Roles of Chloride and Sulfate Ions in Controlling Cadmium Transport in a Soil-Rice System as Evidenced by the Cd Isotope Fingerprint

Eco-friendly utilization and microbiological characteristics of coal gangue substrate via functional microbial fermentation

The development of the coal mining industry increased the production of coal gangue that was one of the most common solid wastes in China. In this study, a simple in situ treatment of gangue was developed. Without the addition of exogenous soil, through composting by using Acinetobacter lwoffii XAQ2297 with a small amount of corn stover powder, the gangue was as a substrate for planting alfalfa. Moreover, the proportion of the formulation was optimized. Results showed that compared with the control group, A. lwoffii XAQ2297 could increase the release of humic acid from gangue. The best growth of alfalfa was achieved when the ratio of gangue to corn stover powder was 9:1, which was able to increase the survival rate of alfalfa to more than 50%. The analysis of substrate properties indicated that urease was the main positive influencing factor, sulfate ions were the main negative influencing factors. In the diversity analysis, Bacillus and Herbinix were the main genera that play a positive role, whereas Pseudomonas and Brevundimonas were the genera involving the main negative influence. This study provides a new strategy for gangue matrixification and reduces the cost of gangue utilization.

Decoupling soil pH and geography: Universal drivers of cadmium bioavailability in rice across terrains

With the accelerating global industrialization, Cadmium (Cd) pollution in rice has become a significant threat to both ecological safety and human health. But the universal factors influencing Cd content across different terrains remain under-investigated. Hence, 300 groups of root system-rice samples were collected from typical rice planting areas in the plains and hills of Southern China to investigate the driving factors of Cd content in rice at a large scale. Moreover, a Cd content prediction model in rice was built. Results showed that although total Cd (T-Cd) and available Cd (DTPA-Cd) contents in rice soils from hilly areas were significantly lower than those in plains, the Cd content in rice was significantly higher (P < 0.05). In a geographical distribution analysis, there was a weak correlation between geographical distance and Cd content in soil (∣R∣<0.30, P < 0.05), although this showed evidence of gradual geographical changes. In addition, this study found that DTPA-Cd (positive correlation, feature importance score of 42.67) and soil pH (negative correlation, 38.91) were the most critical factors that influenced Cd content in rice from different terrains using a network diagram and random forest model computation. In summary, there was evidence of a complicated interaction between terrain and soil pH in rice-Cd pollution at a large regional scale. Soil pH and DTPA-Cd were dominant influencing factors of Cd content in rice when compared to geographical distribution. These results provide an important scientific reference for large-scaled Cd pollution monitoring, control, and risk evaluation.

Manganese oxide application reduces cadmium bioavailability in rice rhizosphere: Insights from desorption kinetics and high-resolution imaging

Cadmium (Cd) contamination in paddy soils threatens global food safety. While manganese (Mn)-based materials show promise in reducing soil Cd bioavailability, their efficacy requires further evaluation. Traditional ex situ sampling methods often overlook metal desorption kinetics and rhizosphere biochemical heterogeneity, potentially misinterpreting Mn's regulatory influence on Cd dynamics. This study employed in situ monitoring tools, including diffusive gradients in thin-films (DGT) measurements, DIFS (DGT-induced fluxes in soils) modeling, and high-resolution DGT and planar optode (PO) imaging, to assess the impact of two Mn oxides (MnO2 and Mn2O3) on Cd bioavailability in rice rhizosphere. Application of MnO2 and Mn2O3 reduced bioavailable Cd by 28.9 % and 15.3 %, respectively, attributed to elevated soil Mn and Fe levels fostering Cd immobilization. DGT-DIFS results revealed that Mn oxide application prolonged Cd replenishment time and reduced its desorption rate from soil solids. PO imaging identified pH heterogeneity in rice rhizosphere, confirming that Mn oxides mediated Cd bioavailability reduction by increasing pH. High-resolution DGT imaging revealed distinct spatial distribution patterns of Cd, Mn, and Fe fluxes, demonstrating Mn's inhibitory effects on Cd bioavailability. These findings highlight the potential of Mn oxides to mitigate Cd uptake by rice, offering a promising strategy for managing Cd-contaminated soils.

