Polluted Paddy Soils Research Articles

Fe3O4/Mulberry Stem Biochar as a Potential Amendment for Highly Arsenic-Contaminated Paddy Soil Remediation

Magnetite-loaded biochar has recently received attention owing to its ability to remove arsenic from contaminated soil. In this study, mulberry stem biochar (MBC) and Fe3O4-loaded mulberry stem biochar (Fe3O4@MBC) were produced and used in a 100-day incubation experiment to investigate their performance in the stabilization of arsenic in paddy soil severely polluted by the As (237.68 mg·kg−1) mechanism. Incubation experiments showed that Fe3O4@MBC was more effective in immobilizing As after incubation for 100 days. Moreover, adding Fe3O4@MBC facilitated the transformation of exchangeable heavy metals into organic-bound and residual forms, thereby reducing As available concentrations, mobility, and bioavailability in the soil, and elevating slightly the soil pH and dissolved organic carbon (DOC). The concentration of TCLP-extractable As (AsTCLP) in contaminated soil was reduced from 93.85 to 7.64 μg·L−1 within 10 d, below the safety limit for drinking water set by the World Health Organization (WHO). The characterization results of Fe3O4@MBC after incubation indicated that the mechanisms for As passivation are linked to redox reactions, complexation, electrostatic attraction, surface adsorption, and coprecipitation. Conclusively, Fe3O4@MBC is a promising amendment in highly As-contaminated soil and provides a theoretical reference in such polluted paddy soil remediation.

Efficient removal of lead from polluted paddy soil by one-pot synthesized Nano-Fe3O4 incorporated stable lignin hydrogel

Lead (Pb) contamination in agricultural soils poses a significant threat to both ecosystems and human health. While nano-Fe3O4 exhibits promising potential for Pb remediation, its practical application in the soil is hindered by its’ biotoxicity, easy aggregation, and the risk of secondary pollution. Thus, this study presents a novel approach wherein Fe3O4 was incorporated into hydrogel via a one-pot synthetic strategy (Fe3O4@LH). This incorporation enhanced the mechanical properties and environmental stability of the hydrogel composites. Based on the mechanical properties, environmental stability, and single-point adsorption results for Pb, we selected Fe3O4@LH-4 for further research. The removal mechanism and the feasibility of employing Fe3O4@LH-4 for Pb removal from paddy soil were investigated through batch adsorption experiments and soil culture studies. Results showed that the adsorption process was primarily governed by swelling adsorption, electrostatic adsorption, ion exchange, precipitation, nanometer effect, and complexation mechanisms. The application of Fe3O4@LH-4 significantly led to the reduction of 16.7 %–25.4 % in soil Pb content, with removal rates escalating alongside increased dosage and application periods of Fe3O4@LH-4. Fitting results of the prediction model indicated that the Pb content in mildly Pb-contaminated soil (186.55 mg/kg) would decrease to be below the risk control standard for soil contamination of agricultural land in China (140 mg/kg) after 112 days of continuous application. Concurrently, cadmium and arsenic contents in the soil decreased by 5.2 %–10.8 % and 7.1 %–16.7 %, respectively. Moreover, the application of Fe3O4@LH-4 positively influenced soil nutrient levels, with total nitrogen and soil organic matter content significant increments of 13.6 %–41.0 % and 4.6 %–16.1 %, respectively. Furthermore, Fe3O4@LH-4 recovery exceeded 88.3 % after a 90-day application period. These findings underscore the potential of Fe3O4 incorporated hydrogel as a promising agent for the sustained removal of heavy metal Pb from paddy soil.

