Arsenic biogeochemical cycling in paddy soil-rice system: Interaction with various factors, amendments and mineral nutrients

Effect of inorganic arsenic in paddy soil on the migration and transformation of selenium species in rice plants

Selenium (Se) in paddy rice is one of the significant sources of human Se nutrition. However, the effect of arsenic (As) pollution in soil on the translocation of Se species in rice plants is unclear. In this research, a pot experiment was designed to examine the effect of the addition of 50 mg As/kg soil as arsenite or arsenate on the migration of Se species from soil to indica Minghui 63 and Luyoumingzhan. The results showed that the antagonism between inorganic As and Se was closely related to the rice cultivar and Se oxidation state in soil. Relative to the standalone selenate treatment, arsenite significantly (p < 0.05) decreased the accumulation of selenocystine, selenomethionine and selenate in the roots, stems, sheaths, leaves, brans and kernels of both cultivars by 21.4%-100.0%, 40.0%-100.0%, 41.0%-100%, 5.4%-96.3%, 11.3%-100.0% and 26.2%-39.7% respectively, except for selenocystine in the kernels of indica Minghui 63 and selenomethionine in the leaves of indica Minghui 63 and the stems of indica Luyoumingzhan. Arsenate also decreased (p < 0.05) the accumulation of selenocystine, selenomethionine and selenate in the roots, stems, brans and kernels of both cultivars by 34.9%-100.0%, 30.2%-100.0%, 11.3%-100.0% and 5.6%-39.6% respectively, except for selenate in the stems of indica Minghui 63. However, relative to the standalone selenite treatment, arsenite and arsenate decreased (p < 0.05) the accumulation of selenocystine, selenomethionine and selenite only in the roots of indica Minghui 63 by 45.5%-100.0%. Our results suggested that arsenite and arsenate had better antagonism toward Se species in selenate-added soil than that in selenite-added soil; moreover, arsenite had a higher inhibiting effect on the accumulation of Se species than arsenate.

Antimony release and volatilization from organic-rich and iron-rich submerged soils

Antimony (Sb) is an poorly understood, increasingly common pollutant, especially in soils susceptible to waterlogging. We investigated the impact of waterlogging on Sb release, methylation, and volatilization from an organic-rich wetland soil and an iron (Fe)-rich floodplain soil in a 27-day microcosm experiment. The release of Sb into the porewaters of the organic-rich soil was environmentally relevant and immediate with waterlogging (3.2 to 3.5 mg L-1), and likely associated with a complex interplay of sulfide precipitation, sorption with organic matter and manganese (Mn) (oxyhydr)oxides in the soil. The release of Sb from the Fe-rich soil was likely associated with Fe-(oxyhydr)oxide reduction and immobilized due to co-precipitation with Fe-sulfides or as Sb-sulfides. Volatile Sb was produced from the soils after waterlogging. The organic-rich soil produced more volatile Sb (409 to 835 ng kgsoil-1), but the Fe-rich soil volatilized Sb more efficiently. The negligible association of Sb volatilization with soil parameters indicates a more complex underlying, potentially microbial, mechanism and that antimony volatilization could be ubiquitous and not dependent on specific soil properties. Future works should investigate the microbial and physiochemical drivers of Sb volatilization in soils as it may be an environmentally relevant part of the biogeochemical cycle.

Construction of algal-bacterial consortia using green microalgae Chlorella vulgaris and As(III)-oxidizing bacteria: As tolerance and metabolomic profiling

Bioremediation became a promising technology to resolve arsenic (As) contamination in aquatic environment. Since monoculture such as microalgae or bacteria was sensitive to environmental disturbance and vulnerable to contamination, green microalgae Chlorella vulgaris and arsenite (As(III)) - oxidizing bacteria Pseudomonas sp. SMS11 were co-cultured to construct algal-bacterial consortia in the current study. The effects of algae-bacteria (A:B) ratio and exposure As(III) concentration on algal growth, As speciation and metabolomic profile were investigated. Algal growth arrested when treated with 100 mg/L As(III) without the co-cultured bacteria. By contrast, co-cultured with strain SMS11 significantly enhanced As tolerance in C. vulgaris especially with A:B ratio of 1:10. All the As(III) in culture media of the consortia were oxidized into As(V) on day 7. Methylation of As was observed on day 14. Over 1% and 0.5% of total As were converted into dimethylarsinic acid (DMA) after 21 days cultivation when the initial concentrations of As(III) were 1 and 10 mg/L, respectively. Metabolomic analysis was further performed to reveal the response of consortia metabolites to external As(III). The enriched metabolomic pathways were associated with carbohydrate, amino acid and energy metabolisms. Tricarboxylic acid cycle and glyoxylate and dicarboxylate metabolism were upregulated under As stress due to their biological functions on alleviating oxidative stress and protecting cells. Both carbohydrate and amino acid metabolisms provided precursors and potential substrates for energy production and cell protection under abiotic stress. Alterations of the pathways relevant to carbohydrate or amino acid metabolism were triggered by energy requirement.

Selenium-oxidizing Agrobacterium sp. T3F4 decreases arsenic uptake by Brassica rapa L. under a native polluted soil

Toxic arsenic (As) and trace element selenium (Se) are transformed by microorganisms but their complex interactions in soil-plant systems have not been fully understood. An As- and Se- oxidizing bacterium, Agrobacterium sp. T3F4, was applied to a native seleniferous As-polluted soil to investigate As/Se uptake by the vegetable Brassica rapa L. and As-Se interaction as mediated by strain T3F4. The Se content in the aboveground plants was significantly enhanced by 34.1%, but the As content was significantly decreased by 20.5% in the T3F4-inoculated pot culture compared to the control (P < 0.05). Similar result was shown in treatment with additional 5 mg/kg of Se(IV) in soil. In addition, the As contents in roots were significantly decreased by more than 35% under T3F4 or Se(IV) treatments (P<0.05). Analysis of As-Se-bacterium interaction in a soil simulation experiment showed that the bioavailability of Se significantly increased and As was immobilized with the addition of the T3F4 strain (P < 0.05). Furthermore, an As/Se co-exposure hydroponic experiment demonstrated that As uptake and accumulation in plants was reduced by increasing Se(IV) concentrations. The 50% growth inhibition concentration (IC50) values for As in plants were increased about one-fold and two-fold under co-exposure with 5 and 10 µmol/L Se(IV), respectively. In conclusion, strain T3F4 improves Se uptake but decreases As uptake by plants via oxidation of As and Se, resulting in decrease of soil As bioavailability and As/Se competitive absorption by plants. This provides a potential bioremediation strategy for Se biofortification and As immobilization in As-polluted soil.

Silicon-nanoparticles loaded biochar for soil arsenic immobilization and alleviation of phytotoxicity in barley: Implications for human health risk

Arsenic (As)-induced environmental pollution and associated health risks are recognized on a global level. Here the impact of cotton shells derived biochar (BC) and silicon-nanoparticles loaded biochar (nano-Si-BC) was explored on soil As immobilization and its phytotoxicity in barley plants in a greenhouse study. The barley plants were grown in a sandy loam soil with varying concentrations of BC and nano-Si-BC (0, 1, and 2%), along with different levels of As (0, 5, 10, and 20 mg kg-1). The FTIR spectroscopy, SEM-EDX, and XRD were used to characterize BC and nano-Si-BC. Results revealed that As treatment had a negative impact on barley plant development, grain yield, physiology, and anti-oxidative response. However, the addition of nano-Si-BC led to a 71% reduction in shoot As concentration compared to the control with 20 mg kg-1 of As, while BC alone resulted in a 51% decline. Furthermore, the 2% nano-Si-BC increased grain yield by 94% compared to control and 28% compared to BC. The addition of 2% nano-Si-BC to As-contaminated soil reduced oxidative stress (34% H2O2 and 48% MDA content) and enhanced plant As tolerance (92% peroxidase and 46% Ascorbate peroxidase activity). The chlorophyll concentration in barley plants decreased due to oxidative stress. Additionally, the incorporation of 2% nano-Si-BC resulted in a 76% reduction in water soluble and NaHCO3 extractable As. It is concluded that the use of BC or nano-Si-BC in As contaminated soil for barley resulted in a low human health risk (HQ < 1), as it effectively immobilized As and promoted higher activity of antioxidants.

Evaluation of iron-modified biochar on arsenic accumulation by rice: a pathway to assess human health risk from cooked rice

Arsenic (As) contamination of rice grain poses a serious threat to human health. Therefore, it is crucial to reduce the bioavailability of As in the soil and its accumulation in rice grains to ensure the safety of food and human health. In this study, mango (Mangifera indica) leaf-derived biochars (MBC) were synthesized and modified with iron (Fe) to produce FeMBC. In this study, 0.5 and 1% (w/w) doses of MBC and FeMBC were used. The results showed that 1% FeMBC enhanced the percentage of filled grains/panicle and biomass yield by 17 and 27%, respectively, compared to the control. The application of 0.5 and 1% FeMBC significantly (p < 0.05) reduced bioavailable soil As concentration by 33 and 48%, respectively, in comparison to the control. The even higher As flux in the control group as compared to the biochar-treated groups indicates the lower As availability to biochar-treated rice plant. The concentration of As in rice grains was reduced by 6 and 31% in 1% MBC and 1% FeMBC, respectively, compared to the control. The reduction in As concentration in rice grain under 1% FeMBC was more pronounced due to reduced bioavailability of As and enhanced formation of Fe-plaque. This may restrict the entry of As through the rice plant. The concentrations of micronutrients (such as Fe, Zn, Se, and Mn) in brown rice were also improved after the application of both MBC and FeMBC in comparison to the control. This study indicates that the consumption of parboiled rice reduces the health risk associated with As compared to cooked sunned rice. It emphasizes that 1% MBC and 1% FeMBC have great potential to decrease the uptake of As in rice grains.

