Arsenic-contaminated Paddy Soil Research Articles

Activated Carbon Application Simultaneously Alleviates Paddy Soil Arsenic Mobilization and Carbon Emission by Decreasing Porewater Dissolved Organic Matter.

Flooding of paddy fields during the rice growing season enhances arsenic (As) mobilization and greenhouse gas (e.g., methane) emissions. In this study, an adsorbent for dissolved organic matter (DOM), namely, activated carbon (AC), was applied to an arsenic-contaminated paddy soil. The capacity for simultaneously alleviating soil carbon emissions and As accumulation in rice grains was explored. Soil microcosm incubations and 2-year pot experimental results indicated that AC amendment significantly decreased porewater DOM, Fe(III) reduction/Fe2+ release, and As release. More importantly, soil carbon dioxide and methane emissions were mitigated in anoxic microcosm incubations. Porewater DOM of pot experiments mainly consisted of humic-like fluorophores with a molecular structure of lignins and tannins, which could mediate microbial reduction of Fe(III) (oxyhydr)oxides. Soil microcosm incubation experiments cospiking with a carbon source and AC further consolidated that DOM electron shuttling and microbial carbon source functions were crucial for soil Fe(III) reduction, thus driving paddy soil As release and carbon emission. Additionally, the application of AC alleviated rice grain dimethylarsenate accumulation over 2 years. Our results highlight the importance of microbial extracellular electron transfer in driving paddy soil anaerobic respiration and decreasing porewater DOM in simultaneously remediating As contamination and mitigating methane emission in paddy fields.

Anoxygenic phototrophic arsenite oxidation by a Rhodobacter strain.

Microbially mediated arsenic redox transformations are key for arsenic speciation and mobility in rice paddies. Whereas anaerobic anoxygenic photosynthesis coupled to arsenite (As(III)) oxidation has been widely examined in arsenic-replete ecosystems, it remains unknown whether this light-dependent process exists in paddy soils. Here, we isolated a phototrophic purple bacteria, Rhodobacter strain CZR27, from an arsenic-contaminated paddy soil and demonstrated its capacity to oxidize As(III) to arsenate (As(V)) using malate as a carbon source photosynthetically. Genome sequencing revealed an As(III)-oxidizing gene cluster (aioXSRBA) encoding an As(III) oxidase. Functional analyses showed that As(III) oxidation under anoxic phototrophic conditions correlated with transcription of the large subunit of the As(III) oxidase aioA gene. Furthermore, the non-As(III) oxidizer Rhodobacter capsulatus SB1003 heterologously expressing aioBA from strain CZR27 was able to oxidize As(III), indicating that aioBA was responsible for the observed As(III) oxidation in strain CZR27. Our study provides evidence for the presence of anaerobic photosynthesis-coupled As(III) oxidation in paddy soils, highlighting the importance of light-dependent, microbe-mediated arsenic redox changes in paddy arsenic biogeochemistry.

Ecological drivers and potential functions of viral communities in flooded arsenic-contaminated paddy soils

This work revealed the profile of viral communities in paddy soils with different levels of arsenic (As) contamination during the flooded period. The structure of viral communities differed significantly in highly and moderately As-contaminated soils. The diversity of soil viral communities under high As contamination decreased. Siphoviridae, Podoviridae, Myoviridae, and Microviridae were the dominant viral families in all samples, and the relative abundances of five of the top 20 viral genera were significantly different between highly and moderately As-contaminated groups. Seventeen dissimilatory As(V)-reducing bacteria were predicted to host 161 viral operational taxonomic units (vOTUs), mainly affiliated with the genera of Sulfurospirillum, Deferribacter, Bacillus and Fusibacter. Among them, 28 vOTUs were also associated with Fe(III)-reducing bacteria, which belonged to different species of the genus Shewanella. Procrustes analysis showed that the community structure of soil viruses was strongly correlated with both prokaryotic community structure and geochemical properties. Random forest analyses revealed that the Total-Fe, DCB-Fe and oxalate-Fe were the most significant variables on viral community richness, while the total-As concentration was an important factor on the Shannon index. Furthermore, As resistance genes (ArsC, ArsR and ArsD), As methylation genes (arsM) and As transporter genes (Pst and Pit) were identified among the auxiliary metabolic genes (AMGs) of the virome. This work revealed that the viruses might influence microbial adaptation in response to As-induced stress, and provided a perspective on the potential virus-mediated biogeochemical cycling of As.

