Elevated CO2 benefits the soil microenvironment in the rhizosphere of Robinia pseudoacacia L. seedlings in Cd- and Pb-contaminated soils

Elevated CO2 and nitrogen addition enhance the symbiosis and functions of rhizosphere microorganisms under cadmium exposure

Soil microbes are fundamental to ecosystem health and productivity. How soil microbial communities are influenced by elevated atmospheric carbon dioxide (eCO2) concentration and nitrogen (N) deposition under heavy metal pollution remains uncertain, despite global exposure of terrestrial ecosystems to eCO2, high N deposition and heavy metal stress. Here, we conducted a four year's open-top chamber experiment to assess the effects of soil cadmium (Cd) treatment (10 kg hm-2 year-1) alone and combined treatments of Cd with eCO2 concentration (700 ppm) and/or N addition (100 kg hm-2 year-1) on tree growth and rhizosphere microbial community. Relative to Cd treatment alone, eCO2 concentration in Cd contaminated soil increased the complexity of microbial networks, including the number links, average degree and positive/negative ratios. The combined effect of eCO2 and N addition in Cd contaminated soil not only increased the complexity of microbial networks, but also enhanced the abundance of microbial urealysis related UreC and nitrifying related amoA1 and amoA2, and the richness of arbuscular mycorrhiza fungi (AMF), thereby improving the symbiotic functions between microorganisms and plants. Results from correlation analysis and structural equation model (SEM) further demonstrated that eCO2 concentration and N addition acted on functions and networks differently. Elevated CO2 positively regulated microbial networks and functions through phosphorus (P) and Cd concentration in roots, while N addition affected microbial functions through soil available N and soil organic carbon (SOC) concentration and microbial network through soil Cd concentration. Overall, our findings highlight that eCO2 concentration and N addition make microbial communities towards ecosystem health that may mitigate Cd stress, and provide new insights into the microbiology supporting phytoremediation for Cd contaminated sites in current and future global change scenarios.

The microbial mechanisms by which long-term heavy metal contamination affects soil organic carbon levels

Globally, reducing carbon emissions and mitigating soil heavy metal pollution pose pressing challenges. We evaluated the effects of lead (Pb) and cadmium (Cd) contamination in the field over 20 years. The five treatment groups featured Pb concentrations of 40 and 250 mg/kg, Cd concentrations of 10 and 60 mg/kg, and a combination of Pb and Cd (60 and 20 mg/kg, respectively); we also included a pollution-free control group. After 20 years, soil pH decreased notably in all treatments, particularly by 1.02 in Cd10-treated soil. In addition to the increase of SOC in Cd10 and unchanged in Pb40 treatment, the SOC was reduced by 9.62%-12.98% under the other treatments. The α diversities of bacteria and fungi were significantly changed by Cd10 pollution (both p < 0.05) and the microbial community structure changed significantly. However, there were no significant changes in bacterial and fungal communities under other treatments. Cd10 pollution reduced the numbers of Ascomycota and Basidiomycota fungi, and enhanced SOC accumulation. Compared to the control, long-term heavy Cd, Pb, and Pb-Cd composite pollution caused SOC loss by increasing Basidiomycota which promoting carbon degradation, and decreasing Proteobacteria which promoting carbon fixation via the Krebs cycle. Our findings demonstrate that heavy metal pollution mediates Carbon-cycling microorganisms and genes, impacting SOC storage.

Alleviation of gadolinium stress on Medicago by elevated atmospheric CO2 is mediated by changes in carbohydrates, Anthocyanin, and proline metabolism

