Effects of elevated CO2 on the phytoremediation efficiency of Noccaea caerulescens

Recent advances in phyto-combined remediation of heavy metal pollution in soil

The global industrialization and modernization have witnessed a rapid progress made in agricultural production, along with the issue of soil heavy metal (HM) pollution, which has posed severe threats to soil quality, crop yield, and human health. Phytoremediation, as an alternative to physical and chemical methods, offers a more cost-effective, eco-friendly, and aesthetically appealing means for in-situ remediation. Despite its advantages, traditional phytoremediation faces challenges, including variable soil physicochemical properties, the bioavailability of HMs, and the slow growth and limited biomass of plants used for remediation. This study presents a critical overview of the predominant plant-based HM remediation strategies. It expounds upon the mechanisms of plant absorption, translocation, accumulation, and detoxification of HMs. Moreover, the advancements and practical applications of phyto-combined remediation strategies, such as the addition of exogenous substances, genetic modification of plants, enhancement by rhizosphere microorganisms, and intensification of agricultural technologies, are synthesized. In addition, this paper also emphasizes the economic and practical feasibility of some strategies, proposing solutions to extant challenges in traditional phytoremediation. It advocates for the development of cost-effective, minimally polluting, and biocompatible exogenous substances, along with the careful selection and application of hyperaccumulating plants. We further delineate specific future research avenues, such as refining genetic engineering techniques to avoid adverse impacts on plant growth and the ecosystem, and tailoring phyto-combined strategies to diverse soil types and HM pollutants. These proposed directions aim to enhance the practical application of phytoremediation and its integration into a broader remediation framework, thereby addressing the urgent need for sustainable soil decontamination and protection of ecological and human health.

Role of Root Exudates on the Soil Microbial Diversity and Biogeochemistry of Heavy Metals

Due to extensive industrialization and escalation in pollution, the world is facing problems related to soil heavy metal pollution. The traditional ways of soil remediation are neither feasible nor cost-effective in most of the real-world scenarios, where metal concentration is relatively low in soil. Therefore, phytoremediation using plants and plant secretions to remediate heavy metal-contaminated soil is recently getting more attention. The plant root exudates act as an ecological driver in the rhizospheric region where they influence and guide the microbial community to function in such a way that can be advantageous for plant growth. They also promote phytoremediation process by altering the bioavailability of pollutants in soil. Root exudates affect the biogeochemical properties of heavy metals as well. In this review, existing literature on the role of root exudates (natural as well as artificial) on the phytoremediation of heavy metal-contaminated (particularly lead) soil is reviewed. The effect of root exudates on the biogeochemistry of lead in soil is also discussed.

Impacts of climate change on the fate of contaminants through extreme weather events

The direct impacts of climate change involve a multitude of phenomena, including rising sea levels, intensified severe weather events such as droughts and flooding, increased temperatures leading to wildfires, and unpredictable fluctuations in rainfall. This comprehensive review intends to examine firstly the probable consequences of climate change on extreme weather events such as drought, flood and wildfire. This review subsequently examines the release and transformation of contaminants in terrestrial, aquatic, and atmospheric environments in response to extreme weather events driven by climate change. While drought and flood influence the dynamics of inorganic and organic contaminants in terrestrial and aquatic environments, thereby influencing their mobility and transport, wildfire results in the release and spread of organic contaminants in the atmosphere. There is a nascent awareness of climate change's influence of climate change-induced extreme weather events on the dynamics of environmental contaminants in the scientific community and decision-making processes. The remediation industry, in particular, lags behind in adopting adaptive measures for managing contaminated environments affected by climate change-induced extreme weather events. However, recognizing the need for assessment measures represents a pivotal first step towards fostering more adaptive practices in the management of contaminated environments. We highlight the urgency of collaboration between environmental chemists and climate change experts, emphasizing the importance of jointly assessing the fate of contaminants and rigorous action to augment risk assessment and remediation strategies to safeguard the health of our environment.