Anaerobic ammonium oxidation coupled to arsenate reduction, a novel biogeochemical process observed in arsenic-contaminated paddy soil

Anaerobic ammonium oxidation represents an important pathway of N loss, which can be coupled with reduction of nitrite and metal(loid)s (e.g., Fe(III) and Mn(IV)). Similar to Fe(III) and Mn(IV), As(V) is also an active metal(loid) and ammonium oxidation coupled with As(V) reduction is thermodynamically feasible. However, little is known about this potential process. In this study, anaerobic ammonium oxidation coupled with As(V) reduction, designated as Asammox, was observed in cultures inoculated from As-contaminated paddy soil using 15N isotope tracer analysis. Compared with the treatment amended with 15N-urea only, the production of 15N-labeled N2 (i.e., 30N2 and 29N2) was significantly greater in the treatment amended with As(V) and 15N-urea. Furthermore, the abundances of the genes encoding for arsenate reductase (arrA) and hydrazine synthase (hzsB) were significantly higher in the treatment amended with As(V) and 15N-urea than those in the treatment amended with 15N-urea only. In addition, putative Asammox bacteria affiliated with Halomonas, Pelagibacterium, and Chelativorans were identified by DNA-stable isotope probing. Members of Ca. Brocadia were the most dominant Anammox bacteria in the soil cultures and may interact with Asammox bacteria in ammonium oxidation, suggesting that the N loss may be attributed to the contribution of Asammox and Anammox in the As-contaminated soil. The observation of Asammox, a novel biogeochemical process, and identification of bacteria responsible for this biogeochemical process expands the fundamental understanding of both N and As biogeochemical cycling. In addition, this study provides a proof-of-concept for investigating anaerobic ammonium oxidation coupled with metal(loid)s reduction by combining stable isotope probing and isotope tracer microcosm incubations.

Methylation of arsenic differs with substrates in Arcticibacter tournemirensis R1 from an As-contaminated paddy soil

Arsenic can be methylated by arsenite (As(III)) S-adenosylmethionine methyltransferases (ArsMs) among various kingdoms of life. The intermediate product methylarsenite (MAs(III)) is highly toxic and can be utilized as an antibiotic by some microbes. ArsM gene is widely distributed in the members of every kingdom from bacteria to humans and displays a high diversity of sequence. Based on arsenic methylating capacity, ArsM proteins can be divided into two phylogenetically distinct clades (Groups 1 and 2). In this study, we show that Arcticibacter tournemirensis R1 isolated from arsenic contaminated paddy soil is resistant to both As(III) and MAs(III), but exhibits different methylation activities for As(III) and MAs(III). The A. tournemirensis R1 shows low As(III) methylation activity and produces an unknown arsenic compound. In contrast, it shows high methylation activity with MAs(III), with the main product of dimethylarsenate (DMAs(V)). An AtarsM gene is found in ars operon of A. tournemirensis R1 genome and is regulated by an atypical transcriptional repressor ArsR. Expressed in Escherichia coli AtArsM confers resistance to As(III) and MAs(III). Both in vivo and in vitro assays show that AtArsM methylates As(III) and MAs(III) to dimethyl- and trimethyl‑arsenicals. AtArsM has four conserved cysteine residues, which are present in most ArsMs and can be classified into phylogenetic group 2 family, producing trimethylated arsenic metabolites. The high arsenic methylation and volatilization activity of AtArsM provides a potential strategy for arsenic bioremediation. The methylation activity differs with As(III) and MAs(III) in A. tournemirensis R1 indicates that there may have different detoxification mechanisms for As(III) and MAs(III), which are worth investigating in the future.

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.