Rare earth elements (REE) like Gadolinium (Gd), are increasingly used in industry and agriculture and this is concomitant with the increasingly leaking of Gd into the environment. Under a certain threshold concentration, REE can promote plant growth, however, beyond this concentration, they exert negative effects on plant growth. Moreover, the effect of Gd on plants growth and metabolism under a futuristic climate with increasingly atmospheric CO2 has not yet been studied. To this end, we investigated the effect of soil contamination with Gd (150 mg/kg soil) on the growth, carbohydrates, proline, and anthocyanin metabolism of Medicago plants grown under ambient (aCO2, 410 ppm) or elevated CO2 (eCO2, 720 ppm) concentration. Gd negatively affected the growth and photosynthesis of plants and imposed oxidative stress i.e., increased H2O2 and lipid peroxidation (MDA) level. As defense lines, the level and metabolism of osmoprotectants (soluble sugars and proline) and antioxidants (phenolics, anthocyanins, and tocopherols) were increased under Gd treatment. High CO2 positively affected the growth and metabolism of Medicago plants. Moreover, eCO2 mitigated the negative impacts of Gd on Medicago growth. It further induced the levels of osmoprotectants and antioxidants. In line with increased proline and anthocyanins, their metabolic enzymes (e.g. OAT, P5CS, PAL, and CS) were also increased. This study advances our understanding of how Gd adversely affects Medicago plant growth and metabolism. It also sheds light on the biochemical mechanisms underlying the Gd stress-reducing impact of eCO2.

Metabolomics analysis reveal the molecular responses of high CO2 concentration improve resistance to Pb stress of Oryza sativa L. seedlings

Rice seedlings were exposed to two CO₂ concentrations (400 ± 20 and 800 ± 20 μmol mol^-1) and three PbNO₃ concentrations (0, 50 and 100 µmol L^-1) for 10 days to explore the regulatory mechanisms of elevated CO₂ for Pb stress resistance. Electrical conductivity, MDA content, SOD, POD, CAT activities and metabolomics changes were studied. Results showed that: Pb stress damaged cell membrane system, electrical conductivity and MDA content increased 49.34 % and 73.27 %, respectively, and some antioxidant enzymes activities increased. Sugar, polyol, amino acid metabolism and fatty acid β-oxidation were all enhanced to improve osmotic adjustments, maintain cell membrane stability, supply energy, nitrogen assimilates and antioxidant capacity; Under composite treatments, cell membrane damage was reduced, activities of protective enzymes increased compared with only Pb stress, POD activity increased the most (49.14 %) under severe Pb composite treatment. High CO₂ caused the enhance of cells antioxidant capacity, TCA cycle intermediate products contents and fatty acid desaturation under mild Pb stress. Many sugars, polyols and amino acids contents were increased as osmotic regulatory substances by high CO₂ under severe Pb stress; Secondary metabolites played an important role under Pb stress and composite treatments. The object of this study is to provide a possible molecular mechanism of rice response to Pb stress under high CO₂ in the future.

Global patterns of plant and microbial biomass in response to CO2 fumigation

Introduction

The stimulation of plant and microbial growth has been widely observed as a result of elevated CO₂ concentrations (eCO₂), however, this stimulation could be influenced by various factors and their relative importance remains unclear.

Methods

A global meta-analysis was performed using 884 lines of observations collected from published papers, which analyzed the eCO₂ impact on plant and microbial biomass.

Results

A significant positive impact of eCO₂ was observed on various biomass measures, including aboveground biomass (20.5%), belowground biomass (42.6%), soil microbial biomass (10.4%), fungal biomass (11.0%), and bacterial biomass (9.2%). It was found that eCO₂ levels above 200 ppm had a greater impact on plant biomass compared to concentrations at or below 200 ppm. On the other hand, studies showed that positive effects on microbial biomass were more prominent at lower eCO₂ levels (≤200 ppm) than at higher levels (>200 ppm), which could be explained by soil nitrogen limitations. Importantly, our results indicated that aboveground biomass was controlled more by climatic and experimental conditions, while soil properties strongly impacted the stimulation of belowground and microbial biomass.

Discussion

Our results provided evidence of the eCO₂ fertilization effect across various ecosystem types, experimental methods, and climates, and provided a quantitative estimate of plant and soil microbial biomass sensitivity to eCO₂. The results obtained in this study suggest that ecosystem models should consider climatic and edaphic factors to more accurately predict the effects of global climate change and their impact on ecosystem functions.

