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Athyrium yokoscense is hypertolerant to cadmium (Cd) and can grow normally under a high Cd concentration despite Cd being a highly toxic heavy metal. To mitigate Cd stress in general plant species, Cd is promptly chelated with a thiol compound and is isolated into vacuoles. Generated active oxygen species (ROS) in the cytoplasm are removed by reduced glutathione. However, we found many differences in the countermeasures in A. yokoscense. Thiol compounds accumulated in the stele of the roots, although a long-term Cd exposure induced Cd accumulation in the aerial parts. Synchrotron radiation-based X-ray fluorescence (SR-XRF) analysis indicated that a large amount of Cd was localized in the cell walls of the roots. Overexpression of AyNramp5a, encoding a representative Fe and Mn transporter of A. yokoscense, increased both Cd uptake and Fe and Mn uptake in rice calli under the Cd exposure conditions. Organic acids are known to play a key role in reducing Cd availability to the plants by forming chelation and preventing its entry in free form into the roots. In A. yokoscense roots, Organic acids were abundantly detected. Investigating the chemical forms of the Cd molecules by X-ray absorption fine structure (XAFS) analysis detected many compounds with Cd-oxygen (Cd-O) binding in A. yokoscense roots, whereas in the aerial parts, the ratio of the compounds with Cd-sulfur (Cd-S) binding was increased. Together, our results imply that the strong Cd tolerance of A. yokoscense is an attribute of the following two mechanisms: Cd-O compound formation in the cell wall is a barrier to reduce Cd uptake into aerial parts. Thiol compounds in the region of root stele are involved in detoxication of Cd by formation of Cd-S compounds.

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Due to high humification, hyperthermophilic composting products (HP) show potential for remediating heavy metal pollution. However, the interaction between HP and heavy metals remains unclear. This study investigated the adsorption mechanism and soil remediation effect of HP on heavy metals. The results showed that the maximum adsorption capacity of HP increased by an average of 30.74 % compared to conventional composting products. HP transformed 34.87 % of copper, 42.55 % of zinc, and 35.63 % of lead from exchangeable and reducible forms into residual and oxidizable forms, thus reducing the soil risk level. In conclusion, HP significantly enhanced the adsorption of heavy metals and their transformation from unstable to stable forms, primarily due to the higher content of hydroxyl and carboxyl groups. This study aims to demonstrate the effectiveness of HP for remediating heavy metal pollution and to enhance the understanding of the underlying mechanism, which lays a foundation for waste utilization.

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Plants communicate underground by secreting multiple amino acids (AAs) through their roots, triggering defense mechanisms against cadmium (Cd) stress. However, the specific roles of the individual AAs in Cd translocation and detoxification remain unclear. This study investigated how exogenous AAs influence Cd movement from the roots to the shoots in Cd-resistant and Cd-sensitive Chinese cabbage cultivars (Jingcui 60 and 16-7 cultivars). The results showed that methionine (Met) and cysteine (Cys) reduced Cd concentrations in the shoots of Jingcui 60 by approximately 44% and 52%, and in 16-7 by approximately 43% and 32%, respectively, compared to plants treated with Cd alone. However, threonine (Thr) and aspartic acid (Asp) did not show similar effects. Subcellular Cd distribution analysis revealed that AA supplementation increased Cd uptake in the roots, with Jingcui 60 preferentially storing more Cd in the cell wall, whereas the 16-7 cultivar exhibited higher Cd concentrations in the organelles. Moreover, Met and Cys promoted the formation of Cd-phosphate in the roots of Jingcui 60 and Cd-oxalate in the 16-7 cultivar, respectively. Further analysis showed that exogenous Cys inhibited Cd transport to the xylem by downregulating the expression of HMA2 in the roots of both cultivars, and HMA4 in the 16-7 cultivar. These findings provide insights into the influence of exogenous AAs on Cd partitioning and detoxification in Chinese cabbage plants.