Comparison of the arsenic protective effects of four nanomaterials on pakchoi in an alkaline soil

Accurately applying engineered nanoparticles (NPs) in farmland stress management is important for sustainable agriculture and food safety. We investigated the protective effects of four engineered NPs (SiO2, CeO2, ZnO, and S) on pakchoi under arsenic (As) stress using pot experiments. The results showed that CeO2, SiO2, and S NPs resulted in biomass reduction, while ZnO NPs (100 and 500 mg kg-1) significantly increased shoot height. Although 500 mg kg-1 S NPs rapidly dissolved to release SO42-, reducing soil pH and pore water As content and further reducing shoot As content by 21.6 %, the growth phenotype was inferior to that obtained with 100 mg kg-1 ZnO NPs, probably due to acid damage. The addition of 100 mg kg-1 ZnO NPs not only significantly reduced the total As content in pakchoi by 23.9 % compared to the As-alone treatment but also enhanced plant antioxidative activity by increasing superoxide dismutase (SOD) and peroxidase (POD) activities and decreasing malondialdehyde (MDA) content. ZnO NPs in soil might inhibit As uptake by roots by increasing the dissolved organic carbon (DOC) by 19.12 %. According to the DLVO theory, ZnO NPs were the most effective in preventing As in pore water from entering plant roots due to their smaller hydrated particle size. Redundancy analysis (RDA) further confirmed that DOC and SO42- were the primary factors controlling plant As uptake under the ZnO NP and S NP treatments, respectively. These findings provide an important basis for the safer and more sustainable application of NP-conjugated agrochemicals.

Illuminated fulvic acid stimulates denitrification and As(III) immobilization in flooded paddy soils via an enhanced biophotoelectrochemical pathway

Fulvic acid (FA) is a representative photosensitive dissolved organic matter (DOM) compound that occurs naturally in paddy soils. In this study, the effect of a FA + nitrate treatment (0, 4 and 8 mg/L FA + 20 mmol/L nitrate) on denitrification and As(III) immobilization in flooded paddy soils was assessed under dark and intermittently illuminated conditions (12 h light+12 h dark). The FA input stimulated denitrification in illuminated soils (~100 % of nitrate removal within 6 days) compared to dark conditions (~92 % nitrate removal after 6 days). Meanwhile, As(III) (initial concentration of 0.1 mmol/L) was nearly completely immobilized (~100 %) under illuminated conditions after 4 days for the FA + nitrate treatment compared to 90- 93 % retention in the dark. Denitrification and As immobilization were positively related to the FA dosage in the illuminated assays. The stronger denitrification in illuminated soils was ascribed to denitrifiers harvesting photoelectrons from photosensitive substrates/semiconducting minerals. FA addition also increased the activities of denitrifying enzymes (e.g., NAR, NIR and NOR) and the denitrification electron transport system by nearly 0.6-0.7 and 1.5-1.8 times that of the nitrate-alone treatment under illuminated and dark conditions, thereby fostering stronger denitrification. Upon irradiation, As(III) immobilization was not only stimulated by the interactions with the denitrification pathway whereby As(III) acts as an electron donor for denitrifiers, but was also modulated by Fe(III)/oxidative reactive species-derived photooxidation of As(III). Moreover, the FA + nitrate treatment promoted the enrichment of metal-oxidizing bacteria (e.g., Stenotrophomonas and Acidovorax) that are responsible for nitrate-dependent As(III)/Fe(II) oxidation. The results of this study enhance our understanding of interactions among the biogeochemical cycles of As, Fe, N and C, which are intricately linked by a biophotoelectrochemical pathway in flooded paddy soils.

Unveiling the significance of foliar-applied silicon, selenium and phosphorus for the management and remediation of arsenic in two different rice genotypes

Under paddy soil conditions, rice plants are vulnerable to arsenic (As) accumulation, thus causing potential threat to human health. Here we investigated the influence of foliar-applied phosphorus (P: 10 and 20 mg L-1), silicon (Si: 0.6 and 1.5 g L-1) and selenium (Se: 5 and 10 mg L-1) on As accumulation, morphological and physiological attributes of two contrasting rice genotypes (KSK-133 and Super Basmati) under As stress (25 mg kg-1 as arsenate). Silicon foliar dressing significantly (p < 0.05) reduced grain As uptake (up to 67%) and improved rice growth and chlorophyll content (28-66%) in both rice genotypes over their controls. Phosphorus foliar application resulted in a notable decrease (17%) in grain As uptake of coarse rice genotype (KSK-133), while it slightly increased grain As uptake in the fine one (Super Basmati; 6%) compared to controls. However, foliar-applied Se did not show significant effects on rice plants growth attributes and As uptake in both genotypes. Similarly, biochemical and enzymatic attributes (i.e., lipid peroxidation, electrolyte leakage, peroxidase and catalase) were improved with Si application in rice plants, except for P treatment that was only effective for coarse one. Foliar-applied Si also resulted in reduced cancer risk and hazard quotient (< 0.10) for both rice genotypes. This study advances our understanding on critical role of different foliar-applied nutrients and rice genotypes, which is imperative to develop effective As remediation and management strategies in coarse and fine rice genotypes and protect human health.

Reduced arsenic availability in paddy soil through Fe-organic ligand complexation mediated by bamboo biochar

The reuse of arsenic (As)-contaminated paddy fields is a global challenge because long-term flooding would result in As release due to the reductive dissolution of iron minerals. Biochar amendment is a common and effective remediation technique for As-contaminated paddy soil. However, the literature is still lacking in systematic research on the function of biochar in controlling the complexation of released dissolved organic matter (DOM) and iron oxides and its synergistic impact on the availability of As in flooded paddy soil. In the present study, bamboo biochar was prepared at different pyrolysis temperatures (300, 450 and 600 °C), as BB300, BB450 and BB600. Four paddy soil treatments including BB300, BB450, BB600 applications (1% ratio, m/m, respectively) and control (CK, no biochar application) were set and incubated for 60 d in flooding condition. The results showed that As availability represented by adsorbed As species (A-As) was mitigated by BB450 amendment compared with CK. The amendment of BB450 in paddy soil facilitated the complexation of HCl extractable Fe(III)/(II) and DOM and formation of amorphous iron oxides (e.g. complexed Fe species). Moreover, the abundance of Geobacteraceae and Xanthomonadaceae, as common electroactive bacteria, was promoted in the BB450 treated paddy soil in comparison to CK, which assisted to form amorphous iron oxides. The formed amorphous iron oxides then facilitated the formation of ternary complex (As-Fe-DOM) with highly stability, which could be considered as a mechanism for As immobilization after biochar was applied to the flooding paddy soil. Thus, the synergistic effect between amorphous iron oxides and electroactive stains could make main contribution to the passivation of released As in paddy soil under long-term flooding condition. This study provided a new insight for As immobilization via regulating iron-organic ligand complexation amendment with biochar in flooding paddy soil.

Metabolomic profiling reveals key factors and associated pathways regulating the differential behavior of rice (Oryza sativa L.) genotypes exposed to geogenic arsenic

Arsenic (As) toxicity is an escalating problem; however, information about the metabolic events controlling the varied pattern of As accumulation in rice genotypes within their natural environment is still lacking. The present study is thus an advancement in unravelling the response of such rice genotypes. Soil-water-rice samples were analyzed for As accumulation using ICP-MS. Furthermore, we implemented metabolomics through LC-MS/MS and UHPLC to identify metabolic signatures regulating As content by observing the metalloid's composition in rice agrosystem. Results showed that rice genotypes differed significantly in their levels of metabolites, with Mini mansoori and Pioneer having the highest levels. Mini mansoori contained least As which might have been regulated by Ala, Ser, Glu, Phe, Asn, His, Ile, Lys, Gln, Trp, Tyr, chlorogenic, p-coumaric, trans-ferulic, rutin, morin, naringenin, kampferol, and myricetin, while Asp, Arg, Met, syringic, epigalocatechin, and apigenin contributed to the greater As acclimatization ability of Pioneer. Multivariate tools separated the rice genotypes into two major clusters: Pioneer-Mini mansoori and Damini-Sampoorna-Chintu. KEGG identified three major metabolic pathways (aminoacyl-tRNA, phenylpropanoid, and secondary metabolites biosynthesis route) linked with As tolerance and adaptation mechanisms in rice. Overall, these two genotypes symbolize their As hostile and accommodating attitudes probably due to the accumulated metabolites and the physicochemical attributes of the soil-water. Thus, thorough understanding of the metabolic reactions to As may facilitate the emergence of As tolerant/resilient genotypes. This will aid in the selection of molecular markers to cultivate healthier rice genotypes in As-contaminated areas.

Exploring the potential: Can arsenic (As) resistant silicate-solubilizing bacteria manage the dual effects of silicon on As accumulation in rice?