Effect of arbuscular mycorrhizal fungi (Glomus mosseae) and elevated air temperature on Cd migration in the rhizosphere soil of alfalfa

Cadmium (Cd) migration in the rhizosphere soil is easily affected by plants and microorganisms. Global warming significantly affects plant growth, and arbuscular mycorrhizal fungi (AMF) can chelate heavy metals by mycelium, cell wall components, and mycelial secretion. Here, we investigated the regulation of Glomus mosseae on Cd migration in the rhizosphere soil of alfalfa under elevated temperature (ET, + 3 °C). Elevated temperature significantly decreased G. mosseae colonization rate in the roots by 49.5% under Cd exposure. Under ET + G. mosseae + Cd relative to ET + Cd, the contents of free amino acids, total and easily extractable glomalin-related soil protein (GRSP), and root Cd increased significantly; however, the changes in DTPA-Cd in the rhizosphere soil and Cd in the shoots were insignificant. In addition, G. mosseae colonization enhanced the bioconcentration factor of Cd in the roots and the total removal rate of Cd in the rhizosphere soil by 63.4% and 16.3%, respectively, under ET + Cd. However, the changes in the expression of iron-regulated transport 1 (IRT1) and natural resistance-associated macrophage protein 1 genes were insignificant under ET + G. mosseae + Cd relative to ET + Cd. In summary, temperature and G. mosseae significantly affected Cd fate in the rhizosphere soil, and IRT1 gene and rhizosphere soil pH, N, and C/N ratio were significant factors influencing Cd migration. Additionally, G. mosseae improved the remediation efficiency of Cd-contaminated soils by alfalfa under ET. The results will help us understand the regulation of AMF on the phytoremediation of heavy metal-contaminated soils under global warming scenarios.

Recent Progress and Prospects in Catalytic Water Treatment

Presently, conventional technologies in water treatment are not efficient enough to completely mineralize refractory water contaminants. In this context, the implementation of catalytic processes could be an alternative. Despite the advantages provided in terms of kinetics of transformation, selectivity, and energy saving, numerous attempts have not yet led to implementation at an industrial scale. This review examines investigations at different scales for which controversies and limitations must be solved to bridge the gap between fundamentals and practical developments. Particular attention has been paid to the development of solar-driven catalytic technologies and some other emerging processes, such as microwave assisted catalysis, plasma-catalytic processes, or biocatalytic remediation, taking into account their specific advantages and the drawbacks. Challenges for which a better understanding related to the complexity of the systems and the coexistence of various solid-liquid-gas interfaces have been identified.

Salinization depresses soil enzyme activity in metal-polluted soils through increases in metal mobilization and decreases in microbial biomass

Salinity may increase metal mobilization with a potentially significant consequence for soil enzymatic activity and nutrient cycling. The goal of this study was to investigate changes in soil enzyme activity in response to salinization of a clay loam soil artificially polluted with cadmium (Cd) and lead (Pb) during a 120-day incubation experiment. Soil samples were polluted with Cd (10 mg Cd kg^-1), Pb (150 mg Pb kg^-1), and a combination of Cd and Pb, then preincubated for aging and eventually salinized with three levels of NaCl solution (control, low and high). NaCl salinity consistently increased the mobilization of Cd (12-22%) and Pb (5-16%) with greater increases at high (17-22% for Cd, 9-16% for Pb) than low (12% for Cd, 5-7% for Pb) salinity levels. While the increased Cd mobilization was greater in co-polluted (22%) than Cd-polluted (17%) soils, the increase of Pb mobilization was lower in co-polluted (9%) than Pb-polluted (16%) soils at high salinity level. The salinity-induced increases in metal mobilization significantly depressed soil microbial respiration (up to 43%), microbial biomass content (up to 63%), and enzymatic activities (up to 87%). The multivariate analysis further supported that the increased soil electrical conductivity, Cd mobilization, and pH after salinization were the most important factors governing microbial activity and biomass in metal-polluted soils. Results showed that changes in microbial biomass and mobile metal pool with increasing salinity had a major effect on enzyme activities, particularly under the combined metals. This study indicated that the secondary salinization of metal-polluted soils would impose an additional stress on enzymatic activities as biochemical indicators of soil quality, and therefore should be avoided for the maintenance of soil microbial and biochemical functions, especially in arid regions. In metal-polluted soils, the observed responses of extracellular and intracellular enzymes to salinity can be used to advance our knowledge of microbial processes when modeling the carbon and nutrient cycling.

