Effects of elevated CO2 on the Cd uptake by rice in Cd-contaminated paddy soils

Alleviation of cadmium toxicity by improving antioxidant defense mechanism and nutrient uptake in wheat (Triticum aestivum L.) through foliar application of 24-epibrassinolide under elevated CO2

Heavy metals like cadmium (Cd) contamination occur in conjunction with the rising CO2 threatening food security and safety. Foliar application of 24-Epibrassinolide (EBR) was found to ameliorate Cd stress and improve nutrient availability in crops. However, its role under elevated CO2 is currently unknown. Accordingly, a pot experiment was conducted in open-top chambers (CO2 at 400 and 600 μmol mol-1) to determine the protective effect of EBR on wheat plants under different Cd concentrations (0, 2, and 4 mg kg-1) in soil. The foliar application of EBR significantly improved growth, biomass, photosynthesis, proline, total phenol, and total soluble protein in Cd stress treatments under elevated CO2. Simultaneously, it significantly (p ≤ 0.05) increased catalase (42.89 %), superoxide dismutase (26.53 %), peroxidase (28.10 %), and ascorbate peroxidase (61.70 %) while reduced malondialdehyde (35.53 %), hydrogen peroxide (19.94 %), and electrolyte leakage (23.55 %) under elevated CO2 compared to ambient CO2 conditions. Furthermore, EBR and elevated CO2 interactively showed a maximum reduction in Cd concentrations and accumulation in the wheat roots (39.74,41.63 %), shoots (46.83,44.87 %), and grains (27.52,29.06 %) respectively. Elevated CO2 and Cd stress interactively showed a significant reduction in nutrient content. Conversely, the EBR application recovered and significantly increased calcium, magnesium, iron, zinc, and copper content in wheat roots, shoots, and grains. Our findings inferred that EBR foliar application reduced Cd toxicity and improved plant growth and nutritional quality under elevated CO2.

Machine learning and structural equation modeling for revealing the influence factors and pathways of different water management regimes acting on brown rice cadmium

Excessive cadmium (Cd) in brown rice has detrimental effects on rice growth and human health. Water management is a cost-effective, eco-friendly measure to suppress Cd accumulation in rice. However, there is no acknowledged water management regime that reduces Cd accumulation in brown rice without compromising the yield. Meanwhile, the major factors affecting brown rice Cd and the pathways of water management affecting rice Cd are not clear. This study explored major factors affecting brown rice Cd using machine learning (ML) and examined the pathways of water management affecting rice Cd using a structural equation model (SEM). Three water management systems were set up, namely flooding, water-saving, and wetting irrigation. Results showed that water-saving irrigation increased dry matter and reduced Cd content and translocation. Root uptake during the grain filling stage and Cd remobilization before the grain filling stage contributed 36 % and 64 % of the Cd accumulation in brown rice, respectively. ML explained 97 % of the variance, suggesting that crop covariates were the most important (e.g., the brown rice bioconcentration factor (12 %), stem Cd (9 %), root-to-stem translocation factor (7 %)), followed by soil covariates (e.g., reducing substances 12 %) and water management (3 %). All SEM explanatory variables collectively explained 94 % of the variation, with a predictive power of 76 %. Water treatments indirectly affected soil available Fe and Mn (indirect effect coefficient = 0.909), iron plaques (indirect effect coefficient = 0.866), soil available Cd (indirect effect coefficient = -0.671), and Cd intensity of xylem sap (BICd, indirect effect coefficient = -0.664) via pH and reducing substances. BICd significantly positively affected stem Cd (path coefficient = 0.445). These findings provide insight into the agronomic and environmental effects of water management on brown rice Cd and influence pathways in soil-rice systems, suggesting that water-saving irrigation may alleviate Cd contamination in the paddy soil.