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The migration and transformation mechanisms of arsenic (As) in soil environments necessitate an understanding of its influencing processes. Here, we investigate the subsurface biogeochemical transformation of As and iron (Fe) through amended in the top 20 cm with iron oxidizing bacteria (FeOB) and organic fertilizer (OF). Our comprehensive 400-day field study, conducted in a calcareous soil profile sectioned into 20 cm increments, involved analysis by sequential extraction and assessment of microbial properties. The results reveal that the introduction of additional OF increased the release ratio of As/Fe from the non-specific adsorption fraction (136.47 %) at the subsoil depth (40-60 cm), underscoring the importance of sampling at various depths and time points to accurately elucidate the form, instability, and migration of As within the profile. Examination of bacterial interaction networks indicated a disrupted initial niche in the bottom layer, resulting in a novel cooperative symbiosis. While the addition of FeOB did not lead to the dominance of specific bacterial species, it did enhance the relative abundance of As-tolerant Acidobacteria and Gemmatimonadetes in both surface (39.2 % and 38.76 %) and deeper soils (44.29 % and 23.73 %) compared to the control. Consequently, the amendment of FeOB in conjunction with OF facilitated the formation of poorly amorphous Fe (hydr)oxides in the soil, achieved through abiotic and biotic sequestration processes. Throughout the long-term remediation process, the migration coefficient of bioavailable As within the soil profile decreased, indicating that these practices did not exacerbate As mobilization. This study carries significant implications for enhancing biogeochemical cycling in As-contaminated Sierozem soils and exploring potential bioremediation strategies. ENVIRONMENTAL IMPLICATION: The long-term exposure of sewage irrigation has potential adverse effects on the local ecosystem, causing serious environmental problems. Microorganisms play a vital role in the migration and transformation of arsenic in calcareous soil in arid areas, which highlights the necessity of understanding its dynamics. The vertical distribution, microbial community and fate of arsenic in calcareous farmland soil profile in northwest China were studied through field experiments. The results of this work have certain significance for the remediation of arsenic-contaminated soil in arid areas, and provide new insights for the migration, transformation and remediation of arsenic in this kind of soil.

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In this study, wild barley (Hordeum brevisubulatum) infected (E+) and uninfected (E-) by Epichloë bromicola were used for hydroponic experiments during the seedling stage. Various attributes, such as the effect of fungal endophyte on the growth and development of wild barley, the absorption of cadmium (Cd) and mineral elements (Ca, Mg, Fe, Mn, Cu, Zn), subcellular distribution, and chemical forms were investigated under CdCl2 stress. The results showed that the fungal endophy significantly reduced the Ca content and percentage of plant roots under Cd stress. The Fe and Mn content of roots, the mineral element content of soluble fractions, and the stems in the pectin acid or protein-chelated state increased significantly in response to fungal endophy. Epichloë endophyte helped Cd2+ to enter into plants; and reduced the positive correlation of Ca-Fe and Ca-Mn in roots. In addition, it also decreased the correlation of soluble components Cd-Cu, Cd-Ca, Cd-Mg in roots, and the negative correlation between pectin acid or protein-chelated Cd in stems and mineral elements, to increase the absorbance of host for mineral elements. In conclusion, fungal endophy regulated the concentration and distribution of mineral elements, while storing more Cd2+ to resist the damage caused by Cd stress. The study could provide a ground for revealing the Cd tolerance mechanism of endophytic fungal symbionts.

The present study is the first to study the effect of fungal endophy on essential mineral elements of plants under heavy metal stress, filling a gap in the existing research. The study could be helpful to reveal the mechanism of endophytic fungi to improve the host's tolerance to heavy metals and provide a foundation for the grass-endophyte symbionts to improve heavy metal-contaminated soils as ecological grasses.

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Due to the increasing scarcity of wild resources, most traditional Chinese medicinal materials (TCMMs) in the market are produced via artificial cultivation. The widespread pollution of cadmium (Cd) in soil limits the safe cultivation and use of TCMMs. This study investigated Cd accumulation, distribution, and the medicinal component content under simulated field conditions to clarify the differences in the Cd absorption, transfer and detoxification mechanisms of Polygonatum cyrtonema Hua and Bletilla striata, and provide the preliminary safe utilization conditions of TCMMs based on the analytic hierarchy process (AHP). The results showed that the Cd content of P. cyrtonema Hua was lower than the safety threshold under a high soil Cd concentration of 0.91 mg/kg (Cd-L), while B. striata was safe only at a low Cd concentration of 0.25 mg/kg (CK). Cd at 0.91 mg/kg induced hormesis affecting the net increase in biomass and medicinal component content for both TCMMs, while P. cyrtonema Hua showed better potential for safe utilization. Additionally, P. cyrtonema Hua had stronger resistance to Cd stress, exhibiting superior characteristics for synergistic absorption of Cd with mineral elements, transfer to nonmedical part and safer fixation forms in subcellular components. In contrast, B. striata showed insufficient Cd tolerance, and Cd was easily accumulated in organelles to inhibit plant growth. Our findings may attract more attention to the safe cultivation of TCMMs and provide insight into guidance for the safe utilization of slightly Cd-contaminated soil.

