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Root exudates are carriers for the transfer of material, energy and information between plant roots and soils. Plants encountering environmental stresses such as heavy metal pollution adapt to the environment by producing and secreting root exudates. In this study, laboratory soil culture experiment and pot experiment with Sedum plumbizincicola were used to study the effects of single and combined application of three root exudates, citric acid, glycine, and fructose, on the Cd-activation and phytoremediation of Cd-contaminated paddy soil. Results from the soil culture experiment showed that for the single application of root exudates, all three root exudates significantly activated the Cd in soil as presented by the increased content of diethylenetriamine pentaacetic acid extracted Cd (DTPA-Cd). In Particular, citric acid (SC) at a relatively low concentration (2 mmol/kg) exhibited the best Cd activation efficiency by increasing DTPA-Cd in the soil by 66.12%. For the combined application of root exudates, citric acid in combination with glycine (SC + G, 1:3) had the best activation effect on the Cd in the soil. In the phytoremediation pot experiment, both the single application of citric acid at a low concentrate (1 mmol/kg) and the combined application of citric acid and glycine (1:1) significantly reduced the total Cd and DTPA-Cd in the soil and increased the biomass and the content of Cd in S. plumbizincicola ; thus, the phytoremediation efficiency of Cd-contaminated soil increased by 42.33% and 35.61%. The results from this study suggest that citric acid plays a crucial role in Cd activation and phytoremediation with single or combined applications with glycine. However, the mechanisms under the synergetic interaction between citric acid and glycine require further investigation.

The geochemistry of major and trace elements (including heavy metals and rare earth elements) of the fresh and weathered black shales, and the soils derived from black shales in the Ganziping mine area in western Hunan province (China) were studied using the following techniques: X-ray fluorescence (XRF), inductively coupled plasma mass spectrometer (ICP-MS) and X-ray diffraction (XRD). The results show that the black-shale soils are significantly enriched with Al₂O₃ and Fe₂O₃, and depleted of mobile elements CaO, Na₂O and K₂O. The soils are also highly enriched with heavy metals U, V, Ni, Ba, Cu, Zn and Pb, that may cause potential heavy-metal contamination of the soils. Composition of the soils is homogeneous compared to the weathered black shales, for which the concentrations of major elements except CaO and Na₂O, and trace elements except heavy metals (U, V, Ni, Ba, Cu, Zn and Pb) as well as the mobile Sr, show lower variations than in the weathered black shales. Ratios of Zr/Hf, Ta/Nb, Y/Ho, Nd/Sm, and Ti/(Ti + Zr), of the soils are also less variable, with values constantly similar to that of the fresh and weathered black shales correspondingly. Thus, components of the soils are believed to be contributed from the parent black shales through weathering and pedogenesis. It is concluded that the soils were formed by at least two stages of geochemical processes: the early stage of chemical differentiation and the later stage of chemical homogenization. The chemical differentiation that was taken during black-shale weathering might have caused the depletion of CaO and Na₂O, and the enrichment of Al₂O₃ and Fe₂O₃; while the chemical homogenization that was taken during pedogenesis led to the depletion of SiO₂ and K₂O, and to the further enrichment of Al₂O₃ and Fe₂O₃. The heavy-metal enrichment (contamination) of the soils was then genetically related to the enrichment of Al₂O₃ and Fe₂O₃ in the soils.

Considering the eastern part of Hunan Province as the research area, 34 sampling sites were set up, 198 samples were collected from representative paddy soil, the distribution characteristics of antimony (Sb) were studied. The results showed that: (1) The content of Sb on the surface of paddy soil ranging from 0.07 to 11.00 mg/kg and the geometric mean was 1.56 mg/kg. (2) The distribution of contents of Sb in paddy soil in different areas was shown as Yueyang > Changsha > Zhuzhou > Xiangtan. (3) Sb showed a strong migration in paddy soil in the research area and its content increased initially and then decreased or gradually decreased with the increase of profile depth. (4) The content of Sb in the substratum was significantly affected by parent materials.