Elevated atmospheric CO2 combined with Epichloë endophyte may improve growth and Cd phytoremediation potential of tall fescue (Festuca arundinacea L.)

Complex environmental conditions like heavy metal contamination and elevated CO2 concentration may cause numerous plant stresses and lead to considerable crop losses worldwide. Cadmium is a non-essential element and potentially highly toxic soil metal pollution, causing oxidative stress in plants and human toxicity. In order to assess a combination of complex factors on the responses of two genotypes of Festuca arundinacea (75B and 75C), a greenhouse experiment was conducted on plants grown in two Cd-contaminated soil conditions and two soil textures under combined effects of elevated ambient CO2 (700 ppm) and Epichloë endophyte infection. Plant biomass, Cd, Fe, Cu, Zn, and Mn concentrations in the plant shoots and roots, Fv/Fm, chlorophyll (a & b), and carotenoid contents were measured after 7 months of growth in pots. Our results showed that endophyte-infected plants (E+) grown in elevated CO2 atmosphere (CO2+), clay-loam soil texture (H) with no Cd amendment (Cd-) in the genotype 75B had significantly greater shoot and root biomass than non-infected plants (E-) grown in ambient CO2 concentration (CO2-), sandy-loam soil texture (L) with amended Cd (Cd+) in the genotype 75C. Increased CO2 concentration and endophyte infection, especially in the genotype 75B, enabled Festuca for greater phytoremediation of Cd because of higher tolerance to Cd stress and higher biomass accumulation in the plant genotype. However, CO2 enrichment negatively influenced the plant mineral absorption due to the inhibitory effects of high Cd concentration in shoots and roots. It is concluded that Cd phytoremediation can be positively affected by the increased atmospheric CO2 concentration, tolerant plant genotype, heavy soil texture, and Epichloë endophyte. Using Taguchi and AIC design methodologies, it was also predicted that the most critical factors affecting Cd phytoremediation potential were CO2 concentration and plant genotype.

Plant–Metal Interactions in the Context of Climate Change

Expanding fundamental understanding of the complex and far-reaching impacts of anthropogenic climate change is essential for formulating mitigation strategies. There is abundant evidence of ongoing damage and threat to plant health across both natural and cultivated ecosystems, with potentially immeasurable cost to humanity and the health of the planet. Plant–soil systems are multi-faceted, incorporating key variables that are individually and interactively affected by climatic factors such as rainfall, solar radiation, air temperature, atmospheric CO2, and pollution. This synthesis focuses on climate effects on plant–metal interactions and related plant–soil dynamics. Ecosystems native to metalliferous soils incorporate vegetation well adapted to metal oversupply, yet climate-change is known to induce the oversupply of certain immobile soil metals by altering the chemistry of non-metalliferous soils. The latter is implicated in observed stress in some non-metal-adapted forest trees growing on ‘normal’ non-metalliferous soils. Vegetation native to riverine habitats reliant on flooding is increasingly at risk under drying conditions caused by anthropogenic water removal and climate change that ultimately limit plant access to essential trace-metal nutrients from nutrient poor sandy soils. In agricultural plant systems, it is well known that environmental conditions alter soil chemistries and plant responses to drive plant metal toxicity stress. These aspects are addressed with reference to specific scenarios and studies linking climate to plant–metal interactions, with emphasis on land plants.

Soil Remediation: Towards a Resilient and Adaptive Approach to Deal with the Ever-Changing Environmental Challenges

Pollution from numerous contaminants due to many anthropogenic activities affects soils quality. Industrialized countries have many contaminated sites; their remediation is a priority in environmental legislation. The aim of this overview is to consider the evolution of soil remediation from consolidated invasive technologies to environmentally friendly green strategies. The selection of technology is no longer exclusively based on eliminating the source of pollution but aims at remediation, which includes the recovery of soil quality. “Green remediation” appears to be the key to addressing the issue of remediation of contaminated sites as it focuses on environmental quality, including the preservation of the environment. Further developments in green remediation reflect the aim of promoting clean-up strategies that also address the effects of climate change. Sustainable and resilient remediation faces the environmental challenge of achieving targets while reducing the environmental damage caused by clean-up interventions and must involve an awareness that social systems and environmental systems are closely connected.