Delineating the mechanisms of elevated CO2 mediated growth, stress tolerance and phytohormonal regulation in plants

Global climate change has drastically affected natural ecosystems and crop productivity. Among several factors of global climate change, CO₂ is considered to be the dynamic parameter that will regulate the responses of all biological system on earth in the coming decade. A number of experimental studies in the past have demonstrated the positive effects of elevated CO₂ on photosynthesis, growth and biomass, biochemical and physiological processes such as increased C:N ratio, secondary metabolite production, as well as phytohormone concentrations. On the other hand, elevated CO₂ imparts an adverse effect on the nutritional quality of crop plants and seed quality. Investigations have also revealed effects of elevated CO₂ both at cellular and molecular level altering expression of various genes involved in various metabolic processes and stress signaling pathways. Elevated CO₂ is known to have mitigating effect on plants in presence of abiotic stresses such as drought, salinity, temperature etc., while contrasting effects in the presence of different biotic agents i.e. phytopathogens, insects and herbivores. However, a well-defined crosstalk is incited by elevated CO₂ both under abiotic and biotic stresses in terms of phytohormones concentration and secondary metabolites production. With this background, the present review attempts to shed light on the major effects of elevated CO₂ on plant growth, physiological and molecular responses and will highlight the interactive effects of elevated CO₂ with other abiotic and biotic factors. The article will also provide deep insights into the phytohormones modulation under elevated CO₂.

Salinity-induced changes in cadmium availability affect soil microbial and biochemical functions: Mitigating role of biochar

Biochar may improve soil microbial and biochemical functions under abiotic stresses. In this research, we studied changes in soil microbial properties and processes after sugarcane bagasse biochar (SCB) application (1% w/w) to a soil contaminated with Cd under saline conditions during an incubation experiment. SCB produced at 400 °C (B400) and 600 °C (B600) increased soil organic carbon (SOC) content by 89-127% and dissolved organic carbon content by 21-70%. NaCl salinity mobilized Cd by 16-19%, while biochar immobilized Cd by 14-18%, indicating the use of biochar would offset the increase in Cd availability induced by salinity. SCB application improved microbial and biochemical functions (up to 280%) in the soils contaminated with Cd under salinity stress. B400 biochar was often more effective in improving the soil microbial properties and functioning than B600 biochar. SCB application reduced the detrimental effects of salinity-induced Cd toxicity on soil microbial community and enzyme activity mainly through retaining Cd and supplying C substrate for microbial uptake and activity. The factor analysis and redundancy analysis results also confirmed that SOC and Cd availability was the most important factors and accounted for a large portion of the variation in soil microbial properties and enzyme activities in saline Cd-contaminated soils amended with SCB. This study indicated that B400 applied at 1% could be used in saline Cd-contaminated soils to protect the soil microbial communities from Cd toxicity, and to mitigate the potential stresses associated with the co-occurrence of Cd contamination and salinity on critical soil microbial and biochemical functions.

Effects of elevated CO2 on arbuscular mycorrhizal fungi associated with Robinia pseudoacacia L. grown in cadmium-contaminated soils

As symbionts capable of reciprocal rewards, arbuscular mycorrhizal fungi (AMF) can alleviate heavy metal toxicity to host plants and are easily influenced by elevated CO₂ (ECO₂). Although the individual effects of ECO₂ and cadmium (Cd) on AMF have been widely reported, the response of AMF to ECO₂ + Cd receives little attention. We evaluated the combined effects of ECO₂ and Cd on AMF in the rhizosphere soil and roots of Robinia pseudoacacia L. seedlings. Under ECO₂ + Cd relative to Cd, AMF gene copies and richness in rhizosphere soils increased (p < 0.05) and the diversity reduced (p < 0.05) at 4.5 mg Cd kg^-1 dry soil; whereas root AMF abundance at 4.5 mg Cd kg^-1 dry soil and the diversity and richness reduced (p < 0.05). Elevated CO₂ caused obvious differences in the dominant genera abundance between rhizosphere soils and roots upon Cd exposure. Responses of C, water-soluble organic nitrogen (WSON), pH, and diethylene triamine penta-acetic acid (DTPA)-Cd in rhizosphere soils and root N to ECO₂ shaped dominant genera in Cd-polluted rhizosphere soils. Levels of DTPA-Cd, WSON, C and pH in rhizosphere soils and C/N ratio, N, and Cd in roots to ECO₂ affected (p < 0.05) dominant genera in roots under Cd exposure. AMF richness and diversity were lower in roots than in rhizosphere soils. Elevated CO₂ altered AMF communities in rhizosphere soils and roots of R. pseudoacacia seedlings exposed to Cd. AMF associated with R. pseudoacacia may be useful/interesting to be used for improving the phytoremediation of Cd under ECO₂.