This paper reports a geochemical study of trace metals and Pb isotopes of sediments from the lowermost Xiangjiang River, Hunan province (P. R. China). Trace metals Ba, Bi, Sc, V, Cr, Mn, Co, Ni, Cu, Zn, Mo, Cd, Sn, Sb, Pb, Tl, Th, U, Zr, Hf, Nb and Ta were analyzed using ICP-MS, and Pb isotopes of the bulk sediments were measured by MC-ICP-MS. The results show that trace metals Cd, Bi, Sn, Sc, Cr, Mn, Co, Ni, Cu, Zn, Sb, Pb and Tl are enriched in the sediments. Among these metals, Cd, Bi and Sn are extremely highly enriched (EF values >40), metals Zn, Sn, Sb and Pb significantly highly (5 < EF < 20), and metals Sc, Cr, Mn, Co, Ni, Cu and Tl moderately highly (2 < EF < 5) enriched in the river sediments. All these metals, however, are moderately enriched in the lake sediments. Geochemical results of trace metals Th, Sc, Co, Cr, Zr, Hf and La, and Pb isotopes suggest that metals in the river sediments are of multi-sources, including both natural and anthropogenic sources. Metals of the natural sources might be contributed mostly from weathering of the Indosinian granites (GR) and Palaeozoic sandstones (PL), and metals of anthropogenic sources were contributed from Pb–Zn ore deposits distributed in upper river areas. Metals in the lake sediments consist of the anthropogenic proportions, which were contributed from automobile exhausts and coal dusts. Thus, heavy-metal contamination for the river sediments is attributed to the exploitation and utilization (e.g., mining, smelting, and refining) of Pb–Zn ore mineral resources in the upper river areas, and this for the lake sediments was caused by automobile exhausts and coal combustion. Metals Bi, Cd, Pb, Sn and Sb have anthropogenic proportion of higher than 90%, with natural contribution less than 10%. Metals Mn and Zn consist of anthropogenic proportion of 60–85%, with natural proportion higher than 15%. Metals Sc, Cr, Co, Cu, Tl, Th, U and Ta have anthropogenic proportion of 30–70%, with natural contribution higher than 30%. Metals Ba, V and Mo might be contributed mostly from natural process.

Widespread arsenic (As) contamination in paddy rice ( Oryza sativa ) from both geologic and anthropogenic origins is an increasing concern globally. Substantial efforts have been made to elucidate As transformation and uptake processes in rhizosphere and metabolism in rice plant, which provides an essential foundation for the development of mitigation strategies. However, a range of crucial mechanisms from As mobilization in rhizosphere to transport to grains remain poorly understood. To provide new insight into the underlying mechanisms of As accumulation in rice, a range of new perspectives on As bioavailability, transport pathways, and in situ speciation are reviewed here. Specifically, the prominent effects of water regime, Fe plaque, and biochar on As mobilization in rice rhizosphere are discussed critically. An updated understanding of arsenite (AsIII) and methylated As transport from root to vascular bundle and grain is integrated and discussed in detail. Special attention is given to As speciation and distribution in rice grain with potential coping strategies being provided and discussed. Future research priorities are also identified. The new insight into As bioavailability, transport and speciation in rice would lead to a better understanding of As contamination in rice. They would also provide useful strategies from agronomic measures to genetic engineering for more effective restriction of As transport and accumulation in food chain.