Remediation of arsenic-contaminated paddy soil by iron oxyhydroxide and iron oxyhydroxide sulfate-modified coal gangue under flooded condition

Flooded condition enhances arsenic (As) mobility in paddy soils, posing an imminent threat to food safety and human health. Hence, iron oxyhydroxide and iron oxyhydroxide sulfate-modified coal gangue (CG-FeOH and CG-FeOS) were synthesized for remediation of As-contaminated paddy soils under a flooded condition. Compared to the control, CG-FeOH and CG-FeOS application decreased the soil pH by 0.10–0.80 and 0.13–1.63 units, respectively. CG-FeOH and CG-FeOS application significantly (P < 0.05) decreased available As concentration by 13.46–43.44% and 21.31–54.37%, respectively. CG-FeOH and CG-FeOS significantly (P < 0.05) reduced the non-specifically adsorbed and specifically adsorbed As fractions and increased As(V) proportion by 22.61–26.53% and 29.10–36.51%, respectively. Our results showed that CG-FeOH and CG-FeOS could change As geochemical fraction and valence state, consequently reducing available As concentration in paddy soils. Moreover, the sulfate could enhance the oxidation and co-precipitation of As with CG-FeOH. Compared to CG-FeOH, CG-FeOS was more effective in decreasing available As concentration and oxidizing As(III) to As(V). This study revealed that CG-FeOS is a potential amendment for As immobilization in paddy soils.

Anaerobic As(III) Oxidation Coupled with Nitrate Reduction and Attenuation of Dissolved Arsenic by Noviherbaspirillum Species

Arsenic pollution is a worldwide problem. In flooded arsenic-contaminated paddy soils, arsenic bioavailability to rice plants is elevated due to the reductive dissolution of arsenate (As(V)) contai...

Remediation of arsenic-contaminated paddy soil: Effects of elemental sulfur and gypsum fertilizer application

Heavy metal(loid) contamination represents an immense challenge in sustainable agriculture. Arsenic, in particular, poses a great risk to the quality of agricultural products (e.g., rice grain). The sulfur amendment is recommended as an effective practice to remediate heavy metal(loid)–polluted soil, given its function in enhancing crop production and alleviating heavy metal(loid) accumulation in the plant. This study aims to investigate the roles of sulfur fertilizer on arsenic accumulation in rice and to explore the key mechanisms. In this study, Elemental sulfur (ES) and gypsum sulfur (GS) were chosen as sulfur fertilizers, with different application rates (0, 0.15, and 0.30 g S kg−1 soil). The results showed that ES and GS treatment significantly increased rice grain yield by 46.6–59.7% and significantly reduced the rice grain arsenic content by more than 39.1%. The sulfur treatment decreased soil pe + pH values. ES treatment increased the availability of arsenic in the bulk soil, whereas GS showed little effect. Sulfur application promoted the formation of iron and manganese plaques, which could suppress the migration of arsenic from soil to rice root. In addition, the sulfur treatment decreased the arsenic that migrating from rice roots to grains by 33.3–66.7%. This study indicates that sulfur application could increase arsenic availability in paddy soil; however, it can inhibit arsenic accumulation in rice grains via increasing the root plaques content and inhibiting the translocation of arsenic from roots to grains.

Isolation, Morphological and Biochemical Characterization of Rhizobacteria from Arsenic Contaminated Paddy Soils in Bangladesh: An In vitro Study

Soil-plant–microbes relations within the plant rhizosphere are the determinants of plant and soil health, which is important for soil ecological environment for plant-microbe interactions. Plant growth-promoting rhizobacteria (PGPR) are considered to encourage plant growth and development directly or indirectly in soil. PGPR can demonstrate a diversity of characteristics responsible .for influencing plant growth and development. During this study, Twenty four different bacterial isolates were isolated, and detailed morphological, biochemical, and physiological characterizations of those isolates were accomplished. This experiment was performed with the 24 bacterial isolates to see their gram stain test, KOH test, catalase activity, cellulose degradation capability, in dole acetic acid (IAA) production, and phosphate solubilization activities, and also tested for growth within the different arsenic and salt stress conditions and 37°C temperature. Results revealed that among the rhizobacterial isolates, fifteen bacterial isolates were negative and nine was positive in gram reaction, while some were showed high IAA production ability, phosphate solubility capability, and cellulose degradation capacity within the culture media. The isolates were isolated from paddy soils and a few were characterized by a yellow color, flat elevation, and gram-positive, while some were characterized because of the yellowish color with round colony shape, raised elevation, gram-negative, and every one the isolates were positive in catalase production capacity and phosphate solubilization activity which is able to increase the available phosphorus within the soil for plants and also produced indole acetic acid that may use as a hormone to be used in growth enhancement of plants. Hence, these isolates need to be tested further for their effect on arsenic dynamics at the plant rhizosphere, selection of suitable plant species for the bacterial association, bacterial effect on arsenic uptake by plants, and potentials for field applications for sustainable agriculture.