Non-targeted metabolomics reveal the impact of phenanthrene stress on root exudates of ten urban greening tree species

Different root exudations can modify the bioavailability of persistent organic pollutants (POPs). Among these exudations, the low molecular weight organic acids play an imperative role in this process. The study was conducted to analyze the effect of phenanthrene (PHE) stress on root exudation variations and changes in its chemical composition in ten urban greening tree species, namely Loropetalum chinense, Gardenia ellis, Photinia fraseri, Ligustrum japonicum, Rhododendron simsii, Osmanthus fragrans, Gardenia jasminoides, Buxus sinica, Camellia sasanqua, and Euonymus japonicas. The experiment was carried out in three PHE concentration treatments (0 mg kg^-1 (CK), 200 mg kg^-1 (PHE_L), 2000 mg kg^-1 (PHE_H)). The root exudates were collected and analyzed by GC-MS method. In total, 673 compounds were identified either with high or low abundance among all species and treatments. Compounds identified in CK, PHE_L, and PHE_H were 240, 180, and 256, respectively. The results illustrated that carbohydrates, phenols, and esters were the dominant compounds, accounted for more than 92%. Principal component analysis depicted that tree species grown in PHE_H showed obvious alteration in compounds of root exudation, whereas little difference was noticed between PHE_L and CK. Phenols (80%) were the most abundant, while nitriles contributed a small portion. Moreover, among all species, R. simsii released the maximum number of compounds, and L. japonicum released the least number of compounds accounting for 89 and 46, respectively. The results achieved here to illustrate that plant type, and PHE stress can significantly change the concentrations and species of root exudates. This study provides the scientific reference for understanding the phenanthrene responsive changes in root exudates and phytoremediation of polycyclic aromatic hydrocarbons (PAHs), as well as a screening of urban greening tree species.

The combined effects of elevated atmospheric CO2 and cadmium exposure on flavonoids in the leaves of Robinia pseudoacacia L. seedlings

Flavonoids participate in several plant processes such as growth and physiological protection in adverse environments. In this study, we investigated the combined effects of eCO₂ and cadmium (Cd)-contaminated soils on the total flavonoid and monomer contents in the leaves of Robinia pseudoacacia L. seedlings. Elevated CO₂, Cd, and eCO₂+ Cd increased the total flavonoids in the leaves relative to the control, and eCO₂ mostly increased (p < 0.05) the total flavonoid content under Cd exposure. Elevated CO₂ increased (p < 0.05) robinin, rutin, and acacetin contents in the leaves of 45-day seedlings and decreased (p < 0.05) the content of robinin and acacetin at 90 and 135 d under Cd exposure except for robinin at day 45 under Cd1 and acacetin on day 135 under Cd1. Quercetin content decreased (p < 0.05) under the combined conditions relative to Cd alone. Kaempferol in the leaves was only detected under eCO₂ on day 135. The responses of total chlorophyll, total soluble sugars, starch, C, N, S, and the C/N ratio in the leaves to eCO₂ significantly affected the synthesis of total flavonoids and monomers under Cd exposure. Overall, rutin was more sensitive to eCO₂+ Cd than the other flavonoids. Cadmium, CO_2, and time had significant interactive effects on the synthesis of flavonoids in the leaves of R. pseudoacacia L. seedlings. Elevated CO₂ may improve the protection and defense system of seedlings grown in Cd-contaminated soils by promoting the synthesis of total flavonoids, although robinin, rutin, quercetin, and acacetin yields may reduce with time. Additionally, increased Cd in the leaves suggested that eCO₂ could improve the phytoremediation of Cd-contaminated soils.

Cadmium Immobilization in the Rhizosphere and Plant Cellular Detoxification: Role of Plant-Growth-Promoting Rhizobacteria as a Sustainable Solution

Food is the major cadmium (Cd)-exposure pathway from agricultural soils to humans and other living entities and must be reduced in an effective way. A plant can select beneficial microbes, like plant-growth-promoting rhizobacteria (PGPR), depending upon the nature of root exudates in the rhizosphere, for its own benefits, such as plant growth promotion as well as protection from metal toxicity. This review intends to seek out information on the rhizo-immobilization of Cd in polluted soils using the PGPR along with plant nutrient fertilizers. This review suggests that the rhizo-immobilization of Cd by a combination of PGPR and nanohybrid-based plant nutrient fertilizers would be a potential and sustainable technology for phytoavailable Cd immobilization in the rhizosphere and plant cellular detoxification, by keeping the plant nutrition flow and green dynamics of plant nutrition and boosting the plant growth and development under Cd stress.