Light intensity can significantly regulate cadmium transformation into CdS nanoparticles in microalgae (Dunaliella salina and Phaeodactylum tricornutum)

Light is a critical factor influencing algal growth and contributes to the uptake of metal elements by algae. However, the impact of light on the bioavailability and transformation of heavy metals requires further exploration, particularly in the context of bioremediation efforts. This study explores how varying light intensities (1000, 2000, and 3000 lux) influence the ability of these algae to absorb Cd, distribute it within cells, and transform Cd (II) into CdS NPs. By using ICP-MS, it was found that increasing the light intensity to 2000 lux could increase the Cd uptake capacity of Dunaliella salina and Phaeodactylum tricornutum by 28 % and 14 %, respectively. Changes in the percentage of Cd (II) in each component (medium, intracellular, and adsorption on cell surface) with the different light intensities supported the interpretation that the increase in Cd uptake by algal cells was a result of increased cellular adsorption and accumulation. Further analyses by HRTEM-EDS and SEC-ICP-MS showed that increasing light intensity not only influenced the size of CdS NPs but also significantly enhanced the algae's efficiency in transforming Cd(II) into CdS NPs. It is found that the transformation ratio of CdS NPs by D. salina and P. tricornutum increased to 16 % and 52 % respectively, after 10 days of Cd exposure under 2000 lux light intensity. These findings underscore the significance of light intensity as an environmental factor in the uptake and transformation of Cd by algae, with profound implications for its application in bioremediation.

Exploration of the bio-availability and the risk thresholds of cadmium and arsenic in contaminated paddy soils

Cadmium (Cd) and arsenic (As) contamination risk in paddy soils has raised global concern. In order to scientifically and objectively evaluate the bioavailability of soil Cd, As and the risk of Cd or As threshold in contaminated farmland, this study was conducted to investigate different types of extractants for their potential extraction efficiency of Cd and As. Soils from two different parent materials in Hunan, Yueyang and Yiyang, typical double-cropping rice production areas in the south of China, were used as test soils. The extraction capabilities of 10 extractants (ultrapure water, 0.1 mol/L HCl, 1.0 mol/L NH4OAc, CaCl2-DTPA, 0.01 mol/L CaCl2, 0.1 mol/L CaCl2, 0.5 mol/L NaH2PO4, 0.05 mol/L NaHCO3, 0.1 mol/L NaNO3, 0.1 mol/L HNO3), were compared for their extraction capabilities of soil available Cd and As. Meanwhile, the content of Cd and As in plants issues and grains of rice was monitored during harvest, and the Cd, As content correlation between extracted forms and rice was analyzed. The results showed that the HCl, CaCl2, HNO3, and CaCl2-DTPA solutions exhibited high extraction efficiency for Cd (42.2-88.4 %); for As, NaH2PO4, HCl, and HNO3 have the extraction efficiency (0.85-23.4 %). The concentration of Cd extracted by 0.01 mol/L CaCl2 was significantly positively correlated with Cd levels in rice. The potential risk extraction threshold of CaCl2 in sandy loam soil was 0.178 mg/kg, while it was 0.312 mg/kg in clay soil. The concentration of As extracted by CaCl2-DTPA and 0.05 mol/L NaHCO3 in clay soil was significantly positively correlated with As levels in rice, the potential risk extraction thresholds were 0.115 mol/L and 0.106 mg/kg, respectively. These investigations indicated that the heavy metals extraction methods by 0.01 mol/L CaCl2, CaCl2-DTPA, and 0.05 mol/L NaHCO3 could reflect the Cd and As pollution degree in farmland and suggest their potential to serve as methods for assessing the risk of Cd and As pollution in sandy loam and clay paddy soil.

