translocation index, metal and drought tolerance, fast growth rate, high lignocellulosic content, good biomass production, adequate calorific value, second generation attribute, and a good rooting system. In this study, we proposed a multi-criteria decision analysis (MCDA) to aid decision-making on plant species based on information generated from a systematic review survey. Eight species Helianthus annuus (sunflower), Brassica juncea (Indian mustard), Glycine max (soybean), Salix spp. (willow), Populus spp. (poplar), Panicum virgatum (switchgrass), Typha latifolia (cattails), and Miscanthus sinensis (silvergrass) were examined based on the amount of hits on a number of scientific search databases. The data was normalized by estimating their min-max values and their suitability. These criteria/indicators were weighted based on stipulated research objectives/priorities to form the basis of a final overall utility scoring. Using the MCDA, sunflower and silvergrass emerged as the top two candidates for both phytoremediation and bioenergy production. The multi-criteria matrix scores assist the process of making decisions because they compile plant species options quantitatively for all relevant criteria and key performance indicators (KPIs) and its weighing process helps incorporate stakeholder priorities to the selection process.

Synergistic enhancement of the phytostabilization of a semiarid mine tailing by a combination of organic amendment and native microorganisms (Funneliformis mosseae and Bacillus cereus)

The goal of this work was to evaluate the effects of fermented sugar beet residue and inoculation with a native arbuscular mycorrhizal (AM) fungus, Funneliformis mosseae (Nicol. and Gerd.) Gerd. and Trappe, or a native bacterium, Bacillus cereus Frankland & Frankland, alone or in combination, on the establishment of Lygeum spartum L. seedlings grown in a mine tailing under semiarid conditions. We conducted a field study to analyse root and shoot dry biomass, shoot nutrient contents, mycorrhization, plant nitrate reductase (NR) and acid phosphomonoesterase activities, soil enzyme activities and aggregate stability. Ten months after field transplanting, it was found that the three experimental factors had interacted synergistically with regard to shoot and root biomass, with increases of about 410% and 370%, respectively relative to plants in the untreated soil. The treatment combining all three factors increased the root content of all heavy metals, and the levels of nitrogen (N), phosphorus, potassium and NR activity in shoot tissues, whereas it decreased root acid phosphomonoesterase activity. Soil dehydrogenase, protease and β-glucosidase activities, total N content and aggregate stability were increased by the combined treatment. In conclusion, the combination of the organic amendment, the native AM fungus and the native bacterium can be regarded as a suitable tool for phytostabilization with L. spartum due to its ability to enhance the tolerance of plants to heavy metals, improve the plant nutritional status and increase the soil microbial function related to the C cycling.

Electro-enhanced phytoremediation system on the removal of trace metal concentration from contaminated water

The combination of electro-enhanced and hydroponic phytoremediation hereinafter referred to as electro-enhanced phytoremediation (EP) system, has been employed for rapid removal of trace metal concentration of lead (II) from contaminated water using Kentucky bluegrass (Poa pratensis L.) as accumulator plant. In this study, for rapid assessment the effectiveness of two-dimensional (2D) electrode configuration in electro-enhanced system was evaluated by agar media for 48h period of time. Furthermore, these configurations were applied to enhance the EP system for 9d period of time. Also, a common agrochemical-urea as chaotropic agent to facilitate the healthy growth of plant in contaminated water was evaluated. The results showed that the accumulation of lead (II) concentration was higher in the plant roots (i.e. high bioaccumulation coefficient (BC) value) than in aerial parts of plant (i.e. low translocation factor (TF) value). Also, the accumulation of lead (II) concentration in plant was higher under the treated urea of EP system. The chlorophyll content, biomass accumulation productivity, and water content (i.e. dry weight-fresh weight (DW/FW) ratio) of plant either under the treated urea or untreated urea with high accumulation of lead (II) concentration revealed that the Kentucky bluegrass has able to hold out the plant stress.

Tissue Culture and Somatic Embryogenesis in Warm-Season Grasses—Current Status and Its Applications: A Review

Warm-season grasses are C4 plants and have a high capacity for biomass productivity. These grasses are utilized in many agricultural production systems with their greatest value as feeds for livestock, bioethanol, and turf. However, many important warm-season perennial grasses multiply either by vegetative propagation or form their seeds by an asexual mode of reproduction called apomixis. Therefore, the improvement of these grasses by conventional breeding is difficult and is dependent on the availability of natural genetic variation and its manipulation through breeding and selection. Recent studies have indicated that plant tissue culture system through somatic embryogenesis complements and could further develop conventional breeding programs by micropropagation, somaclonal variation, somatic hybridization, genetic transformation, and genome editing. This review summarizes the tissue culture and somatic embryogenesis in warm-season grasses and focus on current status and above applications including the author’s progress.

Nature-Based Solutions for the Sustainable Management of Urban Soils and Quality of Life Improvements

The rehabilitation and restoration of land-based ecosystems is a key strategy for recovering the services (goods and resources) ecosystems offer to humankind. The use of nature-based solutions (NBSs) to restore degraded soil functions and improve soil quality can be a sustainable and successful strategy to enhance their ecosystem services by working together with the forces of nature and using well-designed measures that require less maintenance, are more cost-effective, and if constructed in the right way may even be more effective over long periods because nature’s forces can increase the structural efficiency. In this study, we aimed to (i) evaluate the bioremediation capacity of some grasses and their suitability for lawn planting in settlements (in residential and non-residential areas, along roads, etc.) and (ii) propose technological solutions for their practical application in an urban environment. Emphasis was placed on the potential of some perennial grasses and their application for the bioremediation of polluted urban soils, including perennial ryegrass (Lolium perenne L.), crested wheatgrass (Agropyron cristatum L.), tall fescue (Festuca arundinacea Schreb), and bird’s foot trefoil (Lotus corniculatus L.). A case study from the city of Plovdiv (Bulgaria) is presented, together with an effective technological solution for the establishment of urban lawns and the roadside green buffer patches.

Buffer Green Patches around Urban Road Network as a Tool for Sustainable Soil Management

Urban areas are facing a range of environmental challenges including air, water and soil pollution as a result of industrial, domestic and traffic emissions. In addition, global climate change is likely to aggravate certain urban problems and disturb the urban ecology by increasing the frequency and severity of extreme weather events. In the context of urbanization growth and the consequent impact on the environment, there is a growing interest in maintaining urban soil quality and functions as they are the medium for green infrastructure development. Furthermore, urban soils are becoming one of the key factors in the delivery of many ecosystem services such as carbon storage, climate regulation, water flow regulation, etc. On the other hand, urban soils are well-known to be a major sink of air pollutants due to the wet and dry atmospheric deposition and recirculation. Soil has the ability to degrade some chemical contaminants but when the levels are high, urban soils could hold on large amounts and pose a risk to human health. A cost-effective technological solution is to use the ability of some plant species to metabolize, accumulate and detoxify heavy metals or other harmful organic or inorganic compounds from the soil layer. The establishment of urban lawns (grass covered surfaces) is a helpful, environmentally friendly, economically sustainable and cost-effective approach to remove contaminants from polluted soils (terrains), which also has some aesthetic benefits. In this paper, an overview of the benefits and limitations of urban lawn construction is presented. The focus is on the perspectives for sustainable management of urban lawns, especially as buffer green patches in the road network surroundings, that can represent strategies to provide ecological and social multifunctionality of urban soils, and thus, increasing their ecosystem services capacity. Specifically, the paper highlights (i) the possibilities for phytoremediation of urban soils, (ii) potential of some perennial grasses and (iii) key issues that should be considered in the planning and design of urban lawns.