Rice (Oryza sativa L.) cultivation in regions marked by elevated arsenic (As) concentrations poses significant health concerns due to As uptake by the plant and its subsequent entry into the human food chain. With rice serving as a staple crop for a substantial share of the global population, addressing this issue is critical for food security. In flooded paddy soils, where As availability is pronounced, innovative strategies to reduce As uptake and enhance agricultural sustainability are mandatory. Silicon (Si) and Si nanoparticles have emerged as potential candidates to mitigate As accumulation in rice. However, their effects on As uptake exhibit complexity, influenced by initial Si levels in the soil and the amount of Si introduced through fertilization. While low Si additions may inadvertently increase As uptake, higher Si concentrations may alleviate As uptake and toxicity. The interplay among existing Si and As availability, Si supplementation, and soil biogeochemistry collectively shapes the outcome. Adding water-soluble Si fertilizers (e.g., Na2SiO3 and K2SiO3) has demonstrated efficacy in mitigating As toxicity stress in rice. Nonetheless, the expense associated with these fertilizers underscores the necessity for low cost innovative solutions. Silicate-solubilizing bacteria (SSB) resilient to As hold promise by enhancing Si availability by accelerating mineral dissolution within the rhizosphere, thereby regulating the Si biogeochemical cycle in paddy soils. Promoting SSB could make cost-effective Si sources more soluble and, consequently, managing the intricate interplay of Si's dual effects on As accumulation in rice. This review paper offers a comprehensive exploration of Si's nuanced role in modulating As uptake by rice, emphasizing the potential synergy between As-resistant SSB and Si availability enhancement. By shedding light on this interplay, we aspire to shed light on an innovative attempt for reducing As accumulation in rice while advancing agricultural sustainability.

Risk assessment of toxic and hazardous metals in paddy agroecosystem by biochar-for bio-membrane applications

Toxic and carcinogenic metal (loid)s, such arsenic (As) and cadmium (Cd), found in contaminated paddy soils pose a serious danger to environmental sustainability. Their geochemical activities are complex, making it difficult to manage their contamination. Rice grown in Cd and As-polluted soils ends up in people's bellies, where it can cause cancer, anemia, and the deadly itai sickness. Solving this issue calls for research into eco-friendly and cost-effective remediation technology to lower rice's As and Cd levels. This research delves deeply into the origins of As and Cd in paddy soils, as well as their mobility, bioavailability, and uptake mechanisms by rice plants. It also examines the current methods and reactors used to lower As and Cd contamination in rice. Iron-modified biochar (Fe-BC) is a promising technology for reducing As and Cd toxicity in rice, improving soil health, and boosting rice's nutritional value. Biochar's physiochemical characteristics are enhanced by the addition of iron, making it a potent adsorbent for As and Cd ions. In conclusion, Fe-BC's biomembrane properties make them an attractive option for remediating As- and Cd-contaminated paddy soils. More efficient mitigation measures, including the use of biomembrane technology, can be developed when sustainable agriculture practices are combined with these technologies.

Regulating sludge composting with percarbonate facilitated the methylation and detoxification of arsenic mediated via reactive oxygen species

This study purposed to demonstrate the impact of reactive oxygen species (ROS) on arsenic detoxification mechanism in sludge composting with percarbonate. In this study, sodium percarbonate was used as an additive. Adding sodium percarbonate increased the content of H2O2 and OH, which the experimental group (SPC) was higher than the control group (CK). In addition, it decreased the bioavailability of arsenic by 19.10%. Metagenomic analysis found that Firmicutes and Pseudomonas took an active part in the overall compost as the dominant bacteria of arsenic methylation. ROS positively correlated with arsenic oxidation and methylation genes (arsC, arsM), with the gene copy number of arsC and arsM increasing to 7.74 × 1012, 5.24 × 1012 in SPC. In summary, the passivation of arsenic could be achieved by adding percarbonate, which promoted the methylation of arsenic, reduced the toxicity of arsenic, and provided a new idea for the harmless management of sludge.

Gene regulation and ionome homeostasis in rice plants in response to arsenite stress: potential connection between transcriptomics and ionomics

Ionomics and transcriptomics were applied to demonstrate response of rice to arsenite [As(III)] stress in the current study. Rice plants were cultured in nutrient solutions treated with 0, 100 and 500 μg/L As(III) coded as CK, As1 and As5, respectively. The rice ionomes exhibited discriminatory response to environmental disturbances. Solid evidence of the effects of As(III) stress on binding, transport or metabolism of P, K, Ca, Zn and Cu was obtained in this work. Differentially expressed genes (DEGs) in the shoots were identified in three datasets: As1 vs CK, As5 vs CK and As5 vs As1. DEGs identified simultaneously in two or three datasets were selected for subsequent interaction and enrichment analyses. Upregulation of genes involved in protein kinase activity, phosphorus metabolic process and phosphorylation were detected in the rice treated with As(III), resulting in the maintenance of P homeostasis in the shoots. Zn and Ca binding genes were up-regulated since excess As inhibited the translocation of Zn and Ca from roots to shoots. Increased expression of responsive genes including HMA, WRKY, NAC and PUB genes conferred As tolerance in the rice plants to cope with external As(III) stress. The results suggested that As(III) stress could disturb the uptake and translocation of macro and essential elements by rice. Plants could regulate the expression of corresponding genes to maintain mineral nutrient homeostasis for essential metabolic processes.

As(III)-Oxidizing Bacteria Alleviate Arsenite Toxicity via Reducing As Accumulation, Elevating Antioxidative Activities and Modulating Ionome in Rice (Oryza sativa L.)

Paddy rice trends to accumulate more arsenic (As) from soils than other terrestrial crops. The toxicity and mobility of As mainly depend on its chemical species. Transformation of arsenite [As(III)] into arsenate [As(V)] would be a promising method to mitigate As toxicity. In the current study, As(III)-oxidizing strain SMS11 isolated from As-contaminated soils was employed for As remediation. Co-cultured with SMS11 alleviated As(III) stress to the rice plants by increasing the length and biomass of rice shoots up to 10% and 15%, respectively. Evaluation of oxidative stress indices showed that the activity of catalase in the rice shoots was weakened when exposed to As(III), increasing the risk of hydroxyl radical (·OH) formation. When co-cultivated with the bacteria, ·OH formation was significantly inhibited in the rice shoots. The ionomes of the rice plants were impacted by the external conditions. As(III) stress significantly disturbed ionome homeostasis in the rice plants. Uptake of As simultaneously elevated the levels of macro and nutrient elements such as Mg, P, K, Ca, and Zn in the rice shoots. The ionomic variation in the rice plants under As(III) stress was mitigated by inoculated with SMS11. The results represented that the As(III)-oxidizing bacteria alleviated external As(III) stress to the rice plants through elevating antioxidative activities and modulating ionome homeostasis.

Detoxification and removal of arsenite by Pseudomonas sp. SMS11: Oxidation, biosorption and bioaccumulation

Arsenite [As(III)] oxidizing bacteria have been widely studied for their detoxification ability through transforming As(III) into arsenate [As(V)]. However, few was focused on removal capacity of arsenic (As). In the current study, As(III) oxidation accompanied with removal of total As was observed in Pseudomonas sp. SMS11. The biosorption (unbinding and surface binding) and bioaccumulation (intracellular uptake) of As by the cells were investigated. Biosorption isotherm was defined adequately by Langmuir and Freundlich models. Biosorption kinetics was recommended by pseudo second-order model. For comparison, the bacteria were inoculated in pure water or culture media amended with different concentrations of As(III) to evaluate the remediation capacity without or with bacterial growth. After removing unbound As, surface bound and intracellular As were sequentially separated using EDTA elution and acidic extraction from bacterial cells. Without bacterial growth, oxidation of As(III) was retarded and the maximum values of surface bound and intracellular As were 4.8 and 10.5 mg/g, respectively. Efficient oxidation and high adsorption capacity were observed after bacterial growth. The surface bound and intracellular As achieved up to 555.0 and 2421.5 mg/g, respectively. Strain SMS11 exhibited great accumulation capacity of As in aqueous solutions, indicating potential application in detoxification and removal of As(III) contamination. The results also suggested that bioremediation via bacteria should be based on living cells and bacterial growth rate.

Can simultaneous immobilization of arsenic and cadmium in paddy soils be achieved by liming?

Liming acidic paddy soils to near-neutral pH is the most cost-effective strategy to minimize cadmium (Cd) accumulation by rice. However, the liming-induced effect on arsenic (As) (im)mobilization remains controversial and is called upon for further investigation, particularly for the safe utilization of paddy soils co-contaminated with As and Cd. Here, we explored As and Cd dissolution along pH gradients in flooded paddy soils and extracted key factors accounting for their release discrepancy with liming. The minimum As and Cd dissolution occurred concurrently at pH 6.5-7.0 in an acidic paddy soil (LY). In contrast, As release was minimized at pH < 6 in the other two acidic soils (CZ and XX), while the minimum Cd release still appeared at pH 6.5-7.0. Such a discrepancy was determined largely by the relative availability of Fe under overwhelming competition from dissolved organic carbon (DOC). A mole ratio of porewater Fe/DOC at pH 6.5-7.0 is suggested as a key indicator of whether co-immobilization of As and Cd can occur in flooded paddy soils with liming. In general, a high mole ratio of porewater Fe/DOC (≥ 0.23 in LY) at pH 6.5-7.0 can endow co-immobilization of As and Cd, regardless of Fe supplement, whereas such a case is not in the other two soils with lower Fe/DOC mole ratios (0.01-0.03 in CZ and XX). Taking the example of LY, the introduction of ferrihydrite promoted the transformation of metastable As and Cd fractions to more stable ones in the soil during 35 days of flooded incubation, thus meeting a class I soil for safe rice production. This study demonstrates that the porewater Fe/DOC mole ratio can indicate a liming-induced effect on co-(im)mobilization of As and Cd in typical acidic paddy soils, providing new insights into the applicability of liming practice for the paddy soils.