Aggravation of Cd availability in the plastisphere of paddy soil

Soil plastisphere has attracted many concerns, however, its influence on cadmium (Cd) availability in paddy soil was still unclear. This study carried out batch microcosmic and bagging experiments to explore the influence of microplastic (MPs) on Cd availability in paddy soil under flooding conditions in the view of plastisphere. Results showed that the presence of MPs could act as plastisphere micro-environment. The bacterial community composition changed dramatically around the plastisphere compared with MPs-contaminated bulk soil and control soil. The relative abundance of Symbiobacteraceae, Rhodocyclaceae and Bryobacteraceae was improved in the plastisphere which contributed to the enhanced the reduction of Fe(III) and sulfate in flooding paddy soil. The higher content of Fe(II) and S content contributed to the enrichment of Cd in the plastisphere which aggravated Cd availability in paddy soil under flooding conditions. The partial least squares structure equation modeling results confirmed the presence of MPs in paddy soil could act as plastisphere which could change the bacterial community composition and improve the content Fe and S that was conductive to gather Cd in plastisphere. This study shed lights on the understanding of the role of plastisphere on Cd availability in paddy field ecosystem under flooding conditions.

Cadmium immobilization by mercapto-palygorskite in alkaline soil: Impacts on soil microbial communities and wheat rhizosphere metabolism

Weakly alkaline cadmium (Cd) contaminated soil in China has aroused great concern regarding its impact on food security and human health. Mercapto-modified palygorskite (MP) has exhibited good potential to minimize Cd accumulation in wheat, it is imperative to understand the underlying mechanisms within the soil-wheat-microbial system for sustainable development of agrochemicals. This study evaluated the effects of various MP dosages on soil Cd bioavailability, rhizosphere metabolomics, microbial community structure and wheat growth. The results indicated that MP (0.05-0.2 %) application significantly reduced Cd accumulation in wheat grains by 59.0-83.2 % (p < 0.05) and inhibited Cd translocation from root to grain. MP also promoted Mn oxide formation and redistributed the exchangeable Cd to Fe-Mn oxide-bound forms (44.2-109.6 %), thus lowering soil Cd bioavailability by 17.9-32.5 %. Additionally, MP reduced wheat rhizosphere organic acid levels, altered rhizosphere carbon and nitrogen pools, and stimulated the growth of Cd-tolerant Alternaria and Cladosporium, while inhibiting the growth of Fusarium. These findings highlight the potential of MP to modulate soil rhizosphere metabolism and microbial communities, offering a novel perspective on its environmental implications and supporting agrochemical sustainability.

Milk vetch returning combined with lime materials alleviates soil cadmium contamination and improves rice quality in soil-rice system

Milk vetch (Astragalus sinicus L.) returning and lime materials is employed as an effective strategy for remediating cadmium (Cd)-contaminated paddy fields. However, the combined effects of them on alleviating Cd pollution and the underlying mechanisms remain poorly explored. Therefore, this study investigated the impact of these combined treatments on soil properties, iron oxides, iron plaque, mineral elements, and amino acids through a field experiment. The following treatments were employed: lime (LM), limestone (LS), milk vetch (MV), MV + LM (MVLM), and MV + LS (MVLS), and a control (CK) group with no materials. Results demonstrated that treatments significantly decreased soil available Cd by 19.40-32.55 %, 10.20-39.58 %, and 25.36-40.66 % at tillering, filling, and maturing stages compared to CK, respectively. Moreover, exchangeable Cd was transformed into more stable fractions. Compared with individual treatments, MVLM and MVLS treatments further decreased available Cd and exchangeable Cd. Overall, Cd in brown rice was reduced by 18.97-77.39 % compared with CK. And the Cd in iron plaque decreased by 14.12-31.14 %, 24.65-61.60 %, 2.6-38.28 % across three stages. Furthermore, soil pH, dissolved organic carbon, and cation exchange capacity increased, along with 0.22-62.09 % and 0.57-10.66 % increases in free and amorphous iron oxide contents at all stages, respectively. Compared with lime alone, the integration of MV returning facilitated increased formation of Fed, Feo and enhanced the antagonistic effect among grain Ca with Cd; Additionally, it increased AAs in brown rice, improving rice quality and potentially reducing Cd transport. Mantel tests and Partial least squares path modeling revealed a significant positive correlation between Cd in IP and rice Cd uptake and a significant negative correlation between available Cd, Fed and Feo. These findings provide valuable insights into the mechanisms involved in mitigating soil Cd bioavailability using integrated approaches with MV returning and lime materials.