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Magnesium (Mg) and calcium (Ca) are two essential macronutrients in plants; however, the characteristics of Mg and Ca concentrations in organ, subcellular and chemical forms and their relationships in citrus plants, especially under varying Mg supply, are not well understood. In this study, Citrus sinensis seedlings (cv. Xuegan) were cultivated in conditions of Mg deficiency (0 mmol Mg2+ L-1) and Mg sufficiency (2 mmol Mg2+ L-1) to investigate the responses of Mg and Ca homeostasis in different organs and fractions. Compared with Mg sufficiency, Mg deficiency significantly decreased root and shoot growth, with the shoot biomass reduction of branch organs was greater than that of parent organs. In addition to increasing the Ca concentration in the parent stem and lateral root organs, Mg deficiency significantly decreased the concentrations and accumulations of Mg and Ca in citrus seedlings, further altering their distribution in different organs. More than 50% of Ca and Mg were sequestrated in the cell wall and soluble fractions, respectively, with Mg concentration decreasing by 15.4% in roots and 46.9% in leaves under Mg deficiency, while Ca concentration decreased by 27.6% in roots and increased by 23.6% in parent leaves. Approximately 90% of Mg exists in inorganic, water-soluble, and pectate and protein-bound forms, and nearly 90% of Ca exists in water-soluble, pectate and protein-bound, phosphate and oxalate acid forms. Except for the decreased inorganic Mg in roots and water-soluble Mg and Ca in leaves, Mg deficiency increased the proportions of Mg and Ca in all chemical forms. However, Mg deficiency generally increased the Ca/Mg ratio in various organs, subcellular and chemical forms, with negative relationships between Mg concentration and Ca/Mg ratio, and the variations of Mg and Ca were highly separated between Mg supply and organs. In conclusion, our results provide insights into the effects of Mg supply on Mg and Ca homeostasis in citrus plants.

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Arbuscular mycorrhizal fungi (AMF) can alleviate heavy metal phytotoxicity and promote plant growth, while the underlying mechanisms of AMF symbiosis with host plants under manganese (Mn) stress remain elusive. A pot experiment was carried out to investigate the plant growth, micro-structure, Mn accumulation, subcellular distribution, chemical forms, and physiological and biochemical response of Rhus chinensis inoculated with Funneliformis mosseae (FM) under different Mn treatments. The results showed that compared with plants without FM, FM-associated plants exhibited higher growth status, photosynthetic pigments, and photosynthesis under Mn stress. FM-associated plants were able to maintain greater integrity in mesophyll structure, higher thickness of leaf, upper epidermis, and lower epidermis under Mn treatment, and promote leaf growth. Mn accumulation in leaves (258.67-2230.50 mg kg-1), stems (132.67-1160.00 mg kg-1), and roots (360.92-2446.04 mg kg-1) of the seedlings inoculated with FM was higher than non-inoculated ones. FM-associated plants exhibited higher osmotic regulating substances and antioxidant enzymes' activities under Mn exposure, suggesting lower Mn toxicity in FM inoculated seedlings, despite the augment in Mn accumulation. After FM inoculation, Mn concentration (151.04-1211.32 mg kg-1) and percentage (64.41-78.55%) enhanced in the cell wall, whilst the transport of Mn to aerial plant organs decreased. Furthermore, FM symbiosis favored the conversion of Mn from high toxic forms (2.17-15.68% in FEthanol, 11.37-24.52% in Fdeionized water) to inactive forms (28.30-38.15% in FNaCl, 18.07-28.59% in FHAc, 4.41-17.99% in FHCl) with low phytotoxicity. Our study offers a theoretical basis for remediation of the FM- R. chinensis symbiotic system in Mn-contaminated environments.