Nocardiopsis lucentensis and thiourea co-application mitigates arsenic stress through enhanced antioxidant metabolism and lignin accumulation in rice

Arsenic (As) is a group-1 carcinogenic metalloid that threatens global food safety and security, primarily via its phytotoxicity in the staple crop rice. In the present study, ThioAC, the co-application of thiourea (TU, a non-physiological redox regulator) and N. lucentensis (Act, an As-detoxifying actinobacteria), was evaluated as a low-cost approach for alleviating As(III) toxicity in rice. To this end, we phenotyped rice seedlings subjected to 400 mg kg^-1 As(III) with/without TU, Act or ThioAC and analyzed their redox status. Under As-stress conditions, ThioAC treatment stabilized photosynthetic performance, as indicated by 78 % higher total chlorophyll accumulation and 81 % higher leaf biomass, compared with those of As-stressed plants. Further, ThioAC improved root lignin levels (2.08-fold) by activating the key enzymes of lignin biosynthesis under As-stress. The extent of reduction in total As under ThioAC (36 %) was significantly higher than TU (26 %) and Act (12 %), compared to those of As-alone treatment, indicating their synergistic interaction. The supplementation of TU and Act activated enzymatic and non-enzymatic antioxidant systems, respectively, with a preference for young (TU) and old (Act) leaves. Additionally, ThioAC activated enzymatic antioxidants, specifically GR (∼3-fold), in a leaf-age specific manner and suppressed ROS-producing enzymes to near-control levels. This coincided with 2-fold higher induction of polyphenols and metallothionins in ThioAC-supplemented plants, resulting in improved antioxidant defence against As-stress. Thus, our findings highlighted ThioAC application as a robust, cost-effective ameliorative strategy, for achieving As-stress mitigation in a sustainable manner.

Arsenic (As) resistant bacteria with multiple plant growth-promoting traits: Potential to alleviate As toxicity and accumulation in rice

A currently serious agronomic concern for paddy soils is arsenic (As) contamination. Paddy soils are mostly utilized for rice cultivation. Arsenite (As(III)) is prevalent in paddy soils, and its high mobility and toxicity make As uptake by rice substantially greater than that by other food crops. Globally, interest has increased towards using As-resistant plant growth-promoting bacteria (PGPB) to improve plant metal tolerance, promote plant growth, and immobilize As to prevent its uptake and accumulation in the edible parts of rice as much as possible. This review focuses on the As-resistant PGPB characteristics influencing rice growth and the mechanisms by which they function to alleviate As toxicity stress in rice plants. Several recent examples of mechanisms responsible for decreasing the availability of As to rice and coping with As stresses facilitated by the PGPB with multiple PGP traits (e.g., phosphate and silicate solubilization, the production of 1-aminocyclopropane-1-carboxylate deaminase, phytohormones, and siderophore, N₂ fixation, sulfate reduction, the biosorption, bioaccumulation, methylation, and volatilization of As, and arsenite oxidation) are also reviewed. In addition, future research needs about the application of As-resistant PGPB with PGP traits to mitigate As accumulation in rice plants are described.

The adsorption of arsenate and p-arsanilic acid onto ferrihydrite and subsequent desorption by sulfate and artificial seawater: Future implications of sea level rise

Sea level rise (SLR) is estimated to impact 25% of the world's population along coastal areas leading to an increase in saltwater intrusion. Consequently, changes in the soil biogeochemistry of currently non-saline and/or well-drained soils due to saltwater intrusion are of major concern. Saltwater intrusion is expected to affect farmland across large broiler producer regions, where large amounts of manure containing organic arsenicals were applied over the past decades. To determine how SLR may impact the speciation and mobility of adsorbed inorganic and organic As, we used in situ real-time attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) to determine the adsorption and desorption mechanisms of As(V) and 4-aminophenylarsonic (p-ASA, a poultry feed additive) on ferrihydrite (Fh) in the presence of sulfate at varying pH. The adsorption of As(V) and p-ASA increased at lower pH, with As(V) showing IR features consistent with the formation of inner-sphere of As-Fh surface complexes, while p-ASA also formed others structures as H-bonded As-surface complexes, likely mediated by outer-sphere complexes, based on our FTIR and batch experiments data. No observable As(V) or p-ASA desorption from the Fh surface was promoted by sulfate, however sulfate adsorption on the Fh surface was remarkably larger for p-ASA than for As(V). Complimentary, we carried out batch studies of As(V) and p-ASA desorption by Fh, using artificial seawater (ASW) at varying concentrations. The 1% ASW desorbed ∼10% of initially sorbed p-ASA, while at 100% ASW desorbed ∼40%. However, <1% of As(V) was desorbed by 1% ASW solution and only ∼7.9% were desorbed at 100% ASW. The spectroscopic data support the more extensive desorption of p-ASA compared to As(V) observed in batch experiments, suggesting that organoarsenicals may be easily desorbed and, after conversion to inorganic forms, pose a risk to water supplies.

Meta-analyses of arsenic accumulation in Indica and Japonica rice grains

Arsenic (As) is a worldwide concern because of its toxic effects on crop yield and prevalence in the food chain. Rice is consumed by half of the world's population and is known to accumulate As. The present study reviews the available literatures on As accumulation in different subspecies of rice grains (indica, japonica and aromatic) and performs meta-analyses for grain size and texture; these data include 120 studies conducted over the last 15 years across different parts of the world. Aromatic rice varieties accumulate less As with its 95% confidence interval (CI) being 73.90 - 80.94 μg kg^-1 which is significantly lower than the As accumulation by either indica or japonica rice varieties with their overall 95% CI being 135.48 - 147.78 μg kg^-1 and 204.71 - 212.25 μg kg^-1, respectively. Japonica rice varieties accumulate higher As than indica rice grains and within each subspecies polished and/or shorter rice grains accumulated significantly lower As compared to larger and/or unpolished grains; 95% CIs for the polished indica and japonica rice varieties are seen to be 96.33 - 111.11 μg kg^-1 and 203.34 - 211.09 μg kg^-1, respectively, whereas the same for unpolished varieties are seen to be 215.99 - 238.18 μg kg^-1 and 215.27 - 248.63 μg kg^-1, respectively. This shows that rice-based As bioaccumulation in humans could be lowered by increased use of aromatic or polished indica rice varieties, followed by the cultivation of shorter polished grains of japonica rice. These findings will be important to inform policy on rice cultivation and dietary uptake of As for a large portion of the global population.

Dark Side of Ammonium Nitrogen in Paddy Soil with Low Organic Matter: Stimulation of Microbial As(V) Reduction and As(III) Transfer from Soil to Rice Grains

The bioavailability of arsenic (As) is influenced by ammonium (NH₄⁺-N) fertilization, but the underlying mechanisms controlling As transformation in soil-rice systems are still not fully understood. The effects of two NH₄⁺-N fertilizers, urea and NH₄HCO₃, on the transformation of As in a paddy soil with low organic matter content and transfer in rice plants were investigated. Treatments with urea and NH₄HCO₃ significantly increased arsenite (As(III)) concentration in porewater, bioavailable As in rhizosphere soil, and the relative abundance of the As(V) respiratory reductase gene (arrA) and As(III) methyltransferase gene (arsM). Furthermore, the relative expression of As transporter genes in rice roots, such as OsLsi1, OsLsi2, and OsLsi3, was upregulated, and the translocation efficiency of As(III) from rice roots to brown rice was promoted. Subsequently, As(III) accumulation in brown rice significantly increased. Therefore, attention should be paid to As-contaminated paddy fields with NH₄⁺-N fertilization.

A chrysotile-based Fe/Ti nanoreactor enables efficient arsenic capture for sustainable environmental remediation

Iron-based materials for arsenic (As) immobilization in practical groundwater and soil remediation suffer from a low removal capacity and an insufficient long-term stability. Herein, a unique chrysotile-based nanoreactor has been developed by incorporating iron/titanium oxides into the cylindrical cavity of chrysotile (TiFe-Chy), providing sufficient internal reaction sites for As immobilization. Results reveal that the adsorption capacities of TiFe-Chy for As(III) and As(V) are considerably higher than the commonly used amendments, i.e., layered double hydroxide (LDH) and Phoslock®, respectively. More importantly, TiFe-Chy exhibits a strong anti-interference capability of As immobilization in soils compared to those commercial products due to this unique incorporation approach. Fixed-bed leaching experiments indciate that this TiFe-Chy nanoreactor can efficiently decrase the As(III) and As(V) concentrations by 81.8-87.3% within a period of ten years, significantly improving the long-term stability of As immobilization in soils. Life cycle assessment analysis reveals that TiFe-Chy can reduce negative environmental impacts (such as carbon emissions), resulting in a low cost for soils and groundwater remediation. The findings of this work open a new avenue for sustainable heavy metal(loid)s remediation in groundwater and soils.