Iron-modified biochars and their aging reduce soil cadmium mobility and inhibit rice cadmium uptake by promoting soil iron redox cycling

Iron (Fe) modified biochar has been widely used for cadmium (Cd) contaminated soil remediation. However, the accompanying anions introduced during the modification process potentially affect the behavior of Cd in soil. In this study, we investigated the distinct Cd immobilization mechanisms by Fe2(SO4)3 modified biochar (FSBC) and Fe(NO3)3 modified biochar (FNBC) in a two-year pot experiment. Results showed that both FSBC and FNBC significantly reduced Cd concentrations in rice grains by 23%-42% and 30%-37% compared to pristine biochar (BC). Specifically, NFBC promoted the formation of amorphous Fe oxides by enhancing the NO3--reducing Fe(II) oxidation process, which significantly increased Fe/Mn oxide-bound Cd and decreased soil CaCl2-extractable Cd. For FSBC, the introduction of SO42- significantly promoted the formation of Fe plaques by enhancing the Fe(III) reduction process, which blocked the Cd transfer from the soil to the rice roots. More importantly, after two years of biochar application, an organo-mineral complex layer is formed on the biochar surface, which immobilized a large amount of Cd. The Cd immobilization on the surface of aged biochar could be due to the fixation by the secondary Fe oxides within the organo-mineral layer and the complexation by the surface functional groups. The result of laser ablation inductively coupled plasma mass spectrometry showed that the Cd content on aged FNBC and FSBC was 5.9 and 2.6 times higher than on aged BC. This might be attributed to the Fe-modified biochar's higher electron exchange capability (EEC), which promoted the development of organo-mineral complexes. Notably, the EEC of biochar was maintained during its aging process, which may keep the biochar surface active and facilitate continual Cd immobilization. This study revealed the complex mechanisms of soil Cd immobilization with Fe-modified biochar, providing new insights into sustainable biochar environmental remediation.

Hydroxamate Siderophores Intensify the Co-Deposition of Cadmium and Silicon as Phytolith-Like Particulates in Rice Stem Nodes: A Natural Strategy to Mitigate Grain Cadmium Accumulation

Sequestration of cadmium (Cd) in rice phytolith can effectively restrict its migration to the grains, but how hydroxamate siderophore (HDS) affects phytolith formation within rice plants especially the fate of Cd and silicon (Si) remains poorly understood. Here, we found that the addition of HDS increased the content of dissolved Si and Cd in soil pore water as well as its absorption by the rice roots during the reproductive growth stage. HDS effectively trapped orthosilicic acid and Cd ions at the third stem nodes of rice plants via hydrogen bonds and chelation interactions, which then rapidly deposited on the xylem cell wall through hydrophobic interactions. Ultimately, Cd was immobilized as phytolith-like particulates in the form of CdSiO3. Field experiments verified that Cd accumulation was significantly reduced by 46.4% in rice grains but increased by 41.2% in rice stems after HDS addition. Overall, this study advances our understanding of microbial metabolites enhancing the instinctive physiological barriers within rice plants.