Assessing impact of elevated CO2 on heavy metal accumulation in crops: meta-analysis and implications for food security

Human activities led to elevation in carbon dioxide (CO2) concentrations in atmosphere. While such increase per se may be beneficial for the growth of some crops, it comes with a caveat of affecting crop nutritional status. Here, we present a comprehensive analysis of changes in concentration of essential (Cu, Fe, Mn, Zn, Mo, Ni) and non-essential (Ba, Cd, Cr, Hg, Pb, and Sr) heavy metals in response to elevated CO2, drawing on a meta-analysis of 1216 paired observations. The major findings are as follows: (1) Elevated CO2 leads to reduced concentrations of Cu, Fe, Mn, and Zn in crops; (2) the extent of above reduction varies among plants species and is most pronounced in cereals and then in legumes and vegetables; (3) reduction in accumulation of non-essential (toxic) metals is less pronounced, potentially leading to an unfavorable essential/non-essential metal ratio in plants; (4) the above effects will come with significant implication to human health, exacerbating effects of the "hidden hunger" caused by the lack of Fe and Zn in the human diets. The paper also analyses the mechanistic basis of nutrient acquisition (both at physiological and molecular levels) and calls for the changes in the governmental policies to increase efforts of plant breeders to create genotypes with improved nutrient use efficiency for essential micronutrients while uncoupling their transport from non-essential (toxic) heavy metals.

Prediction of As and Cd dissolution in various soils under flooding condition

Although the mobility of arsenic (As) and cadmium (Cd) in soils during the flooding-drainage process has been intensively studied, predicting their dissolution among various soils still remains a challenge. After comprehensively monitoring multiple parameters related to As and Cd dissolution in 8 soils for a 60-day anaerobic incubation, the redundancy analysis (RDA) and structural equation model (SEM) were employed to identify the key factors and influencing pathways controlling the dynamic release of As and Cd. Results showed that pH alone explained 90.5 % Cd dissolution, while the dissolved-Fe(II) and 5 M-HCl extractable Fe(II) jointly only explained 50.6 % As dissolution. After data normalization, the ratio of Fe(II) to 5 M-HCl extracted total Fe (i.e. FetotII/Fetot) significantly improved the correlation to R2 = 0.824 (p < 0.001) with a fixed slope of 0.393 among the 8 soils. Our results highlight the crucial role played by the reduction degree of total iron contents in determining both the reduction and dissolution of As during flooding. In contrast, dissolved-Fe(II) was too vulnerable to soil properties to be a stable indicator of As dissolution. Therefore, we propose to replace the dissolved-Fe(II) with this novel ratio as the key index to quantitatively assess the kinetic change of As solubility potential across various soils under flooding conditions.

Speciation, bioaccumulation, and toxicity of the newly deposited atmospheric heavy metals in soil-earthworm (Eisenia fetida) system near a large copper smelter

The speciation, bioaccumulation, and toxicity of the newly deposited atmospheric heavy metals in the soil-earthworm (Eisenia fetida) system were investigated by a fully factorial atmospheric exposure experiment using soils exposed to 0.8-year and 1.8-year atmospheric depositions. The results shown that the newly deposited metals (Cu, Cd, and Pb) primarily accumulated in the topsoil (0-6 cm) and were present as the highly bioavailable speciation. They can migrate further to increase the concentrations of Cu, Cd, and Pb in soil solution of the deeper layer (at 10 cm) by 12 %-436 %. Earthworms tended to preferentially accumulate the newly deposited metals, which contributed 10 %-61 % of Cu, Cd, and Pb in earthworms. Further, for the unpolluted and moderately polluted soils, the newly deposited metals induced the significant oxidative stress in earthworms, resulting in significant increases in antioxidant enzyme activities (SOD, CAT, and GSH-Px). No significant differences were observed in the levels of heavy metals in soil solutions, bioaccumulation, and enzyme activities in earthworms exposed to 0.8-year and 1.8-year depositions, indicating the bioavailability of atmospheric metals deposited into soils was rapidly decreased with time. This study highlights the high bioaccumulation and toxicity of heavy metals to earthworm from the new atmospheric deposition during the earthworm growing period.

Microbiological Mechanisms of Collaborative Remediation of Cadmium-Contaminated Soil with Bacillus cereus and Lawn Plants