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Salt interference significantly affects the behavior of heavy metals in the environment. This study compared and analyzed the response process, migration, and transformation of cadmium (Cd) in the hyperaccumulator Solanum nigrum (S. nigrum) under different NaCl levels to reveal the interference mechanisms of salt in plant remediation of Cd-contaminated soil. The results showed that Cd and salt stress significantly inhibited the growth of plants. The stress effect had more potent growth inhibition at the root than aboveground, thus inducing changes in the spatial configuration of the plants (decreased root-to-aboveground biomass ratio). Salt could activate Cd in plants, enhancing the inhibitory effect on plant growth. Salt increased Cd bioavailability due to the rhizosphere acidification effect, increasing plants' Cd accumulation. The Cd bioconcentration factor in plant roots peaked during the high Cd-high salt treatment (117.10), but the Cd accumulation of plants peaked during the high Cd-low salt treatment (233.04 µg plant-1). Salt additions and increased Cd concentrations enhanced root compartmentalization, inhibiting Cd transport to the aboveground. Changes in Fourier-transform infrared spectroscopy (FTIR) measurements confirmed that the functional groups in plants provided binding sites for Cd. These findings can help guide the phytoremediation of Cd contamination under saline soil conditions.

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Cadmium (Cd) and polystyrene microplastics (PS) co-contamination always occurs in environment; however, the trophic transfer of Cd and PS is still poorly understood. A hydroponic experiment was conducted to investigate the behavior of Cd in lettuce, together with the root or foliar exposure of different sized PS. Accumulation and chemical form distributions of Cd in leaves were distinguished into young and mature leaves. Subsequently, a 14-day snail feeding experiment was performed. Data showed that Cd accumulation in roots, rather than in leaves, are significantly affected by PS coexistence. However, mature leaves had a higher Cd content than young leaves under the root exposure of PS, while a reverse effect was observed in the foliar exposure. There existed a positive correlation between the food-chain transfer associated Cd (CdFi+Fii+Fiii) in mature leaves and Cd content in snail soft tissue (r = 0.705, p < 0.001), but not in young leaves. Though no bio-amplification of Cd in food chain was observed, an increase of Cd transfer factor (TF) from lettuce to snail was noted in the root exposure of 5 µm PS and the foliar exposure of 0.2 µm PS. Moreover, we observed a highest increase rate of 36.8 % in TF values from lettuce to snail viscera, and a chronic inflammatory response in snail stomach tissue. Therefore, more attentions should be paid to study the ecological risks of heavy metals and microplastics co-contamination in environment.

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Dark septate endophytes (DSEs) can improve the performance of host plants grown in heavy metal-polluted soils, but the mechanism is still unclear. A sand culture experiment was performed to investigate the effects of a DSE strain (Exophiala pisciphila) on maize growth, root morphology, and cadmium (Cd) uptake under Cd stress at different concentrations (0, 5, 10, and 20 mg·kg-1). The results indicated that the DSE significantly improved the Cd tolerance of maize, causing increases in biomass, plant height, and root morphology (length, tips, branch, and crossing number); enhancing the Cd retention in roots with a decrease in the transfer coefficient of Cd in maize plants; and increasing the Cd proportion in the cell wall by 16.0-25.6%. In addition, DSE significantly changed the chemical forms of Cd in maize roots, resulting in decreases in the proportions of pectates and protein-integrated Cd by 15.6-32.4%, but an increase in the proportion of insoluble phosphate Cd by 33.3-83.3%. The correlation analysis revealed a significantly positive relationship between the root morphology and the proportions of insoluble phosphate Cd and Cd in the cell wall. Therefore, the DSE improved the Cd tolerance of plants both by modifying root morphology, and by promoting Cd binding to the cell walls and forming an insoluble phosphate Cd of lower activity. These results of this study provide comprehensive evidence for the mechanisms by which DSE colonization enhances Cd tolerance in maize in root morphology with Cd subcellular distribution and chemical forms.

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Bermudagrass is a perennial herb with the potential to remediate Pb pollution in soils, and it has mechanical resistance to shearing. However, the effects of mowing on Pb absorption and accumulation in bermudagrass are still unclear. In this study, we investigated the effects of different quantities (0, 1, 2, 4 applications) of mowing treatments under 200 mg L-1 Pb application on Pb accumulation and transport in bermudagrass and explored the related mechanisms. Compared to the Pb treatment, all of the mowing treatments greatly decreased root Pb concentration/accumulation, significantly enhanced Pb concentrations/accumulations in stubble stems and stubble leaves, and ultimately promoted Pb enrichment and transport. Of the treatments in this study, two applications of mowing best promoted Pb enrichment, and four applications of mowing best promoted Pb transport efficiency. Furthermore, mowing mediated the microdistribution and physiological patterns of Pb in bermudagrass and affected the Pb transport by changing the subcellar distribution patterns and chemical forms of Pb in various tissues. Additionally, mowing promoted the transport of all mineral elements and showed a synergistic relationship with Pb absorption and transport. The change in mineral element metabolism patterns may be an important reason why mowing promoted Pb accumulation in bermudagrass. Our study provides the first comprehensive evidence regarding mowing facilitating the absorption, accumulation and transport of Pb in bermudagrass. Moderate mowing may be an effective strategy to assist in soil Pb remediation using bermudagrass.