Elevated atmospheric CO2 enhances the phytoremediation efficiency of tall fescue (Festuca arundinacea) in Cd-polluted soil

With the economic development of society, concentrations of atmospheric CO₂ and heavy metals in soils have been increasing. The physiological responses of plants to the interaction between soil pollution and climatic change need to be understood. Pot experiments were designed to assess variations in Festuca arundinacea dry weight, leaf type, chlorophyll content, antioxidase activities, and Cd accumulation ability, under different atmospheric CO₂ treatments. The results showed that the total dry weights increased with increasing CO₂, and Cd concentrations in falling leaf tissues increased with raised atmospheric CO₂, before reaching a peak at 600 ppm, above which they remained constant. Compared with the control (400 ppm), 600, 650, and 700 ppm CO₂ treatments increased the proportions of the falling tissues by 1.7%, 3.3%, and 4.5%, respectively. Antioxidant enzyme activities in plant leaves increased with increasing atmospheric CO₂ levels. The concentration of H₂O₂ in leaf tissues increased with increasing CO₂, reaching a peak at 600 ppm, and then decreased significantly as the CO₂ content increased further, to 700 ppm. The results in this study suggest that F. arundinacea could be regarded as a potential candidate for phytoremediation of Cd-polluted soil; especially if senescent and dead leaf tissues could be harvested, and that raised atmospheric CO₂ levels could improve its soil remediation efficiency.Novelty statement Extrapolation of results from experiments of environmental impacts in greenhouse to real scale field requires to be considered cautiously. External factors such as water, temperature, humidity, and pollution are variable in real field. Plants will face a lot of beneficial or detrimental conditions which will influence the magnitude of the results. However, the elevation of CO₂ is an inevitable phenomenon in future. Therefore, findings from experiments under artificial conditions are sometime a good choice to obtain knowledge about elevated CO₂ related impacts on phytoremediation efficiency of a specific plant. The final goal of this work is to find a suitable CO₂ fumigation strategy optimized for soil remediation. We report on that elevated atmospheric CO₂ can increase the phytoremediation efficiency of Festuca arundinacea for Cd. This is significant because the combined influences of elevated atmospheric CO₂ and metal pollution in terms of biomass yield, pollutant uptake, and phytoremediation efficiency would be more complex than the effects of each individual factor.

Geographical distribution of As-hyperaccumulator Pteris vittata in China: Environmental factors and climate changes

Understanding the distribution of hyperaccumulators helps to implement more efficient phytoremediation strategies of contaminated sites, however, limited information is available. Here, we investigated the geographical distribution of the first-known arsenic-hyperaccumulator Pteris vittata in China and the key factors under two climate change scenarios (SSP 1-2.6 and SSP 5-8.5) at two time points (2030 and 2070). Species distribution model (MaxEnt) was applied to examine P. vittata distribution based on 399 samples from field surveys and existing specimen records. Further, among 23 environmental factors, 11 variables were used in the MaxEnt model, including temperature, precipitation, elevation, soil property, and UV-B radiation. The results show that P. vittata can grow in ~23% of the regions in China. Specifically, it is mainly distributed in 11 provinces of southern China, including Hainan, Guangdong, Guangxi, Yunnan, Guizhou, Hunan, Hubei, Jiangxi, Fujian, Zhejiang, and Jiangsu. Besides, eastern Sichuan, and southern Henan, Shaanxi, and Anhui are suitable for P. vittata growth. Under two climate change scenarios, P. vittata distribution in China would decrease by ~5.76-7.46 × 10⁴ km² in 2030 and ~3.22-4.68 × 10⁴ km² in 2070, with southern Henan and most Jiangsu being unsuitable for P. vittata growth. Among the 11 environmental variables, the minimum temperature of coldest month (bio6) and temperature annual range (bio7) are the two key factors limiting P. vittata distribution. At bio6 <-5 °C and/or bio7 >33 °C, the regions are unsuitable for P. vittata growth. Based on the MaxEnt model, precipitation had limited effects, so P. vittata can probably survive under both dry and moist environments. This study helps guide phytoremediation of As-polluted soils using P. vittata and provides an example to evaluate habitat suitability of hyperaccumulators at international scales.