Remediation of arsenic-contaminated paddy soil by intercropping aquatic vegetables and rice

Soil contamination by arsenic (As) is an important environmental issue globally. Intercropping of hyperaccumulators with main crop is typically applied for remediation of As-contaminated soil. Most hyperaccumulators are wild plants with small biomass and slow growth rates. Thus, remediation is slow. Here, we propose an effective intercropping system for remediation of As-contaminated paddy soil. Four treatments—intercropping with water spinach (Ipomoea aquatica Forsk) (T1), water celery (Oenanthe javanica (Blume) DC.) (T2), or Guangdong white arrowhead (Sagittaria sagittifolia L. var) (T3), with rice (Oryza sativa L.) monoculture (control, CK)—were used. Compared with the CK, grain yield per plant of rice under T1 and T2 increased by 58.13% and 10.48%, respectively, but decreased by 46.90% in T3. As concentration, bioaccumulation factor, and translocation factor in brown rice were significantly lower in the intercropping treatments than in CK. As removal by water spinach was 7.04 and 1.47 times that by water celery and arrowhead, respectively. The pH of paddy soil was significantly higher in all treatments than in CK, and iron plaque on rice roots under T1 and T2 decreased significantly but increased significantly under T3 compared with that of CK. Rice intercropped with water spinach had the best remediation effect. Novelty Statement: We believe that the following highlights of this manuscript will make it interesting to general readers of this journal. First, in recent years, many articles about intercropping system for the remediation of soil heavy metal pollution focus on dry land, and few studies have focused on paddy soil. The present study was on arsenic-contaminated paddy soil remediation. Second, water spinach, water celery, and arrowhead have great potential for phytoremediation. Studies have shown that these three aquatic vegetables play a role in the removal of certain pollutants, such as heavy metals. Moreover, when intercropped with rice, they can effectively increase rice yield and reduce rice diseases and insect pests. However, studies on remediation of arsenic-contaminated soil by intercropping aquatic vegetables and rice have not been reported. We propose here a rice-aquatic vegetables (water spinach, water celery and arrowhead) intercropping pattern for remediation of arsenic in soil. Third, according to the arsenic concentration and removal rate, we used a bioaccumulation factor, translocation factor, and arsenic removal per unit area of plants for the quantitative evaluation of the remediation effects of the intercropping systems. We found that the intercropping of rice and water spinach could be used to remediate arsenic-contaminated soil. Moreover, the extraction contents of arsenic using intercropping with water spinach was higher than that achieved in a previous study that applied intercropping with the arsenic hyperaccumulator Pteris vittata over the same growth time. This study provides a reference for realizing both remediation and increased production in arsenic-contaminated soil and for promoting sustainable development of agriculture.

Effect of riboflavin on active bacterial communities and arsenic-respiring gene and bacteria in arsenic-contaminated paddy soil

Riboflavin (RF) is a widely distributed micronutrient secreted by many bacteria and can serve as a redox mediator to facilitate extracellular electron transfer. However, studies on the effect of RF on the dynamic shift of active bacterial communities and transcription of arsenate [As(V)]-respiring genes and associated bacteria in arsenic-contaminated paddy fields remain rare. In this study, anaerobic microcosms were established with arsenic-contaminated paddy soil amended with RF and lactate. Arsenic and iron transformation showed that microbial As(V) and Fe(III) reduction took place principally within the first 12 days of incubation, when both occurred simultaneously and were substantially enhanced by RF. Dynamics of the active bacterial community (16S rRNA gene based) revealed that RF mainly increased the relative abundance of Geobacter, Clostridium, and Pelobacteraceae on days 0–12 and that of Dechloromonas on days 12–40, whereas RF had no influence on bacterial diversity. A transcript clone library of arrA gene was constructed to profile the dynamic shift of As(V)-respiring bacteria stimulated by RF, which indicated that As(V)-respiring Geobacter spp. dominated in the RF and lactate microcosms during the incubation period. Quantitative reverse transcription-PCR (RT-qPCR) analyses revealed that the transcription of arrA gene and Geobacter spp. predominantly increased during the first 5 days and was substantially enhanced by RF relative to microcosms with lactate alone. Co-occurrence network analysis indicated a positive correlation between the transcript abundance of arrA gene/Geobacter spp. and dissolved As(V) in the presence of RF and lactate. Our results demonstrate that RF stimulates the activity of indigenous dominant metal-reducing bacteria; moreover, it increases transcription of the As(V)-respiring gene and bacteria to facilitate arsenic reduction and release in arsenic-contaminated paddy soil.