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BACKGROUND: Manganese (Mn) is an essential micronutrient for plants, whereas excess Mn(II) in soils leads to its toxicity to crops. Mn(II) is adsorbed onto plant roots from soil solution and then absorbed by plants. Root charge characteristics should affect Mn(II) toxicity to crops and Mn(II) uptake by the roots of the crops. However, the differences in the effects of root surface charge on the uptake of Mn(II) among various crop species are not well understood. RESULTS: The roots of nine legumes and six non-legume poaceae were obtained by hydroponics and the streaming potential method and spectroscopic analysis were used to measure the zeta potentials and functional groups on the roots, respectively. The results indicate that the exchangeable Mn(II) adsorbed by plant roots was significantly positively correlated with the Mn(II) accumulated in plant shoots. Legume roots carried more negative charges and functional groups than non-legume poaceae roots, which was responsible for the larger amounts of exchangeable Mn(II) on legume roots in 2 h and the Mn(II) accumulated in their shoots in 48 h. Coexisting cations, such as Ca2+ and Mg2+ , were most effective in decreasing Mn(II) taken up by roots and accumulated in shoots than K+ and Na+ . This was because Ca2+ and Mg2+ could compete with Mn(II) for active sites on plant roots more strongly compared to K+ and Na+ . CONCLUSION: The root surface charge and functional groups are two important factors influencing Mn(II) uptake by roots and accumulation in plant shoots. © 2023 Society of Chemical Industry.

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Cadmium (Cd), a ubiquitous and highly toxic heavy metal pollutant, is toxic to animals and plants. Calcium (Ca) is an essential component for plant growth and reduces plant Cd absorption by competing with Cd. To gain deeper insight into the effects of Ca on Cd absorption, translocation, subcellular distribution, and chemical forms in S. matsudana seedlings under Cd stress, an investigation was conducted on these properties. Adding Ca alleviated Cd physiological toxicity in S. matsudana, reduced Cd absorption, increased the translocation from roots to shoots, lead to subcellular redistribution of Cd by increasing the proportion of Cd in soluble fractions but decreasing Cd in the cell wall and changed the chemical forms of Cd from 0.6 mol/L HCl- and 2% HAc-extracted Cd to 1 mol/L NaCl-extracted Cd. The energy dispersive X-ray analyses (EDXA) results revealed that after adding Ca, Cd was transferred through the root epidermis, cortex, endodermis, and vascular cylinder, transported to the shoots, and was highly accumulated in leaf epidermal and mesophyll cells, but less in leaf vein and guard cells. The genes involved in Cd uptake and xylem loading included NRAMP1, ZIP8, HMA2, and HMA4, which were up-regulated significantly (P < 0.05) in the Cd and Cd + Ca treatments compared to the control. The findings of this study provide new insight into the mechanism that Ca alleviates Cd toxicity in woody tree species, as well as propose an important prospect of Ca addition for improving the phytoremediation of Cd contamination.