Severe cadmium contamination poses a serious threat to food security and human health. Plant-microbial combined remediation represents a potential technique for reducing heavy metals in soil. The main objective of this study is to explore the remediation mechanism of cadmium-contaminated soil using a combined approach of lawn plants and microbes. The target bacterium Bacillus cereus was selected from cadmium-contaminated soil in mining areas, and two lawn plants (Festuca arundinacea A'rid III' and Poa pratensis M'idnight II') were chosen as the target plants. We investigated the remediation effect of different concentrations of bacterial solution on cadmium-contaminated soil using two lawn plants through pot experiments, as well as the impact on the soil microbial community structure. The results demonstrate that Bacillus cereus promotes plant growth, and the combined action of lawn plants and Bacillus cereus improves soil quality, enhancing the bioavailability of cadmium in the soil. At a bacterial suspension concentration of 105 CFU/mL, the optimal remediation treatment was observed. The removal efficiency of cadmium in the soil under Festuca arundinacea and Poa pratensis treatments reached 33.69% and 33.33%, respectively. Additionally, the content of bioavailable cadmium in the rhizosphere soil increased by up to 13.43% and 26.54%, respectively. Bacillus cereus increased the bacterial diversity in the non-rhizosphere soil of both lawn plants but reduced it in the rhizosphere soil. Additionally, the relative abundance of Actinobacteriota and Firmicutes, which have potential for heavy metal remediation, increased after the application of the bacterial solution. This study demonstrates that Bacillus cereus can enhance the potential of lawn plants to remediate cadmium-contaminated soil and reshape the microbial communities in both rhizosphere and non-rhizosphere soils.

Molecular engineering of a new method for effective removal of cadmium from water

Cadmium (Cd) is a widespread and highly toxic environmental pollutant, seriously threatening animal and plant growth. Therefore, monitoring and employing robust tools to enrich and remove Cd from the environment is a major challenge. In this work, by conjugating a fluorescent indicator (CCP) with a functionalized glass slide, a special composite material (CCPB) was constructed to enrich, remove, and monitor Cd2+ in water rapidly. Then Cd2+ could be effectively eluted by immersing the Cd-enriched CCPB in an ethylenediaminetetraacetic acid (EDTA) solution. With this, the CCPB was continuously reused. Its recovery of Cd2+was above and below 100 % after multiple uses by flame atomic absorption spectrometry (FAAS), which was excellent for practical use in enriching and removing Cd2+ in real aqueous samples. Therefore, CCPB is an ideal material for monitoring, enriching, and removing Cd2+ in wastewater, providing a robust tool for future practical applications of Cd enrichment and removal in the environment.

Elevated CO2 increases soil redox potential by promoting root radial oxygen loss in paddy field

Soil redox potential (Eh) plays an important role in the biogeochemical cycling of soil nutrients. Whereas its effect soil process and nutrients' availability under elevated atmospheric CO2 concentration and warming has seldom been investigated. Thus, in this study, a field experiment was used to elucidate the effect of elevated CO2 concentration and warming on soil Eh, redox-sensitive elements and root radial oxygen loss (ROL). We hypothesized elevated CO2 and warming could alter soil Eh by promoting or inhibiting ROL. We found that soil Eh in the rhizosphere was significantly higher than that of non-rhizosphere. Elevated CO2 enhanced soil Eh by 11.5%, which corresponded to a significant decrease in soil Fe2+ and Mn2+concentration. Under elevated CO2, the concentration of Fe2+ and Mn2+ decreased by 14.7% and 13.7%, respectively. We also found that elevated CO2 altered rice root aerenchyma structure and promoted rice root ROL. Under elevated CO2, rice root ROL increased by 79.5% and 112.2% for Yangdao 6 and Changyou 5, respectively. Warming had no effect on soil Eh and rice root ROL. While warming increased the concentration of Mn2+ and SO42- by 4.9% and 19.3%, respectively. There was a significant interaction between elevated CO2 and warming on Fe2+ and Mn2+. Under elevated CO2, warming had no effect on the concentration of Fe2+ but decreased Mn2+ concentration significantly. Our study demonstrated that elevated atmospheric CO2 in the future could increase soil Eh by promoting rice root ROL, which will alter some soil nutrients' availability, such as Fe2+ and Mn2+.