A consecutive 4-year elevated air temperature shaped soil bacterial community structure and metabolic functional groups in the rhizosphere of black locust seedlings exposed to lead pollution

Global warming may influence the bioavailability and mobility of heavy metals by stimulating or inhibiting plant growth, thereby influencing rhizosphere soil chemistry and microbial characteristics. Black locust has been widely planted in China as a promising species for afforestation programs, farmland shelterbelt projects, and soil restoration in mined areas because of its rapid growth and adaptability to environmental stressors. Here, we examined soil bacterial community structure and predicted bacterial metabolic function in the rhizosphere of black locust exposed to elevated temperature (+1.99 °C) and Pb for 4 years. Elevated temperature significantly (p < 0.05) reduced total carbon (TC), total nitrogen (TN), and total sulfur (TS) contents in above-ground parts but increased TC and TN contents in roots and seedling height under Pb exposure. Elevated temperature significantly (p < 0.05) increased Pb availability and raised pH, TC, TN, TS and water-soluble organic carbon (WSOC) contents, and the C:H ratio in rhizosphere soils under Pb exposure. The interactive effects between Pb and temperature on pH, TC, TH, TS, WSOC, and the C:H ratio were significant (p < 0.05). Elevated temperature significantly (p < 0.05) reduced the diversity and the richness of bacterial community, altered genus-level bacterial community composition, and improved (p < 0.05) the relative abundances of some bacteria involving in terpenoids and polyketides and xenobiotics biodegradation metabolism under Pb exposure. Canonical correspondence analysis indicated that pH, WSOC, C:N ratio, and soluble Pb were significant (p < 0.05) factors on the relative abundance of bacterial genera, such as Ochrobactrum and Sphingomnas. Overall, long-term elevated temperature resulted in changes in rhizosphere soil characteristics and Pb availability, thus affecting the bacterial community structure and metabolic functional groups. The conclusion helps us understand the response mechanism of soil bacteria in the rhizosphere to heavy metals under global warming scenarios.

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.

Elevated atmospheric CO2 might increase the health risk of long-term ingestion of leafy vegetables cultivated in residual DDT polluted soil

Residual dichlorodiphenyltrichloroethane (DDT) in the environment and a continuously increasing atmospheric carbon dioxide (CO₂) concentration are two issues that have received a lot of attention. This study was conducted using a pot experiment to investigate the interactive effects of elevated CO₂ and DDT on the uptake of DDT, the physiological responses and the resulting health risks in three vegetables. These vegetables included Brassica juncea var. foliosa Bailey (B. Bailey), Brassica campestris L. var. communis Tsen et Lee Suzhou Qing (B. Lee) and Brassica campestris L. ssp. pekinensis (Lour.) Olsson Chun Dawang (B. Olsson). Two levels of CO₂ and four DDT treatment levels were set up. Results showed 5 mg kg^-1 DDT significantly reduced the shoot biomass of B. Bailey when compared to 0 mg kg^-1 DDT treatment under ambient CO₂ condition. Elevated CO₂ concentration stimulated the growth of B. Bailey and B. Lee, increased the DDT uptake in the shoots of both vegetables and the values of some photosynthesis indices, and triggered the activity of peroxidase and catalase in the shoots when compared to the related ambient CO₂ treatment. Elevated CO₂ concentration increased the values of hazard indexes for non-carcinogenic and cancer risks of all vegetables when compared to the individual ambient CO₂ treatment (each of vegetable has an ambient CO₂ treatment), especially for B. Bailey (increase amplitude of 123.81%-127.78% at 5 mg kg^-1 DDT). Long-term ingestion with these DDT-polluted vegetables might result in an elevated carcinogenic risk and elevated atmospheric CO₂ may enhance the non-carcinogenic and carcinogenic risks.