Impact of Rhizosphere Biostimulation on Cd Transport and Isotope Fractionation in Cd-Tolerant and Hyperaccumulating Plants Based on MC-ICP-MS and NanoSIMS

Phytoremediation efficiency can be enhanced by regulating rhizosphere processes, and the Cd isotope is a useful approach for deciphering Cd transport processes in soil-plant systems. However, the effects of adsorption and complexation on Cd isotope fractionation during the rhizosphere processes remain unclear. Here, we cultivated the Cd hyperaccumulator Sedum alfredii and Cd-tolerance Sedum spectabile in three different soils with citric acid applied as a degradable rhizosphere biostimulant. Cellular elemental distributions in the tissues and Cd isotope compositions were determined through NanoSIMS and MC-ICP-MS, respectively. Cd precipitation/adsorption on cell walls and intracellular regional distribution were the main mechanisms of Cd tolerance in S. spectabile. Plant roots became enriched with heavier Cd isotopes relative to the surrounding soils upon increasing secretion of rhizosphere organic acids. This indicates that organic matter with O and N functional groups preferentially chelates heavy Cd isotopes. In addition, Cd isotope fractionation between roots and shoots varies within the three soils, which could be due to the influence of protein and metallothionein contents in roots and leaves. The finding indicates that sulfur-containing ligands preferentially chelate light Cd isotopes. This study suggests that organic ligands play a vital role in Cd isotope fractionation and consequent hyperaccumulation of soil-plant systems.

Classification and regression tree (CART) for predicting cadmium (Cd) uptake by rice (Oryza sativa L.) and its application to derive soil Cd threshold based on field data

The entry of Cd into soil-rice systems is a growing concern as it can pose potential risks to public health. To derive regional soil Cd threshold, a total of 333 paired soil and rice samples was collected in Anhui Province, Eastern China. The results showed that the total soil Cd and soil Zn/Cd were the most significant variables contributing to Cd content in polished rice. The Chinese Soil Quality Standards might overestimate risk posed by Cd-contaminated soil for rice production in the mining area due to high Zn/Cd values of some mining-associated soils. Cd levels in polished rice can be predictable using stepwise multiple linear regression (MLR) model. However, the derived soil Cd threshold based on the MLR model would be unrealistically high. The classification and regression tree method (CART) performed well in simulating Cd levels in polished rice and can be used to derive soil Cd threshold instead of MLR to minimize the uncertainty.

Isotope Evidence for Rice Accumulation of Newly Deposited and Soil Legacy Cadmium: A Three-Year Field Study

Biogeochemical processes of atmospherically deposited cadmium (Cd) in soils and accumulation in rice were investigated through a three-year fully factorial atmospheric exposure experiment using Cd stable isotopes and diffusive gradients in thin films (DGT). Our results showed that approximately 37-79% of Cd in rice grains was contributed by atmospheric deposition through root and foliar uptake during the rice growing season, while the deposited Cd accounted for a small proportion of the soil pools. The highly bioavailable metals in atmospheric deposition significantly increased the soil DGT-measured bioavailable fraction; yet, this fraction rapidly aged following a first-order exponential decay model, leading to similar percentages of the bioavailable fraction in soils exposed for 1-3 years. The enrichment of light Cd isotopes in the atmospheric deposition resulted in a significant shift toward lighter Cd isotopes in rice plants. Using a modified isotopic mass balance model, foliar and root uptake of deposited Cd accounted for 47-51% and 28-36% in leaves, 41-45% and 22-30% in stems, and 45-49% and 26-30% in grains, respectively. The implications of this study are that new atmospheric deposition disproportionately contributes to the uptake of Cd in rice, and managing emissions thus becomes very important versus remediation of impacted soils.

Multifunctional Roles of Zinc in Cadmium Transport in Soil-Rice Systems: Novel Insights from Stable Isotope Fractionation and Gene Expression