Does elevated CO2 enhance the arsenic uptake by rice? Yes or maybe: Evidences from FACE experiments

Elevated CO₂ (eCO₂) strongly affects rice yield and quality in arsenic (As) paddy soils. However, understanding of the As accumulation in rice under coupled stress of eCO₂ and soil As is still limited while data are scarce. It greatly limits the prediction for future rice safety. This study investigated the As uptake by rice grown in different As paddy soils under two CO₂ conditions (ambient and ambient +200 μmol mol^-1) in the free-air CO₂ enrichment (FACE) system. Results showed that eCO₂ lowered soil Eh at the tillering stage and caused higher concentrations of dissolved As and Fe²⁺ in soil pore water. Compared with the control, the increased As transfer abilities in rice straws under eCO₂ contributed to the higher As accumulation in rice grains, and their total As concentrations were increased by 10.3-31.2%. Besides, the increased amounts of iron plaque (IP) under eCO₂ failed to effectively inhibit the As uptake by rice due to the difference in critical stage between As immobilized by IP (mainly in maturing stage) and uptake by rice roots (about 50% contribution before filling stage). Risk assessments suggest that eCO₂ enhanced the human health risks of As intake from rice grains produced in low-As paddy soils (<30 mg kg^-1). In order to alleviate the As threats to rice under eCO₂, we consider that proper soil drainage before filling stage to improve soil Eh can serve as an effective way to reduce As uptake by rice. Pursuing appropriate rice varieties to reduce the As transfer ability may be the other positive strategy.

Dynamic mechanisms of cadmium accumulation and detoxification by Lolium perenne grown in soil inoculated with the cadmium-tolerant bacterium strain Cdq4-2

Cadmium (Cd) contamination is a serious threat to food security and human health. The cost-effective in situ method of remediating Cd-contaminated soil uses Cd-tolerant microorganisms and Cd-enriching plants. The present study investigated the dynamic effects of inoculating soil with a Cd-tolerant bacteria strain Cdq4-2 (Enterococcus sp.) on the physiological and biochemical properties of perennial ryegrass Lolium perenne. The combined effects of remediating Cd-contaminated soil with this plant and these bacteria were also studied. An experiment was used to compare three treatments of L. perenne crops: 1) CK (control soil without Cd), 2) C (20 mg/kg Cd-contaminated soil), and 3) CB (20 mg/kg Cd-contaminated soil inoculated with bacteria Cdq4-2). The results show that compared with treatment C, the aboveground biomass, underground biomass, and total biomass of CB were 46.83-69.31%, 131.76-462.79%, and 62.65-101.53% greater, respectively. The superoxide dismutase activity of CB was 17.62-54.63% lower, while its peroxidase activity was 67.49-146.51% higher. The malondialdehyde concentration in CB was 30.40-40.24% more significant, the ascorbic acid concentration was 6.20-188.22% higher, and its glutathione concentration was 16.25-63.63% lower. The Cd concentrations of aboveground parts of a plant in treatment CB were 18.55% and 30.53% higher than those of C at days 20 and 40, respectively, while that of underground parts was 24.25% higher on day 40. The bioconcentration factors of aboveground and underground parts were higher in treatment CB on day 40. The inoculation of Cd-contaminated soils with bacteria Cdq4-2 promoted growth in L. perenne, improved its antioxidant ability, and promoted the absorption, translocation, and accumulation of Cd. Hence, it improved the effectiveness of L. perenne in remediating Cd-contaminated soils.

The clean energy aspect of plastic waste - hydrogen gas production, CO2 reforming, and plastic waste management coincide with catalytic pyrolysis - an extensive review

Clean hydrogen has future fuel capable of receiving an abundance of carbon-neutral energy from hydrogen. In the recent world, new hydrogen affirmation projects have been launched for a green environment. On another side, plastic waste and CO2 threaten the green environment. Vacuum in plastic waste management, plastic waste leads to exhibiting harmful chemicals to the environment. The growth rate of the CO2 concentration in air is 2.45 ppm per year, steadily increasing in 2022. It is realized that uneven climate change, temperature raising the global level, ocean mean level raising, and frequent acidification are dangerous to living and ecosystems. This review discussed tackling multiple harmful environmental fatly by pyrolysis techniques; catalytic pyrolysis is almost reaching the commercialization stage. Recent pyrolysis upgradation methods with hydrogen gas production and the continuous development and execution of sustainable solutions for plastic waste management and CO2 reforming are discussed. Production of carbon nanotubes by plastic waste, the importance of catalyst modification, and the effect of catalyst deactivation are discussed. From this study, integrating the different applications with catalytic modification creates room for multipurpose pyrolysis, CO2 reforming, and hydrogen gas production by pyrolysis techniques capable of giving a sustainable solution for climate change issues and a clean environment. Additionally, carbon utilization by way of carbon nanotube production is also done. Overall, the review supports achieving clean energy from plastic waste.