Determination of bioavailable arsenic threshold and validation of modeled permissible total arsenic in paddy soil using machine learning

Minimizing arsenic intake from food consumption is a key aspect of the public health response in arsenic (As)-contaminated regions. In many of these regions, rice is the predominant staple food. Here, we present a validated maximum allowable concentration of total As in paddy soil and provide the first derivation of a maximum allowable soil concentration for bioavailable As. We have previously used meta-analysis to predict the maximum allowable total As in soil based on decision tree (DT) and logistic regression (LR) models. The models were defined using the maximum tolerable concentration (MTC) of As in rice grains as per the codex recommendation. In the present study, we validated these models using three test data sets derived from purposely collected field data. The DT model performed better than the LR in terms of accuracy and Matthews correlation coefficient (MCC). Therefore, the DT estimated maximum allowable total As in paddy soil of 14 mg kg^-1 could confidently be used as an appropriate guideline value. We further used the purposely collected field data to predict the concentration of bioavailable As in the paddy soil with the help of random forest (RF), gradient boosting machine (GBM), and LR models. The category of grain As (<MTC and >MTC) was considered as the dependent variable; bioavailable As (BAs), total As (TAs), pH, organic carbon (OC), available phosphorus (AvP), and available iron (AvFe) were the predictor variables. LR performed better than RF and GBM in terms of accuracy, sensitivity, specificity, kappa, precision, log loss, F1score, and MCC. From the better-performing LR model, bioavailable As (BAs), TAs, AvFe, and OC were significant variables for grain As. From the partial dependence plots (PDP) and individual conditional expectation (ICE) of the LR model, 5.70 mg kg^-1 was estimated to be the limit for BAs in soil.

Effects of topdressing silicon fertilizer at key stages on uptake and accumulation of arsenic in rice

The booting stage and filling stage have been considered as the key stages for arsenic (As) uptake in rice. In this study, a field study was conducted to investigate the influence of the topdressing different amounts of silicon (Si) fertilizer at the key stages on rice As uptake and accumulation. The results showed that topdressing of a low amount of Si fertilizer at the booting stage and filling stage could increase rice yield, promote the formation of iron plaque and the retention of As on iron plaque, and reduce inorganic As content in brown rice. Compared with the control, the rice grain yield was increased by 22.60% with the topdressing of 20 kg·hm^-2 Si fertilizer at the grain filling stage. As compared with the control, the Fe and As content in iron plaque under the topdressing of 20 kg·hm^-2 Si fertilizer at the booting stage and filling stage was significantly (p < 0.05) increased by 84.34% and 87.78% (Fe content) and 70.96% and 63.80% (As content), respectively. Meanwhile, contents of As in rice roots, stems, and husks at the topdressing of 20 kg·hm^-2 Si fertilizer at the booting stage were significantly (p < 0.05) reduced by 45.10%, 33.34%, and 31.23%, respectively, relative to the control. The lowest inorganic As content (0.21 mg·kg^-1) in brown rice was obtained at the topdressing of 20 kg·hm^-2 Si fertilizer at the booting stage, which was close to the National Food Limit Standard of 0.20 mg·kg^-1 (GB 2762-2017). Therefore, topdressing of 20 kg·hm^-2 Si fertilizer at the booting stage might be considered as an effective method to reduce inorganic As content in brown rice.

The role of various ameliorants on geochemical arsenic distribution and CO2-carbon efflux under paddy soil conditions

Climate change is a global challenge that is accelerated by contamination with hazardous substances like arsenic (As), posing threat to the agriculture, ecosystem and human health. Here, we explored the impact of various ameliorants on geochemical distribution of As in two soils with contrasting textures (sandy clay loam (Khudpur Village) and clay loam (Mattital Village)) under paddy soil conditions and their influence on the CO₂-carbon efflux. The exchangeable As pool in clay loam soil increased as: lignite (0.4%) < biogas slurry (6%) < cow dung (9%), and < biochar (20%). However, in the sandy clay loam soil exchangeable soil As pool was found to be maximum with farmyard manure followed by biogas slurry, biochar and cow dung (17%, 14%, 13% and 7%, respectively). Interestingly, in the sandy clay loam soil the percentage As distribution in organic fraction was: biochar (38%) > cow dung (33%) > biogas slurry (23%) > sugarcane bagasse (22%) > farmyard manure (21%) that was higher compared to the clay loam soil (< 6% for all the amendments). In addition to the highest As immobilization by biochar in sandy clay loam soil, it also led to the lowest CO₂-carbon efflux (1470 CO₂-C mg kg^-1) among all the organic/inorganic amendments. Overall, the current study advances our understanding on the pivotal role of organic amendments, notably biochar, in immobilizing As under paddy soil conditions with low (CO₂) carbon loss, albeit it is dependent on soil and ameliorant types.

Distinct response of arsenic speciation and bioavailability to different exogenous organic matter in paddy soil

Land application of organic waste has been increasingly encouraged since it could sequester carbon to mitigate climate change. Considering the susceptibility of arsenic (As) bioavailability in soils to organic matter, understanding the influence of different exogenous organic matter on As biogeochemical behavior in rice-soil system is crucial to reasonably recycle organic waste on soils and ensure the food safety. In this study, impacts of two typical organic matter amendments, rice straw and humic substance, on the As speciation and bioavailability in paddy soil were investigated. Results showed that addition of both rice straw and humic substance could increase the dissolved organic carbon (DOC) content in soil solution by 16.4%-34.4% and 21.7%-53.2%, respectively, but the response of As speciation and bioavailability was quite different, showing the decoupling between As release and DOC. Rice straw addition increased As release to porewater by 28.0%-28.4%, particularly at the initial 0-18 days after the soil was flooded, but humic substance presented the opposite effect, decreasing As release by 27.4%-43.1% which was mainly attributable to the As^III immobilization. This study suggests that the organic matter with high contents of labile heteroaliphatic/aliphatic carbon, being easily to be biodegraded, should not be applied on As contaminated soils.

Biochar/nano-zerovalent zinc-based materials for arsenic removal from contaminated water

In this study, we explored the potential of a newly prepared nano-zero valent zinc (nZVZn), biochar (BC)/nZVZn and BC/hydroxyapatite-alginate (BC/HA-alginate) composites for the removal of inorganic As species from water. Relatively, higher percentage removal of As(III) and As(V) was obtained by nZVZn at pH 3.4 (96% and 94%, respectively) compared to BC/nZVZn (90% and 88%) and BC/HA-alginate (88% and 80%) at pH 7.2. Freundlich model provided the best fit (R² = up to 0.98) for As(III) and As(V) sorption data of all the sorbents, notably for nZVZn. The pseudo-second order model well-described kinetics of As(III) and As(V) (R² = 0.99) sorption on all the sorbents. The desorption experiments demonstrated that the As removal efficiency, up to the third sorption/desorption cycle, was in the order of nZVZn ∼ BC/HA-alginate (88%) > BC/nZVZn (84%). The Fourier transform infrared spectroscopy depicted that the -OH, -COOH, Zn-O and Zn-OH surface functional groups were responsible for the sorption of As(III) or As(V) on the sorbents investigated here. This study highlights that removal of As species from water by BC/nZVZn composite can be compared with nZVZn, suggesting that integrating BC with nZVZn could efficiently remove As from As-contaminated drinking water.

Arsenic extraction from seriously contaminated paddy soils with ferrihydrite-loaded sand columns

Reductive dissolution of iron oxides in flooded paddy soils is the most important cause of arsenic (As) release into soil aqueous solution and thus entry into rice. From the perspective of soil cleanup, however, As release under flooded condition could facilitate labile As removal. In this study, a porous column pre-loaded with ferrihydrite (Fh) was constructed, and its efficiency of soil As extraction was investigated using a purpose-designed mesocosm coupled with diffusive gradients in thin films (DGT) for in situ visualization. With Fh-column deployed in aqueous solution, >90% removal of As(III) was achieved within 5 days at initial As (100 mg L^-1) of two orders of magnitude higher than in most paddy soil solutions (1-1538 μg L^-1). By applying Fh-column in a seriously contaminated paddy soil (102 mg As kg^-1), porewater As showed stepwise decreases from 2727 μg L^-1 to 129-1455 μg L^-1 at a distance-dependent manner over four intermittent extractions during 91 days. Soil DGT-As exhibited similar spatiotemporal changes to porewater As. After four extractions, 17.8% of total soil As was removed by Fh-column in a 10 cm radius range on average and ∼1/3 of As bound to amorphous and crystalline Fe/Al oxides was depleted, which accounted for 88.7% of decline in total soil As. With the post-extracted soil, a 48% lower As accumulation in rice seedlings and a 65% decline in bulk soil DGT-As were attained. This study provides a conceptual foundation for rapid removal of high soluble As by Fh-columns from flooded soils, improving seriously As-contaminated paddies to sustainable resources for safe food production.

Carcinogenic effects of heavy metals by inducing dysregulation of microRNAs: A review

Heavy metal exposure has soared due to the twentieth century's industrial activity. The most common heavy metals that lead to human poisoning are mercury, cadmium, and arsenic. Acute or chronic poisoning may develop following exposure to water, air, or food, so the bioaccumulation of these heavy metals causes harmful consequences in various human tissues and organs. Heavy metals interfere with biological functions such as growth, proliferation, differentiation, damage repair, and apoptosis. The mechanisms of action for these metals to cause toxicity are similar, including forming reactive oxygen species (ROS), weakening antioxidant defenses, enzyme inactivation, and oxidative stress. Heavy metal exposure is mainly associated with skin, liver, prostate, lung, urinary bladder, thyroid, and kidney cancers, as well as causing gastrointestinal malignancies. Several microRNAs (miRNAs or miRs) have been involved in various human cancers due to the dysregulation of miRNA function. Recent investigations have confirmed that microRNA dysregulation plays a role in the carcinogenesis of many tissues. This review presents the data concerning arsenic, cadmium, and mercury metals and their contamination sources, human exposure, toxicity, and inducing malignant transformations such as carcinogenicity in in-vitro or in-vivo specimens or dysregulated expression of microRNAs.