Enhancement of the Cd phytoremediation efficiency of Festuca arundinacea by sonic seed treatment

It has been reported that both naturally occurring and artificially created sounds can alter the physiological parameters of various plants. A series of experiments were designed in the present study to estimate the physiological responses and the variation in the Cd decontamination capacity of Festuca arundinacea under sonic wave treatments. Plant seeds were treated by sound waves of frequency 200, 300, 400, 500, and 1000 Hz, and the germinated seedlings were transplanted to Cd-polluted soil. The results showed that all the sonic treatments increased the whole plant dry weight of F. arundinacea compared with that of the control, and the highest value was observed in the 200 Hz treatment. The Cd content in below-ground and aerial tissues of the species increased with increasing frequency till 400 Hz, after which they became constant. A higher proportion of senescent and dead leaf tissues was observed in the high-frequency treatment (1000 Hz), and more Cd was transferred to these failing tissues. Therefore, in the 1000 Hz treatment, a significantly greater amount of Cd could be eliminated by harvesting the senescent and dead leaf tissues of the species compared with that of the other treatments. The concentrations of dissolved organic matter (DOM) and the proportions of hydrophilic fractions which have a strong Cd affinity, in the rhizosphere soil of F. arundinacea increased with the increase in sound frequency. Cd extraction ability of DOM also increased with increasing frequency. This study indicated that a suitable sonic treatment can improve the phytoextraction efficiency of F. arundinacea, and also explained the mechanism from the perspective of the variations in soil DOM.

Impacts of root pruning intensity and direction on the phytoremediation of moderately Cd-polluted soil by Celosia argentea

Root pruning can impact the physiological functions of various plants, which influence phytoremediation. A series of root pruning treatments with different combinations of direction (two-side pruning and four-side pruning) and intensity (10, 25, and 33% pruning) were performed on Celosia argentea L. All two-side pruning treatments, regardless of intensity, decreased the dry biomass of the C. argentea roots at the end of the experiment relative to that of the control. However, the two-side-10% and two-side-25% pruning treatments stimulated the growth rate of the plant leaves significantly by 58.6 and 41.4%, respectively, relative to that of the control, and even offset the weight loss of the plant roots. Contrastingly, the two-side-33% pruning treatment reduced the biomass yield of leaves by 24.1%. For the four-side pruning treatments, the low intensity increased the dry weight of both the plant roots and leaves, while both decreased under high-intensity root pruning. The dry weight, Cd content, pigment level, and photosynthetic efficiency in the four-side-10% treatment were higher than those in the other treatments during the experiment. This study indicates that root pruning with a suitable combination of direction and intensity can positively influence the Cd removal ability of C. argentea.