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Kentucky bluegrass (Poa pratensis L.) hyperaccumulates cadmium (Cd) and exhibits a hypertolerance. Thus, it has potential for the phytoremediation of Cd-containing soil. Auxin signaling is involved in the response to Cd stress. However, the mechanisms of auxin-mediated detoxification and Cd translocation in plants remain unclear. This study aimed to investigate the effects of exogenous application of indole-3-acetic acid (IAA) on the Cd translocation, subcellular Cd distribution, chemical forms of Cd, and transcriptional regulation of Kentucky bluegrass. The results showed that the exogenous application of IAA increased the amount of organelle-bound Cd and vacuole-compartmentalized Cd in root cells, reduced the Cd concentration in the leaf tissues (epidermis, mesophyll, and vascular bundle) and root tissues (rhizodermis and cortex) but increased in the stele, and alleviate Cd-induced leaf chlorosis and growth inhibition. The expression of genes associated with Cd transporters (ABCs, ZIPs, NASs, OPTs, and YSLs), phosphatases, oxalate decarboxylases and lignin biosynthesis were significantly regulated by exogenous IAA under Cd stress. A positive regulation of phosphatases and oxalate decarboxylases genes related to an increase in phosphate- and oxalate-bound Cd, as well as a decrease in pectate- and protein-bound Cd and inorganic Cd, thereby contributing to a decrease in Cd phytotoxicity. The significant regulation of Cd transporters associated with decreasing the long-distance translocation of Cd, and the activation of lignin biosynthesis may contribute to the development of root endodermal barriers and increase the deposition of undissolved Cd phosphates and oxalate-bound Cd in the stele. These results revealed the important role of auxin in Cd detoxification and translocation in Kentucky bluegrass and they provide a theoretical basis for the phytoremediation of Cd-containing soil.

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This study aimed to investigate the combined ecotoxicological effects of Cd, Pb, Zn, Hg and regulation mechanisms in Solanum nigrum L. In this work, the co-exposure of these four heavy metals hindered the transformation of Cd, Zn, and Hg (except Pb) from available to non-available chemical forms. Individual Cd, Pb, Zn and Hg induced the oxidative damages to S. nigrum L., while their combination further enhanced this ecological toxicity. By internal regulation, the ecological toxicity of metals to S. nigrum L. could be alleviated to a certain extent. Specifically, S. nigrum L. was a hyperaccumulator of Cd with BCF ＞1. Moreover, since BCFroot of Pb, Zn and Hg were all greater than BCFshoot, S. nigrum L. could accumulate Pb, Hg and Zn mainly in plant roots, which was beneficial for the detoxification of plants. Meanwhile, the immobilization by cell wall (the proportions of Cd, Pb, Zn and Hg in the cell wall were 54.46-84.92%, 38.33-49.25%, 48.38-56.19% and 45.97-63.47% in low metal concentration treatments) and the sequestration in vacuole (the proportions of Cd, Pb, Zn, and Hg in the soluble fractions are 50.99-59.00%, 41.05-45.46%, 37.54-61.04% and 33.47-61.35% in high metal concentration treatments) also act as important detoxification pathways. The external regulation was mainly the changes of soil microbial communities influenced by plants. Specifically, the richness and diversity of bacteria in rhizosphere soil were increased, and roots of S. nigrum L. recruited some potentially beneficial microbials. This study provided a theoretical basis and guidance for S. nigrum L. as a phytoremediation plant under combined heavy metal pollution.

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Soil cadmium (Cd) contamination seriously reduces the production and product quality of Tartary buckwheat (Fagopyrum tataricum), and strategies are urgently needed to mitigate these adverse influences. Herein, we investigated the effect of salicylic acid (SA) on Tartary buckwheat seedlings grown in Cd-contaminated soil in terms of Cd tolerance and accumulation. The results showed that 75-100 µmol L-1 SA treatment enhanced the Cd tolerance of Tartary buckwheat, as reflected by the significant increase in plant height and root and shoot biomass, as well as largely mitigated oxidative stress. Moreover, 100 µmol L-1 SA considerably reduced the stem and leaf Cd concentration by 60% and 47%, respectively, which is a consequence of increased root biomass and root Cd retention with promoted Cd partitioning into cell wall and immobile chemical forms. Transcriptome analysis also revealed the upregulation of the genes responsible for cell wall biosynthesis and antioxidative activities in roots, especially secondary cell wall synthesis. The present study determines that 100 µmol L-1 is the best SA concentration for reducing Cd accumulation and toxicity in Tartary buckwheat and indicates the important role of root in Cd stress in this species.

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Daucus carota is a biennial herb of the Umbelliferae family, which is a candidate plant for the phytoremediation of Mn pollution. To reveal the mechanism of this plant to adapt to Mn stress, plant growth, anatomical structure, Mn accumulation characteristic, Mn subcellular distribution, and chemical forms of D. carota under six Mn2+ concentrations by pot culture experiments were studied. The results showed that with the rising Mn concentrations, the total dry weight and leaf area of D. carota increased firstly and then decreased, while the specific leaf area increased. The thickness of the main vein, upper epidermis, and lower epidermis; the thickness of the palisade tissue; and the thickness of the spongy tissue of the leaves increased firstly and then decreased. The Mn content in the aboveground and underground parts of D. carota increased, and the values of the bioconcentration factor (BCF) and translocation factor (TF) were higher than 1. The Mn existing in the cell wall and soluble components accounted for the largest proportion, and the proportion of Mn in the cell wall increased with increasing concentrations of Mn. In addition, Mn mainly existed in ethanol extraction state, deionized water extraction state, and sodium chloride extraction state. The results showed that D. carota could alleviate the damage caused by high manganese concentration by storing most of manganese in the cell wall and vacuole and existing in the form of low-activity state.