Antimony release and volatilization from rice paddy soils: Field and microcosm study

The fate of antimony (Sb) in submerged soils and the impact of common agricultural practices (e.g., manuring) on Sb release and volatilization is understudied. We investigated porewater Sb release and volatilization in the field and laboratory for three rice paddy soils. In the field study, the porewater Sb concentration (up to 107.1 μg L^-1) was associated with iron (Fe) at two sites, and with pH, Fe, manganese (Mn), and sulfate (SO₄^2-) at one site. The surface water Sb concentrations (up to 495.3 ± 113.7 μg L^-1) were up to 99 times higher than the regulatory values indicating a potential risk to aquaculture and rice agriculture. For the first time, volatile Sb was detected in rice paddy fields using a validated quantitative method (18.1 ± 5.2 to 217.9 ± 160.7 mg ha^-1 y^-1). We also investigated the influence of two common rice agriculture practices (flooding and manuring) on Sb release and volatilization in a 56-day microcosm experiment using the same soils from the field campaign. Flooding induced an immediate, but temporary, Sb release into the porewater that declined with SO₄^2-, indicating that SO₄^2- reduction may reduce porewater Sb concentrations. A secondary Sb release, corresponding to Fe reduction in the porewater, was observed in some of the microcosms. Our results suggest flooding-induced Sb release into rice paddy porewaters is temporary but relevant. Manuring the soils did not impact the porewater Sb concentration but did enhance Sb volatilization. Volatile Sb (159.6 ± 108.4 to 2237.5 ± 679.7 ng kg^-1 y^-1) was detected in most of the treatments and was correlated with the surface water Sb concentration. Our study indicates that Sb volatilization could be occurring at the soil-water interface or directly in the surface water and highlights that future works should investigate this potentially relevant mechanism.

Impact of arsenic on microbial community structure and their metabolic potential from rice soils of West Bengal, India

Paddy soil is a heterogenous ecosystem that harbours diverse microbial communities critical for maintaining ecosystem sustainability and crop yield. Considering the importance of soil in crop production and recent reports on its contamination with arsenic (As) across the South East Asia, its microbial community composition and biogeochemical functions remained inadequately studied. We have characterized the microbial communities of rice soil from eleven paddy fields of As-contaminated sites from West Bengal (India), through metagenomics and amplicon sequencing. 16S rRNA gene sequencing showed considerable bacterial diversity [over 0.2 million Operational Taxonomic Units (OTUs)] and abundance (upto 1.6 × 10⁷ gene copies/g soil). Existence of a core-microbiome (261 OTUs conserved out of a total 141,172 OTUs) across the samples was noted. Most of the core-microbiome members were also found to represent the abundant taxa of the soil. Statistical analyses suggested that the microbial communities were highly constrained by As, Fe K, N, PO₄^3-, SO₄^2- and organic carbon (OC). Members of Proteobacteria, Actinobacteria, Acidobacteria, Chloroflexi, Planctomycetes and Thaumarchaeota constituted the core-microbiome. Co-occurrence network analysis displayed significant interaction among diverse anaerobic, SO₄^2- and NO₃^- reducing, cellulose and other organic matter or C1 compound utilizing, fermentative and aerobic/facultative anaerobic bacteria and archaea. Correlation analysis suggested that taxa which were positively linked with soil parameters that maintain soil health and productivity (e.g., N, K, PO₄^3- and Fe) were adversely impacted by increasing As concentration. Shotgun metagenomics highlighted major metabolic pathways controlling the C (3-hydroxypropionate bicycle), N (Denitrification, dissimilatory NO₃^- reduction to ammonium), and S (assimilatory SO₄^2- reduction and sulfide oxidation) cycling, As homeostasis (methylation and reduction) and plant growth promotion (polyphosphate hydrolysis and auxin biosynthesis). All these major biogeochemical processes were found to be catalyzed by the members of most abundant/core-community.

Arsenic accumulation in rice: Alternative irrigation regimes produce rice safe from arsenic contamination

The natural occurrence of arsenic (As) in groundwater & soils and its bioaccumulation in rice grains is a major health concern worldwide. To combat the problem, best combination of irrigation management and suitable rice variety altering As content in grains must be ensured. With this aim, a field trial was conducted with two rice varieties and water management including alternate wetting and drying (AWD) and continuous flooding (CF) irrigation regimes with As contaminated groundwater (AsW) and temporarily stored groundwater (TSG) and river water for only CF (as control). Results revealed that As content in different portions of paddy plant was significantly different (P < 0.001) with irrigation practices and rice varieties. AWD irrigation with TSG accumulated lower As in rice grains compared with CF-AsW for both varieties. Data showed that AWD-TSG practice led to 61.37% and 60.34% grain As reduction for BRRI dhan28 and BRRI dhan29, respectively, compared with CF-AsW. For Principle Component Analysis (PCA), first principle component (PC1) explained 91.7% of the variability and irrigation water As, soil total and available As, straw As, root As and husk As were the dominating parameters. With significant (P < 0.05) variation in yields between the genotypes, AWD increased grain yield by 29.25% in BRRI dhan29 Compared with CF. However, translocation factor (TF) and bioconcentration factor (BCF) for both varieties were less than one for all the treatments. The addition of this study to our knowledge base is that, AWD-TSG with BRRI dhan29 can be an As-safe practice without compromising yields.

Varietal differences influence arsenic and lead contamination of rice grown in mining impacted agricultural fields of Zamfara State, Nigeria

In Zamfara state, Nigeria, rice is cultivated in fields contaminated with Pb (lead) from artisanal and illicit mining activities. Rice grown in such contaminated agricultural areas risks not only Pb contamination but also contamination from other toxic elements, like arsenic (As); co-contamination of Pb and As in rice cultivated in mining impacted areas has been previously reported and rice is a hyperaccumulator of As. A field study was conducted with ten different commonly-cultivated Nigerian rice varieties in the mining-impacted farmlands of Dareta village, Zamfara State. The aim was to determine the optimal rice variety for cultivation on these contaminated farmlands; an optimal variety would have the lowest contaminant concentrations and highest essential elements concentrations in the rice grains. A total of 300 paired soil and rice plants were collected. The mean As and Pb concentrations in paddy soils were 0.91 ± 0.82 mg kg^-1 and 288.5 ± 464.2 mg kg^-1, respectively. Mean As (30.4 ± 15.1 μg kg^-1) content in rice grains was an order of magnitude lower than the Codex recommendation of 200 μg kg^-1 (for milled rice) while the Pb content in all the rice varieties (overall mean of 743 ± 327 μg kg^-1) was approximately four times higher than the Codex recommendation of 200 μg kg^-1. Contrary to previous studies, a negative correlation was observed between As and Pb in rice grains across all the varieties. Rice variety Bisalayi was the variety with the lowest Pb transfer factor (TF = 0.08), but the average Pb concentration in rice grain was still above the Codex recommendation. Bisalayi also had the highest TF for iron. Variety ART_15, which had the lowest As uptake (TF = 0.10), had the highest TF for essential elements (magnesium, potassium, manganese, zinc, and copper). In areas of Pb contamination, Bisalayi rice may therefore be a suitable variety to choose for cultivation.

Toxic Metals and Metalloids in Hassawi Brown Rice: Fate during Cooking and Associated Health Risks

Rice has been a dietary staple for centuries, providing vital nutrients to the human body. Brown rice is well known for its nutrient-dense food profile. However, owing to multiple causes (anthropogenic and non-anthropogenic), it can also be a potential source of toxic heavy metals in the diet. Brown Hassawi rice samples were collected from the Al-Ahsa region and analyzed for its content of toxic metals. The results reveal that all the tested metals varied significantly in the brown rice samples, while As and Pb in all three samples exceeded their respective maximum allowable limits (MALs), followed by Cd, which nearly approached the MAL in two samples out of three. Brown rice samples were cooked in rice:water systems, viz., low rice:water ratios (1:2.5, 1:3.5) and high rice:water ratios (1:5, 1:6), along with soaking as a pre-treatment. Soaking was unproductive in removing the heavy metals from the rice, whereas cooking dissipated all metals from the rice, except for Cd, which was statistically non-significant. The high-water cooking of the rice was more effective in the dissipation of metals from the rice as compared to low-water cooking conditions. Through the consumption of rice, the estimated daily intake (EDI) of heavy metals is 162 g per person per day for As, which is above the provisional maximum tolerable daily intake (PMTDI) regardless of cooking circumstances. The hazard risk index (HRI) also highlighted the fact that As can be a potential health hazard to rice consumers in the Al-Ahsa region of Saudi Arabia. These results indicate the potential health risks caused by the consumption of this rice by humans. Regular monitoring is recommended to manage and control elevated concentrations and related health hazards as a result of the use of Hassawi rice contaminated by the accumulation of metals and metalloids.