Dark septate endophytes isolated from non-hyperaccumulator plants can increase phytoextraction of Cd and Zn by the hyperaccumulator Noccaea caerulescens

Dark septate endophytes (DSEs) can improve plant stress tolerance by promoting growth and affecting element accumulation. Due to its ability to accumulate high Cd, Zn, and Ni concentrations in its shoots, Noccaea caerulescens is considered a promising candidate for phytoextraction in the field. However, the ability of DSEs to improve trace element (TE) phytoextraction with N. caerulescens has not yet been studied. The aim of this study was therefore to determine the ability of five DSE strains, previously isolated from poplar roots collected at different TE-contaminated sites, to improve plant development, mineral nutrient status, and metal accumulation by N. caerulescens during a pot experiment using two soils differing in their level of TE contamination. Microscopic observations revealed that the tested DSE strains effectively colonised the roots of N. caerulescens. In the highly contaminated (HC) soil, a threefold increase in root biomass was found in plants inoculated with the Leptodontidium sp. Pr30 strain compared to that in the non-inoculated condition; however, the plant nutrient status was not affected. In contrast, the two strains Phialophora mustea Pr27 and Leptodontidium sp. Me07 had positive effects on the mineral nutrient status of plants without significantly modifying their biomass. Compared to non-inoculated plants cultivated on HC soil, Pr27- and Pr30-inoculated plants extracted more Zn (+ 30%) and Cd (+ 90%), respectively. In conclusion, we demonstrated that the responses of N. caerulescens to DSE inoculation ranged from neutral to beneficial and we identified two strains (i.e. Leptodontidium sp. (Pr30) and Phialophora mustea (Pr27)) isolated from poplar that appeared promising as they increased the amounts of Zn and Cd extracted by improving plant growth and/or TE accumulation by N. caerulescens. These results generate interest in further characterising the DSEs that naturally colonise N. caerulescens and testing their ability to improve phytoextraction.

Impact of O3 on the phytoremediation effect of Celosia argentea in decontaminating Cd

Elevated atmospheric O₃ can inhibit the growth rate of various plants and increase metal content in their tissues owing to the oxidative damage, thereby affecting their phytoremediation efficiency. In this study, a series of O₃ fumigation treatments were designed to evaluate the dry weight, Cd content, and transpiration rate responses of Celosia argentea to different levels of O₃ (40, 50, 55, 60, 65, and 80 ppb). The dry weight of C. argentea decreased as the atmospheric O₃ level increased, and the Cd concentration of the plant leaves increased until the level of O₃ reached 60 ppb before decreasing slightly. The variations in the transpiration rate followed a similar trend to the Cd content under different O₃ levels. The phytoremediation efficiency of C. argentea increased with O₃ fumigation at low (50 ppb) and moderate (55 and 60 ppb) levels, and significantly decreased at the highest level. The regression curves indicated that the plant species treated with 52 ppb of O₃ exhibited the highest Cd accumulation capacity. Overall, the phytoremediation effect of C. argentea cultivated in Cd-polluted soil might be improved under the high-O₃ conditions. This result might help to choose suitable plants for soil remediation in future atmospheric environment.

The influence of light combination on the physicochemical characteristics and enzymatic activity of soil with multi-metal pollution in phytoremediation

Light irradiation with suitable quality and intensity could influence the success of phytoremediation by improving the biomass yield of plants. However, mechanisms involved in this influence on the contaminant accumulation and translocation ability of plants have rarely been studied. Five light combinations with different red (R) and blue (B) ratios (0, 10, 50, 75 and 100 % blue) at the same intensity (220 μmol m^-2 s^-1) were used to assist phytoremediation using Noccaea caerulescens, and the change in physicochemical characteristics and enzymatic activities of soils after phytoremediation were evaluated. Compared with the control, the light combinations and monochromic blue light significantly increased the activities of soil ureases, invertases, and phosphatases, whereas monochromic red light strongly inhibited the activities of these enzymes, because different light irradiations altered the formation and excretion of carbohydrates from plants for soil microorganism consumption. Plants under B₅₀R₅₀ treatment accumulated the highest concentrations of metals, but their chlorophyll concentrations and lipid peroxidation were similar to those other species with lower metal concentrations. Hence, light with a proper blue/red ratio can simultaneously improve the physicochemical characteristics and enzymatic activities of soils, increase the metal uptake capacity and oxidation resistance of plants, and reduce the leaching risk during phytoremediation processes.