Cadmium (Cd) pollution in farmland soils severely affects agricultural production safety, thereby threatening human health. Sedum plumbizincicola is a Cd and Zn hyperaccumulator commonly used for the phytoremediation of Cd-contaminated soil. This study was aimed to improve the remediation effect of S. plumbizincicola on Cd-contaminated farmland soil and provide a theoretical basis for the enhancement of endophytic bacteria in the repair of Cd-contaminated soil with S. plumbizincicola. Four kinds of endophytic bacteria, namely Buttiauxella , Pedobacter , Aeromonas eucrenophila , and Ralstonia pickettii , were used, and soil culture experiments and pot experiments were conducted to explore the effects of endophytic bacteria on soil Cd speciation and phytoremediation efficiency of Cd-contaminated farmland soils. Under the experimental conditions, after inoculation with endophytic bacteria, the soil pH was effectively reduced, content of weak acid-extracted Cd and oxidizable Cd increased, and content of reducible Cd and residual Cd decreased. Soil Cd activity was increased, and the availability coefficient of soil Cd increased by 1.15 to 6.41 units compared with that of the control (CK 2 ). Compared with CK 2 , the biomass of S. plumbizincicola significantly increased by 23.23–55.12%; Cd content in shoots and roots of S. plumbizincicola increased by 29.63–46.01% and 11.42–84.47%, respectively; and bioconcentration factor was 2.13 to 2.72 times that of CK 2 . The Cd removal rate of S. plumbizincicola monocropping was 48.25%. When S. plumbizincicola was planted with inoculating endophytic bacteria, the Cd removal rate in the soil reached 61.18–71.49%, which was significantly higher than that of CK 2 ( p  < 0.05). The treatment with endophytic bacteria activated soil Cd, promoted the growth of S. plumbizincicola , increased its Cd content, and enhanced the phytoremediation of Cd-contaminated farmland soil. Therefore, endophytic bacteria can be used to improve the remediation efficiency of S. plumbizincicola in Cd-contaminated farmland soils.

Atmospheric cadmium (Cd) deposition contributes to the accumulation of Cd in the soil-plant system. Sedum plumbizincicola is a Cd and Zn hyperaccumulator commonly used for the phytoremediation of Cd-contaminated soil. However, studies on the effects of atmospheric Cd deposition on the accumulation of Cd and physiological response in S. plumbizincicola are still limited. A Cd solution spraying pot experiment was conducted with S. plumbizincicola at three atmospheric Cd deposition concentrations (4, 8, and 12 mg/L). Each Cd concentration levels was divided into two groups, non-mulching (foliar-root uptake) and mulching (foliar uptake). The soil type used in the experiment was reddish clayey soil collected from a farmland. The results showed that compared with the non-mulching control, the fresh weight of S. plumbizincicola in non-mulching with high atmospheric Cd deposition (12 mg/L) increased by 11.35%. Compared with those in the control group, the malondialdehyde (MDA) content in the non-mulching and mulching S. plumbizincicola groups increased by 0.88–11.06 nmol/L and 0.96–1.32 nmol/L, respectively. Compared with those in the non-Cd-treated control group, the shoot Cd content in the mulching group significantly increased by 11.09–180.51 mg/kg. Under high Cd depositions, the Cd in S. plumbizincicola mainly originated from the air and was stored in the shoots (39.7–158.5%). These findings highlight that the physiological response and Cd accumulation of S. plumbizincicola were mainly affected by high Cd deposition and suggest that atmospheric Cd could directly be absorbed by S. plumbizincicola . The effect of atmospheric deposition on S. plumbizincicola cannot be ignored.

Soil contamination with cadmium (Cd) represents a substantial threat to human health and environmental quality. Long-term effectiveness and persistence of remediation are two important criteria for the evaluation of amendment techniques used to remediate soils polluted with potentially toxic metals. In the current study, we investigated the remediation persistence of a natural sepiolite bearing material (NSBM, containing 15% sepiolite) and ground limestone (equivalent to > 98.0% CaO) on soil pH, Cd bioavailability, and Cd accumulation by pak choi ( Brassica chinensis L.) during the growth of four consecutive crops in a Cd-contaminated acid soil with different amounts of NSBM (0, 0.2, 0.5, 1, 2, and 5%). Soil pH levels ranged from 5.21 to 7.76 during the first crop, 4.30 to 7.34 during the second, 4.23 to 7.80 during the third, and 4.33 to 6.98 during the fourth, and increased significantly with increasing the application rate of NSBM. Soil CaCl 2 -Cd and shoot Cd concentrations decreased by 8.11 to 99.2% and 6.58 to 94.5%, respectively, compared with the control throughout the four cropping seasons. A significant negative correlation was found between soil CaCl 2 -Cd and soil pH. Combined use of 0.1% lime and NSBM showed greater effects than NSBM alone, especially, when the application rate of NSBM was ˂ 2%. Moreover, pak choi tissue Cd concentrations in the treatments with NSBM addition alone at ≥ 2% or at ≥ 1% NSBM combined with 0.1% lime met the maximum permissible concentration (MPC) over the four crops, allowed by the Chinese and European regulations. Based on the present study, safe crop production in the test soil is possible at a soil pH > 6.38 and CaCl 2 -Cd < 14 μg kg −1 , and soil Cd immobilization by NSBM without or with lime is a potentially feasible method of controlling the transfer of soil Cd into the food chain.