Mitigating arsenic accumulation in rice (Oryza sativa L.) using Fe-Mn-La-impregnated biochar composites in arsenic-contaminated paddy soil.

Arsenic (As) is a prominent metal contaminant of the soil in China. Pot experiments were conducted to examine the effects of corn stem powder biochar (BC) and Fe-Mn-La-impregnated biochar composites (FMLBC1, FMLBC2, and FMLBC3; BC:Fe:Mn:La at different weight ratios) on As accumulation in an indica cultivar of rice (Oryza sativa L.). The application of FMLBCs and BC improved the dry weight of the grains, leaves, stems, and roots of rice. The As uptake in different rice organs was significantly reduced in the FMLBC-amended soils (FMLBC3 > FMLBC2 > FMLBC1) compared with the BC treatment. Compared to the concentration of As in the control, the concentration of As in rice grains decreased by 56.0-89.4% with the addition of 2% FMLBC3. The application of FMLBCs significantly increased the ratio of essential amino acids in grains and the contents of Fe and Mn plaques on root surfaces. The reduction in As accumulation can be ascribed to the Fe, Mn, and La oxides that enhance the adsorption and retention of As, as well as the FMLBCs that provide nutrients and create a rhizosphere environment, promoting rice growth. This study demonstrated that applications of 2% FMLBC2 and FMLBC3 have the potential to remediate As-contaminated soils, reduce As accumulation in rice plants, and improve rice grain quality.

The mechanism of polystyrene microplastics to affect arsenic volatilization in arsenic-contaminated paddy soils

Different concentrations and sizes of polystyrene microplastics (PS-MP) were added to low (25.9) and high (56.8) mg kg−1 As-contaminated soil to investigate the effects of PS-MP on soil As volatilization. Either S1 (10 μm) or S2 (0.1–1 μm) PS-MP was added to As-contaminated soil at 0.8%, which increased As volatilization by 13.7% and 7.4% in low As-contaminated soil; and 21.8% and 16.5% in high As-contaminated soil, respectively. The addition of PS-MP reduced the water-soluble (WS) As content, increased the non-specifically-sorbed (NSS), specifically-sorbed (SS) As content, soil catalase (CAT) and urease (UE) activities. The abundance of Proteobacteria and Firmicutes showed opposite trends to As volatilization, while the abundance of Bacteroidetes and arsM gene expression exhibited similar variability to As volatilization over the 7-week experiment. Therefore, we postulate that in As-contaminated soil, As volatilization was enhanced in the presence of PS-MP due to two possible mechanisms: 1) PS-MP affects the abundance of Proteobacteria, Firmicutes, Bacteroidetes, and arsM gene in soil; 2) PS-MP increases As volatilization via reducing soil nutrient and increasing the content of SS As. Our results highlighted the importance of investigating impacts of microplastics on the volatility of specific contaminants to implement effective environmental remediation strategies in polluted farmlands.