Effects of CO2 application coupled with endophyte inoculation on rhizosphere characteristics and cadmium uptake by Sedum alfredii Hance in response to cadmium stress

Comparative impact of CO₂ application and endophyte inoculation was investigated on the growth, rhizosphere characteristics, and cadmium (Cd) absorption of two ecotypes of Sedum alfredii Hance in response to Cd stress under hydroponic or rhizo-box culture conditions. The results showed that both CO₂ application and endophyte inoculation significantly (P < 0.05) promoted plant growth (fresh weight and dry weight), improved root morphological properties (SRL, SRA, SRV, ARD and RTN) and exudation (pH, TOC, TN, soluble sugar and organic acids), changed Cd uptake and distribution of both ecotypes of S. alfredii. Meanwhile soil total and DTPA extractable Cd in rhizo-box decreased by biofortification treatments. Superposition biofortification exhibits utmost improvement for the above mentioned parameters, and has potential for enhancing phytoremediation efficiency of hyperaccumulator and sustaining regular growth of non-hyperaccumulator in Cd contaminated soils.

Indirect Effect of Elevated CO2 on Population Parameters and Growth of Agasicles hygrophila (Coleoptera: Chrysomelidae), a Biocontrol Agent of Alligatorweed (Amaranthaceae)

Alligatorweed, Alternanthera philoxeroide (Mart.) Griseb. (Amaranthaceae) is an invasive weed in China that is often kept under control by the alligatorweed flea beetle, Agasicles hygrophila Selman and Vogt (Coleoptera: Chrysomelidae) introduced into China from Argentina in the 1980s. Elevated CO2 levels have been shown to have a direct effect on Ag. hygrophila. In order to fully evaluate the indirect effects of three different atmospheric concentrations of CO2 (420, 550, and 750 ppm) on the population parameters of Ag. hygrophila reared on Al. philoxeroides, we collected life table data for Ag. hygrophila using the age-stage, two-sex life table method. In general, there were no significant differences in the lengths of the preadult parameters among the three treatments. The adult duration and total longevity of males, however, did increase as CO2 increased in concentration. Although the adult preoviposition and total preoviposition periods decreased, the fecundity, oviposition days, eggs per oviposition day, net reproductive rate, intrinsic rate of increase, and finite rate of increase all increased significantly at the high CO2 concentration. Consequently, we determined that the Ag. hygrophila population size will potentially increase rapidly over a short period of time at elevated CO2 concentrations. Our results suggest that 550 and 750 ppm CO2 may also cause physiological changes in Al. philoxeroides that, in turn, provide enhanced nutrition for increasing reproduction in Ag. hygrophila by accelerating maturation of their reproductive system. These results indicate that the efficacy of Ag. hygrophila as a biological control agent against Al. philoxeroides will likely be increased at 550 and 750 ppm CO2.

Soil microbial community structure in the rhizosphere of Robinia pseudoacacia L. seedlings exposed to elevated air temperature and cadmium-contaminated soils for 4 years

The co-occurrence of heavy metal contamination of soils and increasing air temperature can affect the microbial community in rhizosphere soils by altering the allocation of plant photosynthates to roots. Here, we investigated the community structure of bacteria, fungi, ammonia oxidizing bacteria (AOB) and ammonia oxidizing archaea (AOA) in the rhizosphere of Robinia pseudoacacia L. seedlings exposed to elevated air temperature (+1.99 °C) and cadmium (Cd) for 4 years. Elevated temperature increased the richness of bacterial and AOA communities by 15.1% to 43.8% and by 1.4% to 18.6%, respectively, and decreased fungal and AOB richness by 3.7% to 28.7% and by 2.1% to 30.6%, respectively, under Cd exposure. Elevated temperature combined with Cd exposure decreased fungal diversity by 1.5% to 14.0%. However, elevated temperature decreased the diversity of bacteria, AOB and AOA by 1.4%, 17.4% and 10.1%, respectively, under 1.0 mg Cd kg^-1 dry soil and increased the diversity of these taxa by 1.5%, 15.3% and 9.2%, respectively, under 5.0 mg Cd kg^-1 dry soil relative to Cd exposure alone. Elevated temperature led to increased abundance of genera such as Methylobacterium, Stenotrophomonas, and Archangium and decreased abundance of genera including Ramlibacter, Microascus and Nitrosospira under Cd exposure. Over all, 4 years of exposure to elevated temperature had a greater effect on the community structure of bacteria, fungi, AOB and AOA when combined with Cd pollution.