Human health impact due to arsenic contaminated rice and vegetables consumption in naturally arsenic endemic regions

Rice and vegetables cultivated in naturally arsenic (As) endemic areas are the substantial source of As body loading for persons using safe drinking water. However, tracing As intake, particularly from rice and vegetables by biomarker analysis, has been poorly addressed. This field investigation was conducted to trace the As transfer pathway and measure health risk associated with consuming As enriched rice and vegetables. Purposively selected 100 farmers from five sub-districts of Chandpur, Bangladesh fulfilling specific requirements constituted the subjects of this study. A total of 100 Irrigation water, soils, rice, and vegetable samples were collected from those farmers' who donated scalp hair. Socio-demographic and food consumption data were collected face to face through questionnaire administration. The mean As level in irrigation water, soils, rice, vegetables, and scalp hairs exceeded the acceptable limit, while As content was significant at 0.1%, 5%, 0.1%, 1%, and 0.1% probability levels, respectively, in all five locations. Arsenic in scalp hair is significantly (p ≤ 0.01) correlated with that in rice and vegetables. The bioconcentration factor (BCF) for rice and vegetables is less than one and significant at a 1% probability level. The average daily intake (ADI) is higher than the RfD limit for As. Both grains and vegetables have an HQ (hazard quotient) > 1. Maximum incremental lifetime cancer risk (ILCR) showed 2.8 per 100 people and 1.6 per 1000 people are at considerable and threshold risk, respectively. However, proteinaceous and nutritious food consumption might have kept the participants asymptomatic. The PCA analysis showed that the first principle component (PC1) explains 91.1% of the total variance dominated by As in irrigation water, grain, and vegetables. The dendrogram shows greater variations in similarity in rice and vegetables As, while the latter has been found to contribute more to human body loading compared to grain As.

Effect of applying persulfate on the accumulation of arsenic in rice plants grown in arsenic-contaminated paddy soil

Arsenic is known to be a notorious human carcinogen and rice consumption is becoming the primary human exposure route for As, especially in many Asian countries. As one of redox-sensitive elements in soil, sulfur plays an indisputable role in controlling As behaviors. However, information on the effects of persulfe (PS) on the toxicity and accumulation of As in rice plant under flooded conditions is limited. Therefore, a pot experiment was conducted to investigate the effects of PS amendment on the growth and accumulation of As species in rice plants grown in As-contaminated paddy soil. Results revealed that PS application increased the As, Fe, and Mn in porewater at the early stage, and then declined. Application of PS increased the biomass of stem and root, while inhibited the formation of iron plaque on the root surface. The As translocation from root to rice above tissues and accumulation of As species in brown rice were declined by amendment with PS. The inorganic arsenic (iAs) and DMA were the two main species in brown rice, and decreased by 13~26% and 40~60% respectively upon PS application. The results suggested that amendment with PS might be feasible for reducing the accumulation of As in rice grains grown in As-contaminated paddy soil. However, further detailed studies on the potential soil biogeochemical and physiological mechanisms are recommended.

Significance of Arbuscular Mycorrhizal Fungi in Mitigating Abiotic Environmental Stress in Medicinal and Aromatic Plants: A Review

Medicinal and aromatic plants (MAPs) have been used worldwide for thousands of years and play a critical role in traditional medicines, cosmetics, and food industries. In recent years, the cultivation of MAPs has become of great interest worldwide due to the increased demand for natural products, in particular essential oils (EOs). Climate change has exacerbated the effects of abiotic stresses on the growth, productivity, and quality of MAPs. Hence, there is a need for eco-friendly agricultural strategies to enhance plant growth and productivity. Among the adaptive strategies used by MAPs to cope with the adverse effects of abiotic stresses including water stress, salinity, pollution, etc., their association with beneficial microorganisms such as arbuscular mycorrhizal fungi (AMF) can improve MAPs’ tolerance to these stresses. The current review (1) summarizes the effect of major abiotic stresses on MAPs’ growth and yield, and the composition of EOs distilled from MAP species; (2) reports the mechanisms through which AMF root colonization can trigger the response of MAPs to abiotic stresses at morphological, physiological, and molecular levels; (3) discusses the contribution and synergistic effects of AMF and other amendments (e.g., plant growth-promoting bacteria, organic or inorganic amendments) on MAPs’ growth and yield, and the composition of distilled EOs in stressed environments. In conclusion, several perspectives are suggested to promote future investigations.

How different nitrogen fertilizers affect arsenic mobility in paddy soil after straw incorporation?

In straw return fields, nitrogen-fertilizers are added to mitigate microbial competition for nitrogen with plants. However, in arsenic (As)-contaminated paddy fields, the specific effects of different nitrogen fertilizers on As mobility after straw incorporation and the interactions among iron(Fe)/carbon(C)/nitrogen(N)/As are not well understood. In the reported microcosm experiment we monitored As-mobility as a function of different dosages of KNO₃, NH₄Cl and rice straw incorporation. Addition of both KNO₃ and NH₄Cl significantly inhibited the As mobilization induced by straw incorporation. Following the KNO₃ addition, the As concentration in porewater dropped by 51-66% after 2 days of the incubation by restraining Fe reduction and enhancing Fe oxidation. High-dose NH₄Cl addition reduced As in porewater by 22-43% throughout the incubation by decreasing porewater pH. High-throughput sequencing results demonstrated that KNO₃ addition enriches both the denitrifying and Fe-oxidizing bacteria, while diminishing Fe-reducing bacteria; NH₄Cl addition has the opposite effect on Fe-reducing bacteria. Network analysis revealed that As and Fe concentrations in porewater were positively correlated with the abundance of denitrifying and Fe-reducing bacteria. This study broadens our insight into the As biogeochemistry associated with the N/C/Fe balance in soil, which are of great significance for agronomic management and mitigation the risk of As-contaminated paddy fields.

Characterization of the rare microbiome of rice paddy soil from arsenic contaminated hotspot of West Bengal and their interrelation with arsenic and other geochemical parameters

Rare microbial taxa [bacterial and archaeal operational taxonomic units (OTUs) with mean relative abundance ≤ 0.001%] were critical for ecosystem function, yet, their identity and function remained incompletely understood, particularly in arsenic (As) contaminated rice soils. In the present study we have characterized the rare populations of the As-contaminated rice soil microbiomes from West Bengal (India) in terms of their identity, interaction and potential function. Major proportion of the OTUs (73% of total 38,289 OTUs) was represented by rare microbial taxa (henceforth mentioned as rare taxa), which covered 4.5-15.7% of the different communities. Taxonomic assignment of the rare taxa showed their affiliation to members of Gamma-, Alpha-, Delta- Proteobacteria, Actinobacteria, and Acidobacteria. SO₄^2-, NO₃^-, NH₄⁺and pH significantly impacted the distribution of rare taxa. Rare taxa positively correlated with As were found to be more frequent in relatively high As soil while the rare taxa negatively correlated with As were found to be more frequent in relatively low As soil. Co-occurrence-network analysis indicated that rare taxa whose abundance were correlated strongly (R > 0.8) with As also had strong association (R > 0.8) with PO₄^2-, NO₃^-, and NH₄⁺. Correlation analysis indicated that the rare taxa were likely to involved in two major guilds one, involved in N-metabolism and the second involved in As/Fe as well as other metabolisms. Role of the rare taxa in denitrification and dissimilatory NO₃^- reduction (DNRA), As biotransformation, S-, H-, C- and Fe-, metabolism was highlighted from 16S rRNA gene-based predictive analysis. Phylogenetic analysis of rare taxa indicated signatures of inhabitant rice soil microorganisms having significant roles in nitrogen (N) cycle and As-Fe metabolism. This study provided critical insights into the taxonomic identity, metabolic potentials and importance of the rare taxa in As biotransformation and biogeochemical cycling of essential nutrients in As-impacted rice soils.

Loading ferric lignin on polyethylene film and its influence on arsenic-polluted soil and growth of romaine lettuce plant

This work developed a composite (Pe-FeLs) which loaded ferric lignin on polyethylene film (PE film) by chemical modification and physico-chemically characterized by Microscope, FESEM with elemental mapping analysis, and XRD. Microscope pictures showed that chemical modification did not destroy the appearance of PE film. The FESEM images of Pe-FeLs showed the well-distributed clusters could be clearly seen and most of the particles were spherical morphology. Elemental mapping of individual element on Pe-FeLs clearly indicated the existing of iron. The XRD pattern showed the amorphous hydroxides of iron on Pe-FeLs. In arsenic solution, the total arsenic adsorption capacity of Pe-FeLs was much higher than that of ferric lignin and PE, which showed Pe-FeLs had the ability to adsorb arsenic. For making Pe-FeLs work well in the soil, a Pe-FeLs system was set up with plastic grid plate, PE film with holes, Pe-FeLs, PE film, and plastic grid plate from the upper to bottom in order. With applying Pe-FeLs system under the soil, arsenic was significantly reduced by 25.5 ~ 53.4% in heavily, moderately, and lower arsenic-polluted soils, the biomass of the romaine lettuce increased and arsenic accumulation in the romaine lettuce decreased.