Community dynamics of As(V)-reducing and As(III)-oxidizing genes during a wet–dry cycle in paddy soil amended with organic matter, gypsum, or iron oxide

Microbe-mediated redox transformations regulate arsenic mobility in paddy soil. However, the community dynamics of the related genes, which might be affected by soil ameliorants, have not been systematically investigated during a wet–dry cycle. This study incubated arsenic-contaminated paddy soil amended with organic matter (OM), gypsum, or hematite in microcosms under alternate watering conditions. Added gypsum and hematite reduced arsenic mobility in the soil by 8–60% during the wet and dry periods. However, added OM increased arsenic mobility by 70–130% during the first 4 weeks (not the last 4 weeks) of submergence and the dry period. The results of quantitative real-time polymerase chain reaction (qPCR) depended heavily on the primers used, so the contribution of relevant genes to arsenic transformation cannot be compared using only the gene abundance assessed by qPCR. However, correlation analyses showed that the abundance and community members of the arrA gene, which mediates dissimilatory As(V) reduction [i.e., As(V) respiration], were related to soil arsenic concentrations. This was not the case for the arsC gene, which mediates cytoplasmic As(V) reduction, or the aioA gene, which mediates As(III) oxidation. These suggest that the dissimilatory pathway was mainly responsible for arsenic reduction and release in the soil studied.

Nitrite Accumulation Is Required for Microbial Anaerobic Iron Oxidation, but Not for Arsenite Oxidation, in Two Heterotrophic Denitrifiers.

Phylogenetically diverse species of bacteria can mediate anaerobic oxidation of ferrous iron [Fe(II)] and/or arsenite [As(III)] coupled to nitrate reduction, impacting the biogeochemical cycles of Fe and As. However, the mechanisms for nitrate-dependent anaerobic oxidation of Fe(II) and As(III) remain unclear. In this study, we isolated two bacterial strains from arsenic-contaminated paddy soils, Ensifer sp. ST2 and Paracoccus sp. QY30. Both strains were capable of anaerobic As(III) oxidation, but only QY30 could oxidize Fe(II) under nitrate-reducing conditions. Both strains contain the As(III) oxidase gene aioA, whose expression was induced greatly by As(III) exposure. Both strains contain the whole suite of genes for complete denitrification, but the nitrite reductase gene nirK was not expressed in QY30 and nitrite accumulated under nitrate-reducing conditions. When the functional nirK gene was knocked out in strain ST2, its nitrite reduction ability was completely abolished and nitrite accumulated in the medium. Moreover, the ST2ΔnirK mutant gained the ability to oxidize Fe(II). When nirK gene from ST2 was introduced to QY30, the recombinant strain QY30::nirK gained the ability to reduce nitrite but lost the ability to oxidize Fe(II). These genetic manipulations did not affect the ability of both strains to oxidize As(III). Our results indicate that nitrite accumulation is required for anaerobic oxidation of Fe(II) but not for As(III) oxidation in these strains. The results suggest that anaerobic Fe(II) oxidation in the two bacterial strains is primarily driven by abiotic reaction of Fe(II) with nitrite, while reduction of nitrate to nitrite is sufficient for redox coupling with anaerobic As(III) oxidation catalyzed by Aio. Deletion of nirK gene in denitrifiers can enhance anaerobic Fe(II) oxidation.

Effect of Fe–Mn–Ce modified biochar composite on microbial diversity and properties of arsenic-contaminated paddy soils

In this study, we investigated the mechanism of decrease in arsenic (As) bioavailability after addition of biochar (BC) supplemented with iron (Fe)- manganese (Mn)- cerium (Ce) oxide (FMCBC) to As-contaminated paddy soil. We explored the effects of these composites on the oxidation, reduction, microbial community, and soil enzyme activity of As-contaminated paddy soil. Results showed that FMCBCs improve soil pH, significantly improve the redox capacity of soil, and reduce bioavailable forms of As. FMCBCs can convert As from a specifically or non-specifically bound form into amorphous hydrous oxide bound- and crystalline hydrous oxide bound form. The application of FMCBCs increased soil enzyme activity (urease, catalase, alkaline phosphatase, and peroxidase), and greatly influenced the relative abundance of certain microorganisms (Proteobacteria, Acidobacteria, and Gemmatimonadetes), which improved soil enzyme heavy metal tolerance and prevented their denaturation. Thus, FMCBCs can not only change the form and distribution of As in soil but also create an environment suitable for microbial growth, consequently affecting the geochemical cycling of As in soil.