The effect of Zn on Cd accumulation in rice varies under flooding and drainage conditions, and the underlying mechanism during uptake and transport from the soil to grains remains unclear. Isotope fractionation and gene expression were investigated using pot experiments under distinct water regimes and with Zn addition to gain a deeper understanding of the molecular effects of Zn on Cd uptake and transport in rice. The higher OsHMA2 expression but constitutively lower expression of zinc-regulated, iron-regulated transporter-like protein (ZIP) family genes in roots under the drainage regime than the flooding regime caused the enrichment of nonheavy Zn isotopes in the shoots relative to roots but minimally affected Cd isotopic fractionation. Drainage regime seem to exert a striking effect on the root-to-shoot translocation of Zn rather than Cd, and increased Zn transport via OsHMA2. The changes in expression patterns in response to Zn addition were similar to those observed upon switching from the flooding to drainage regime, except for OsNRAMP1 and OsNRAMP5. However, soil solution-to-rice plants and root-to-shoot fractionation toward light Zn isotopes with Zn addition (Δ66Znrice plant-soil solution = -0.49 to -0.40‰, Δ66Znshoot-root = -0.36 to -0.27‰) indicated that Zn transport occurred via nonspecific uptake pathways and OsHMA2, respectively. Accordingly, the less pronounced and minimally varied Cd isotope fractionation suggested that OsNRAMP5 and OsHMA2 are crucial for Cd uptake and root-to-shoot transport, respectively, facilitating Cd accumulation in grains. This study demonstrated that a high Zn supply promotes Cd uptake and root-to-shoot transport in rice by sharing distinct pathways, and by utilizing a non-Zn-sensitive pathway with a high affinity for Cd.

Ferric Oxide Nanomaterials and Plant-Rhizobacteria Symbionts Cogenerate Iron Plaque for Removing Highly Chlorinated Contaminants in Dryland Soils

Rhizosphere iron plaques derived from Fe-based nanomaterials (NMs) are a promising tool for sustainable agriculture. However, the requirement for flooded conditions to generate iron plaque limits the scope of the NM application. In this study, we achieved in situ Fenton oxidation of a highly chlorinated persistent organic pollutant (2,2',4,5,5'-pentachlorobiphenyl, PCB101) through iron plaque mediated by the interaction between α-Fe2O3 NMs and plant-rhizobacteria symbionts under dryland conditions. Mechanistically, the coexistence of α-Fe2O3 NMs and Pseudomonas chlororaphis JD37 stimulated alfalfa roots to secrete acidic and reductive agents as well as H2O2, which together mediated the rhizosphere Fenton reaction and converted α-Fe2O3 NMs into iron plaque rich in Fe(II)-silicate. Further verifications reproduced the Fenton reaction in vitro using α-Fe2O3 NMs and rhizosphere compounds, confirming the critical role of •OH in the oxidative degradation of PCB101. Significant reductions in PCB101 content by 18.6%, 42.9%, and 23.2% were respectively found in stem, leaf, and soil after a 120-d treatment, proving the effectiveness of this NMs-plant-rhizobacteria technique for simultaneously safe crop production and soil remediation. These findings can help expand the potential applications of nanobio interaction and its mediated iron plaque generation for both agricultural practice and soil remediation.

The colonization of Penicillium oxalicum SL2 on rice root surface increased Pb interception capacity of iron plaque and decreased Pb uptake by roots

The exploration of microbial resources to reduce Pb accumulation in rice attracted great attention. In this study, we found Penicillium oxalicum SL2, a Pb-tolerant strain with good capability of dissolving phosphorus and stabilizing Pb in soil, was able to colonize on the root surface of rice seedlings without additional carbon sources, and promoted the secretion of metabolites related to amino acid metabolism, organic acid metabolism, signal transduction and other pathways in rhizosphere exudates, in which the secretion of oxalate increased by 47.7 %. However, P. oxalicum SL2 increased Fe(II) proportion and Fe availability on the root surface, resulting in iron plaque content decrease. Moreover, by converting root surface Pb from Pb-Fe state to PbC2O4 and Pb-P compounds, P. oxalicum SL2 increased Pb intercept capacity of iron plaque by 118.0 %. Furthermore, P. oxalicum SL2 regulated element distribution on the root surface, and reduced the relative content of Pb on the maturation zone of root tip, which was conducive to reducing Pb uptake by apoplastic pathway and the risk of Pb accumulation in root system. Our findings further revealed the interaction between P. oxalicum SL2 and rice root, providing a theoretical basis for the development and application of microbial agents in Pb-contaminated farmland.