Three years of exposure to lead and elevated CO2 affects lead accumulation and leaf defenses in Robinia pseudoacacia L. seedlings

Few studies have explored the long-term effects of elevated atmospheric CO₂ combined with lead (Pb) contamination on plants. The objective of this study was to examine the effects of 3 years of elevated CO₂ (700 ± 23 μmol mol^-1) on Pb accumulation and plant defenses in leaves of Robinia pseudoacacia L. seedlings in exposed to Pb (500 mg kg^-1 soil). Elevated CO₂ increased Pb accumulation in leaves and Pb removal rate in soils. In plants exposed to Pb stress, total chlorophyll and carotenoid contents in leaves were lower under elevated CO₂ than under ambient CO₂, but seedling height and width increased under elevated CO₂ relative to ambient CO₂. Elevated CO₂ significantly (p < .01) stimulated malondialdehyde content in leaves under Pb exposure. Superoxide dismutase and catalase activity increased significantly (p < .01), peroxidase activity decreased significantly (p < .01), and glutathione, cystine, and phytochelatin contents increased under elevated CO₂ + Pb relative to Pb alone. Elevated CO₂ stimulated the production of soluble sugars, proline, flavonoids, saponins, and phenolics in plants exposed to Pb stress. Ove rall, long-term elevation of CO₂ increased Pb-induced oxidative damage in seedlings, but enhanced the phytoextraction of Pb from contaminated soils.

Elevated tropospheric CO2 and O3 concentrations impair organic pollutant removal from grassland soil

The concentrations of tropospheric CO₂ and O₃ have been rising due to human activities. These rising concentrations may have strong impacts on soil functions as changes in plant physiology may lead to altered plant-soil interactions. Here, the effects of eCO₂ and eO₃ on the removal of polycyclic aromatic hydrocarbon (PAH) pollutants in grassland soil were studied. Both elevated CO₂ and O₃ concentrations decreased PAH removal with lowest removal rates at elevated CO₂ and elevated O₃ concentrations. This effect was linked to a shift in soil microbial community structure by structural equation modeling. Elevated CO₂ and O₃ concentrations reduced the abundance of gram-positive bacteria, which were tightly linked to soil enzyme production and PAH degradation. Although plant diversity did not buffer CO₂ and O₃ effects, certain soil microbial communities and functions were affected by plant communities, indicating the potential for longer-term phytoremediation approaches. Results of this study show that elevated CO₂ and O₃ concentrations may compromise the ability of soils to degrade organic pollutants. On the other hand, the present study also indicates that the targeted assembly of plant communities may be a promising tool to shape soil microbial communities for the degradation of organic pollutants in a changing world.

Leaf defense system of Robinia pseudoacacia L. seedlings exposed to 3years of elevated atmospheric CO2 and Cd-contaminated soils

Short-term exposure to elevated CO₂ increases cadmium (Cd) uptake in some plant species (wheat, poplars, and willows), which triggers an increase in antioxidative system activity to deal with additional reactive oxygen species that are generated. Here, we examined leaf defenses in Robinia pseudoacacia L. seedlings exposed to elevated CO₂+Cd for 3years. Three years of elevated CO₂ decreased Cd uptake into leaves and the Cd content in soils and increased the pH of rhizosphere soil relative to ambient CO₂. In plants exposed to Cd stress, leaf chlorophyll content was greater under elevated CO₂ than under ambient CO₂. Superoxide dismutase, peroxidase, and catalase activity increased, glutathione content increased, and malondialdehyde and phytochelatins contents decreased under elevated CO₂+Cd relative to Cd alone. Proline, soluble sugars, flavonoids, saponins, and phenolic acids contents were greater under elevated CO₂+Cd than under Cd alone, and condensed tannin content was lower. Overall, long-term elevation of CO₂ enhanced the leaf defense system of R. pseudoacacia exposed to Cd by stimulating antioxidant enzyme activity, osmotic adjustment, and the production of glutathione, flavonoids and phenolic acids. Future research should focus on understanding the mechanisms involved in the decrease in Cd uptake into leaves and Cd content in soils and the increase in rhizosphere soil pH under long-term exposure to elevated CO₂.