Spraying silicon to decrease inorganic arsenic accumulation in rice grain from arsenic-contaminated paddy soil

Addition of Si to soil can reduce As uptake and accumulation in rice, while also enhancing As release from soil particles via competing sorption sites in soil minerals with As. Foliar application of Si might be an alternative pathway to reduce As accumulation in rice. It is not clear which growing stage would be better for spraying different types of Si solution to reduce inorganic As in edible parts of rice. Soil pot experiments were conducted to investigate whether total As and inorganic As accumulation in rice grains was alleviated via spraying Si in As-contaminated paddy soil. The results showed that foliar Si application at the tillering or jointing stage significantly reduced As concentrations in rice husks and grain via inhibiting the translocation of As from straw to husk or grain compared with other growing stages. Spraying of Si at the tillering stage markedly decreased the concentrations of inorganic As in rice bran and polished rice, accounting for 27.3% and 61.4% respectively. Furthermore, spraying Si solution mixed with surfactant-Tween 80 not only dramatically reduced the total As in rice tissues by 48.8%, but also significantly alleviated the accumulation of inorganic As in rice grain by 49.2%. Spraying Si restricted inorganic As in rice grain through the mechanism by which foliar Si application at tillering stage increased the Si concentrations in shoot and root, which downregulated Si transporters of Lsi1 and Lsi2 in the root and Lsi6 in the blade and sheath significantly, and finally decreased As uptake and transport. Therefore, spraying Si is an alternative and efficient pathway to reduce inorganic As accumulation of rice grain in As-contaminated soil.

Tracing the Dynamic Changes of Element Profiles by Novel Soil Porewater Samplers with Ultralow Disturbance to Soil-Water Interface.

In flooded soils, soil–water interface (SWI) is the key zone controlling biogeochemical dynamics. Chemical species and concentrations vary greatly at micro- to cm-scales. Techniques able to track these changing element profiles both in space and over time with appropriate resolution are rare. Here, we report a patent-pending technique, the Integrated Porewater Injection (IPI) sampler, which is designed for soil porewater sampling with minimum disturbance to saturated soil environment. IPI sampler employs a single hollow fiber membrane tube to passively sample porewater surrounding the tube. When working, it can be integrated into the sample introduction system, thus the sample preparation procedure is dramatically simplified. In this study, IPI samplers were coupled to ICP-MS at data-only mode. The limits of detection of IPI-ICP-MS for Ni, As, Cd, Sb, and Pb were 0.12, 0.67, 0.027, 0.029, and 0.074 μg·L–1, respectively. Furthermore, 25 IPI samplers were assembled into an SWI profiler using 3D printing in a one-dimensional array. The SWI profiler is able to analyze element profiles at high spatial resolution (∼2 mm) every ≥24 h. When deployed in arsenic-contaminated paddy soils, it depicted the distributions and dynamics of multiple elements at anoxic–oxic transition. The results show that the SWI profiler is a powerful and robust technique in monitoring dynamics of element profile in soil porewater at high spatial resolution. The method will greatly facilitate studies of elements behaviors in sediments of wetland, rivers, lakes, and oceans.

Humic Substances Facilitate Arsenic Reduction and Release in Flooded Paddy Soil.

Organic matter is important for controlling arsenic reduction and release under anoxic conditions. Humic substances (HS) represent an important fraction of natural organic matter, yet the manner in which HS affect arsenic transformation in flooded paddy soil has not been thoroughly elucidated. In this study, anaerobic microcosms were established with arsenic-contaminated paddy soil and amended with three extracted humic fractions (fulvic acid, FA; humic acid, HA; and humin, HM). The HS substantially enhanced the extent of arsenic reduction and release in the order FA > HA > HM. It was confirmed that microbially reduced HS acted as an electron shuttle to promote arsenate reduction. HS, particularly FA, provided labile carbon to stimulate microbial activity and increase the relative abundances of Azoarcus, Anaeromyxobacter, and Pseudomonas, all of which may be involved in the reduction of HS, Fe(III), and arsenate. HS also increased the abundance of transcripts for an arsenate-respiring gene ( arrA) and overall transcription in arsenate-respiring Geobacter spp. The increase in both abundances lagged behind the increases in dissolved arsenate levels. These results help to elucidate the pathways of arsenic reduction and release in the presence of HS in flooded paddy soil.