Abstract PurposeThe present study aimed to characterise and identify the sources of polycyclic aromatic hydrocarbons (PAHs) in tobacco and soils and assess the potential carcinogenic risks (CRs) and non-carcinogenic risks (NCRs) of these PAHs to farmers. Understanding the characteristics of PAHs in the soil–tobacco system will provide regulators with a guide for establishing control standards for PAHs in ambient media.MethodsThe sample PAHs were extracted using dichloromethane and measured using gas chromatography-mass spectrometry. Positive matrix factorisation, principal component analysis, and diagnostic ratios were used to identify the primary sources of PAHs. The CRs and NCRs to farmers of the 16 PAHs were estimated based on the methods outlined by the United States Environmental Protection Agency (US EPA).ResultsThe concentrations of ∑16PAH in soils and tobacco ranged from 72.94 to 941.77 μg kg−1 and 130.02 to 1545.35 μg kg−1, respectively. The source analysis results indicated that coal combustion (15–20%), coke combustion (11–19%), oil leakage (9–21%), traffic emissions (19–24%), petroleum combustion (9–27%), and biomass combustion (12–15%) were the dominant sources of PAHs in the soil–tobacco system. Total CR values in soils and tobacco varied from 9.54 × 10−8 to 5.31 × 10−7 and 1.95 × 10−7 to 4.16 × 10−6, respectively, and the total NCR values in soils and tobacco ranged from 1.49 × 10−6 to 1.28 × 10−4 and 1.32 × 10−6 to 4.67 × 10−4, respectively. A total of 60.4% of the tobacco samples exposed to benzo[a]pyrene (BaP) were higher than 1 × 10−6 but lower than 1 × 10−4.ConclusionThe PAH concentrations in the soil samples observed in this study ranged from low to moderately polluted. In soils and tobacco, 3- to 5-ring PAHs were dominant. Petroleum, coal, and biomass combustion, as well as plastic film and fertiliser consumption in the cultivation and management of tobacco, were the primary PAH sources. The CR of BaP in most tobacco samples was low, and the NCR of PAHs was acceptable.

Herbicides (HBCs) play a significant role in modern agriculture, but their prolonged use has raised concerns about their potential adverse impacts on agricultural produce safety and the ecological environment, especially in black soil regions. In this typical region, we are characterizing, for the first time, the exposure of herbicides through drinking and non-potable water routes and assessing their non-carcinogenic and carcinogenic health risks for both children and adults. The findings indicate that following long-term HBC application, the maximum detected concentration of HBCs reached up to 6288 ng/L, with higher detection rates of acetochlor, fomesafen, bentazone, and atrazine (ATZ). Both non-carcinogenic and carcinogenic exposures to HBCs were found to be higher for children in comparison to adults. The non-carcinogenic risk values for both children and adults resulting from HBCs in surface water and groundwater at all sampling sites remained well below 1, signifying that they are within acceptable limits. However, the maximum carcinogenic risk associated with ATZ in water was estimated to be 10 –5 for children and 10 –6 for adults. In terms of surface water samples collected in April, July, and September, it was observed that 63, 100, and 40% of them, respectively, were at low-risk levels for children. This study underscores the significance of monitoring organic contaminants in environmental media, particularly in typical black soil areas where long-term herbicide applications are prevalent, to elucidate residue characteristics and potential health risks to humans. Particular attention is directed towards the presence of atrazine in water, as it poses a potential threat to human health. The findings of this study hold the potential to promote environmentally sustainable green agricultural practices and facilitate the establishment of regulatory standards for controlling herbicide levels in environmental media.