Arsenic in the water and agricultural crop production system: Bangladesh perspectives

The presence of high levels of carcinogenic metalloid arsenic (As) in the groundwater system of Bangladesh has been considered as one of the major environmental disasters in this region. Many parts of Bangladesh have extensively reported the presence of high levels of arsenic in the groundwater due to both geological and anthropogenic activities. In this paper, we reviewed the available literature and scientific information regarding arsenic pollution in Bangladesh, including arsenic chemistry and occurrences. Along with using As-rich groundwater as a drinking-water source, the agricultural activities and especially irrigation have greatly depended on the groundwater resources in this region due to high water demands for ensuring food security. A number of investigations in Bangladesh have shown that high arsenic content in both soil and groundwater may result in high levels of arsenic accumulation in different plants, including cereals and vegetables. This review provides information regarding arsenic accumulation in major rice varieties, soil-groundwater-rice arsenic interaction, and past arsenic policies and plans, as well as previously implemented arsenic mitigation options for both drinking and irrigation water systems in Bangladesh. In conclusion, this review highlights the importance and necessity for more in-depth studies as well as more effective arsenic mitigation action plans to reduce arsenic incorporation in the food chain of Bangladesh.

Inhibition of methanogenesis leads to accumulation of methylated arsenic species and enhances arsenic volatilization from rice paddy soil

Flooded soils are important environments for the biomethylation and subsequent volatilization of arsenic (As), a contaminant of global concern. Conversion of inorganic to methylated oxyarsenic species is thought to be the rate-limiting step in the production and emission of volatile (methyl)arsines. While methanogens and sulfate-reducing bacteria (SRB) have been identified as important regulators of methylated oxyarsenic concentrations in anaerobic soils, the effects of these microbial groups on biovolatilization remain unclear. Here, microcosm and batch incubation experiments with an Arkansas, USA, rice paddy soil were performed in conjunction with metabolic inhibition to test the effects of methanogenic activity on As speciation and biovolatilization. Inhibition of methanogenesis with 2-bromoethanesulfonate (BES) led to the accumulation of methylated oxyarsenic species, primarily dimethylarsinic acid (DMAs(V)), and a four-fold increase in As biovolatilization compared to a control soil. Our results support a conceptual model that methanogenic activity suppresses biovolatilization by enhancing As demethylation rates. This work refines understanding of biogeochemical processes regulating As biovolatilization in anaerobic soil environments, and extends recent insights into links between methanogenesis and As metabolism to soils from the mid-South United States rice production region.

Bioavailability of antimony and arsenic in a flowering cabbage-soil system: Controlling factors and interactive effect

Soil contamination with antimony (Sb) and arsenic (As) has become a well-recognized environmental and human health issue. Consumption of vegetables, especially leafy vegetables, is one of the most important sources of Sb and As exposure in humans. Accordingly, it is necessary to understand the behaviors of Sb and As in the vegetable-soil system. Moreover, although Sb and As are often assumed to have similar biogeochemical behavior, identified differences in the controlling factors affecting mobility and bioavailability of Sb and As in soils need further investigation. In this study, 112 pairs of soil and flowering cabbage samples were collected from typical farmland protection areas and vegetable-producing regions across the Pearl River Delta (PRD), South China. The contamination levels of Sb and As in soils and harvested cabbages across the PRD were investigated. The main factors affecting the mobility and bioavailability of Sb and As in the cabbage-soil system were disentangled using a random forest model. The contamination levels of Sb in the cabbages and soils of the PRD were generally low, but the soils were moderately polluted by As. Increased concentrations of Fe oxides could decrease Sb accumulation in cabbages but increased the mobilization of As in soils to some extent. In contrast, Al oxides contributed strongly to the mobilization of Sb and the immobilization of As. Moreover, an increased sand content promoted the mobility of Sb and As, whereas increased silt and clay contents showed inhibitory effects. The interactions of As and Sb with Fe oxides decreased the mobility of Sb but moderately increased the mobility of As in soils. Overall, the behaviors of Sb and As in the cabbage-soil system under the effect of several important environmental factors showed some differences indicating that these differences should be considered in the remediation of co-contaminated soils.

Molecular insight into arsenic uptake, transport, phytotoxicity, and defense responses in plants: a critical review

A critical investigation into arsenic uptake and transportation, its phytotoxic effects, and defense strategies including complex signaling cascades and regulatory networks in plants. The metalloid arsenic (As) is a leading pollutant of soil and water. It easily finds its way into the food chain through plants, more precisely crops, a common diet source for humans resulting in serious health risks. Prolonged As exposure causes detrimental effects in plants and is diaphanously observed through numerous physiological, biochemical, and molecular attributes. Different inorganic and organic As species enter into the plant system via a variety of transporters e.g., phosphate transporters, aquaporins, etc. Therefore, plants tend to accumulate elevated levels of As which leads to severe phytotoxic damages including anomalies in biomolecules like protein, lipid, and DNA. To combat this, plants employ quite a few mitigation strategies such as efficient As efflux from the cell, iron plaque formation, regulation of As transporters, and intracellular chelation with an array of thiol-rich molecules such as phytochelatin, glutathione, and metallothionein followed by vacuolar compartmentalization of As through various vacuolar transporters. Moreover, the antioxidant machinery is also implicated to nullify the perilous outcomes of the metalloid. The stress ascribed by the metalloid also marks the commencement of multiple signaling cascades. This whole complicated system is indeed controlled by several transcription factors and microRNAs. This review aims to understand, in general, the plant-soil-arsenic interaction, effects of As in plants, As uptake mechanisms and its dynamics, and multifarious As detoxification mechanisms in plants. A major portion of this article is also devoted to understanding and deciphering the nexus between As stress-responsive mechanisms and its underlying complex interconnected regulatory networks.

Exposure to potentially toxic elements through the soil-tobacco-human pathway: causative factors and probabilistic model

High concentrations of potentially toxic elements (PTEs) in tobacco leaves are possible from the soil contamination and would have adverse health risks on residents. A large-scale survey of 306 tobacco fields in southern China was conducted to investigate the accumulation of PTEs in tobacco leaves through the soil-tobacco-human pathway and the associated health risks for local smokers and passive smokers. Significant enrichment of As, Cd, Hg, and Pb was observed in the investigated tobacco fields, with industrial emissions and applied fertilizers as the major potential sources. Dynamic interactions between factors in the soil acidic labile pool showed site-specific effects on the uptake of PTEs by tobacco plants. It was 99.6% and 91.8% probable that exposure of local adult men smokers to Cd and As exceeded the permitted safety limits, respectively. The population of men smokers had a 20-fold higher Cd exposure risk than did passive smokers. A probability-based transfer model was developed to demonstrate that interactions between soil factors could affect the Cd exposure risk of men smokers of locally harvested tobacco. Optimizing the pH (>6.0) and organic matter content (>40 g kg-1) of tobacco-growing soils, and setting a safe tobacco consumption rate of 2.80 g dry weight per day would help protect 90.4% of men smokers from excessive risks of exposure to Cd.

The mechanistic pathways of arsenic transport in rice cultivars: Soil to mouth

Rice cultivars are major conduit of arsenic (As) poisoning to human. We quantified transferability of fifteen rice cultivars representing three groups i.e., high yielding variety (HYV), local aromatic rice (LAR) and hybrid for As from soil to cooked rice and its ingestion led health risk, elucidating the processes of its unloading at five check points. Conducting a field experiment with those cultivars, we sampled roots and shoots at tillering, booting and maturity (with grains), separated the grains into husk, bran and polished rice, cooked it through different methods and analyzed for As. Of the tested groups, As restriction from root to grain followed the order: LARs (94%) > HYVs (88.3%) > hybrids (87.2%). The low As sequestration by LARs was attributed to their higher root biomass (10.20 g hill^-1) and Fe-plaque formation (2421 mg kg^-1), and lower As transfer coefficients (0.17), and higher As retention in husk and bran (84%). On average, based on calculated four major risk indices, LARs showed 4.7-6.8 folds less As toxicity than HYVs and hybrids. These insights are helpful in advocating some remedies for As toxicity of the tested rice cultivars.

Co-oxidative removal of arsenite and tetracycline based on a heterogeneous Fenton-like reaction using iron nanoparticles-impregnated biochar

A highly efficient, eco-friendly and relatively low-cost catalyst is necessary to tackle bottlenecks in the treatment of industrial wastewater laden with heavy metals and antibiotic such as livestock farm and biogas liquids. This study investigated co-oxidative removal of arsenite (As(III)) and tetracycline (TC) by iron nanoparticles (Fe NP)-impregnated carbons based on heterogeneous Fenton-like reactions. The composites included Fe NP@biochar (BC), Fe NP@hydrochar (HC), and Fe NP@HC-derived pyrolysis char (HDPC). The functions of N and S atoms and the loading mass of the Fe NP in the Fe NP@BC in heterogeneous Fenton-like reactions were studied. To sustain its cost-effectiveness, the spent Fe NP@BC was regenerated using NaOH. Among the composites, the Fe NP@BC achieved an almost complete removal of As(III) and TC under optimized conditions (1.0 g/L of dose; 10 mM H₂O₂; pH 6; 4 h of reaction; As(III): 50 μM; TC: 50 μM). The co-oxidative removal of As(III) and TC by the Fe NP@ BC was controlled by the synergistic interactions between the Fe NPs and the active N and S sites of the BC for generating reactive oxygen species (ROS). After four consecutive regeneration cycles, about 61 and 95% of As(III) and TC removal were attained. This implies that the spent carbocatalyst still has reasonable catalytic activities for reuse. Overall, this suggests that adding technological values to unused biochar as a carbocatalyst like Fe NP@BC was promising for co-oxidative removal of As(III) and TC from contaminated water.