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Lability and bioaccessibility of anthropogenic toxic heavy metals in arid calcareous soils are critical to understand their ecological and health risks. This study examined toxic heavy metal speciation in the calcareous soil contaminated by nonferrous metal smelting. Results demonstrated that approximately 70 years' nonferrous metal smelting and mining in Baiyin led to significant contamination of nearby soil down to about 200 cm depth by cadmium (Cd), copper (Cu), lead (Pb), and zinc (Zn), with more serious contamination in the downwind areas of smelting or mining. More than half of Cd, Cu, Pb, and Zn in the soil was present in the labile fractions while more than 75% of cobalt (Co), chromium (Cr), nickel (Ni), and vanadium (V) was present in the residual fraction. Carbonate minerals in this calcareous soil play an important role in the labile fractions, with approximate 25% of Cd and Pb and 15% of Cu and Zn bound in carbonates. Bioaccessible Cd, Cu, Pb, and Zn in the soil were approximately 49.8%, 29.4%, 12.2%, and 33.8% in gastric phase and 13.5%, 15.9%, 4.3%, and 9.1% in intestinal phase of their total concentrations, respectively. Therefore, Cd and Zn were removed from gastric solution to a greater extent than Cu and Pb by neutral intestine environment. However, bioaccessible Co, Cr, Ni, and V in the soil were less than 3% of their total concentrations. Bioaccessibility of these metals but Cu in this calcareous soil was significantly lower than that for the acidic Ultisols and Alfisols in U.S. The concentrations of Cd, Cu, Pb, Zn, and Ni in each chemical and bioaccessible forms were significantly correlated linearly with their total concentrations in the calcareous soil, while only residual concentration was significantly correlated with the total concentration for Co, Cr, and V. These linear slopes showed that relative lability and bioaccessibility increased for Cd, but decreased for Cu, Pb, and Zn with the increase in their total concentrations in the calcareous soil. Direct oral soil ingestion would not pose a non-carcinogenic health risk to local children. However, very high potential ecological risk would be caused by these metals in the soil. These results provide improved insights into the biogeochemical processes of anthropogenic toxic heavy metals in the arid calcareous soils worldwide.

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Aging behaviors of metals in the field differ from those in a controlled laboratory environment. Whether aging conditions influence the fates of metals in soil remains unclear. In this study, distributions of cadmium (Cd) and phosphatase activity were compared in soil aggregates (i.e., >2, 1-2, 0.25-1, and <0.25 mm) along a profile (0-5, 5-10, and 10-15 cm) at the end of 500-day aging experiments under both controlled laboratory and field conditions. Cd concentration in the 0-5 cm layer was lower and Cd concentration in the 5-10 cm layer was higher in field-aged soil compared to laboratory-aged soil. 25.26-35.62% of soil Cd was loaded in >2 mm aggregates of field-aged soils, and 58.41-66.95% was in laboratory-aged soils. Higher loadings of Cd in 0.25-1 and <0.25 mm aggregates were found in field-aged soil. A higher proportion of exchangeable Cd fraction (20.93% of total soil Cd) was found in the 0-5 cm layer of field-aged soil than in laboratory-aged soil (17.63%), while the opposite tendency was found in deeper soil layers. Soil phosphatase activities in field-aged soils were 1.13-1.26 times higher than in laboratory-aged soils. Phosphatase loadings in the >2 mm aggregates were lower and loadings in both the 1-2 and 0.25-1 mm aggregates were higher in field-aged soils than in laboratory-aged soils. Furthermore, correlation analysis and principal component analysis indicated that available Cd fractions accounted for most of the variations in phosphatase activities. In summary, the fates of the exogenous metal Cd differed between field and controlled laboratory conditions. To better understand the behaviors of heavy metals in soil, especially in a seasonal freeze-thaw area, further field studies are needed.

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