Alteration of soil pH induced by submerging/drainage and application of peanut straw biochar and its impact on Cd(II) availability in an acidic soil to indica-japonica rice varieties

The effects of soil pH variations induced by submergence/drainage and biochar application on soil cadmium (Cd) availability to different rice (Oryza sativa L.) varieties are not well understood. This study aims to investigate the possible reasons for available Cd(II) reduction in paddy soil as influenced by biochar and to determine Cd(II) absorption and translocation rates in different parts of various rice varieties. A pot experiment in a greenhouse using four japonica and four indica rice varieties was conducted in Cd(II) contaminated paddy soil with peanut straw biochar. The results indicated that the submerging led to an increase in soil pH due to the consumption of protons (H+) by the reduction reactions of iron/manganese (Fe/Mn) oxides and sulfate (SO42−) and thus the decrease in soil available Cd(II) contents. However, the drainage decreased soil pH due to the release of protons during the oxidation of Fe2+, Mn2+, and S2− and thus the increase in soil available Cd(II) contents. Application of the biochar increased soil pH during soil submerging and inhibited the decline in soil pH during soil drainage, and thus decreased soil available Cd(II) contents under both submerging and drainage conditions. The indica rice varieties absorbed more Cd(II) in their roots and accumulated higher amounts of Cd(II) in their shoots and grains than the japonica rice varieties. The Cd(II) sensitive varieties exhibited a greater absorption and translocation rate of Cd(II) compared to the tolerant varieties of both indica and japonica rice. Biochar inhibited the absorption and accumulation of Cd(II) in the rice varieties, which ultimately lowered the Cd(II) contents in rice grains below the national food safety limit (0.2 mg kg−1). Overall, planting japonica rice varieties in Cd(II) polluted paddy soils combined with the use of biochar can effectively reduce Cd(II) content in rice grains which protects human health against Cd(II) toxicity.

Effect of Cd(II) adsorption onto rice roots on its uptake by different indica and japonica rice varieties and toxicity effect of Cd(II) under acidic conditions.

The rapid increase in soil acidity coupled with the deleterious effects of cadmium (Cd) toxicity had led to a decline in worldwide agricultural production. Rice absorbs and accumulates Cd(II) from polluted paddy soils, increasing human health risks throughout the food chain. A 35-day hydroponic experiment with four japonica and four indica (two each of them tolerant and sensitive cultivars) was conducted in this study to investigate the adsorption and absorption of Cd(II) by rice roots as related with surface chemical properties of the roots. The results showed that the three chemical forms of exchangeable, complexed, and precipitated Cd(II) increased with the increase in Cd(II) concentration for all rice cultivars. The roots of indica rice cultivars carried more negative charges and had greater functional groups and thus adsorbed more exchangeable and complexed Cd(II) than those of japonica rice cultivars. This led to more absorption of Cd(II) by the roots and greater toxicity of Cd(II) to the roots of indica rice cultivars and more inhibition of Cd(II) stress on the growth of the roots and whole plants of indica rice cultivars compared with japonica rice cultivars, which was one of the main reasons for more declines in the biomass and length of indica rice roots and shoots than japonica rice cultivars. Cd(II) stress showed more toxicity to the sensitive rice cultivars and thus greater inhibition on the growth of the cultivars due to more exchangeable and complexed Cd(II) adsorbed by their roots induced by more negative charges and functional groups on the roots compared with tolerant rice cultivar for both indica and japonica, which resulted in greater decreases in the biomass and length of roots and shoots as well as chlorophyll contents of the sensitive cultivars than the tolerant cultivars. The roots of sensitive rice cultivars also absorbed more Cd(II) than tolerant rice cultivars due to the same reasons as above. These findings will provide useful references for the safe utilization and health risk prevention of Cd-contaminated paddy fields.

The Immobilization Mechanism of Inorganic Amendments on Cu and Cd in Polluted Paddy Soil in Short/Long Term.

This study investigated the impact of soil colloidal characteristics on the transfer patterns of different Cu and Cd speciation in contaminated soil treated with three different amendments: lime (L), zero-valent iron (ZVI), and attapulgite (ATP). It seeks to clarify the activation hazards and aging processes of these modifications on Cu and Cd. Compared with the control (CK), the available Cu concentrations treated with amendments reduced in the short term (6 months) by 96.49%, 5.54%, and 89.78%, respectively, and Cd declined by 55.43%, 32.31%, and 93.80%, respectively. Over a 12-year period, there was no significant change in the immobile effect with L, while Cu and Cd fell by 19.06% and 40.65% with ZVI and by 7.63% and 40.78% with ATP. Short- and long-term increases in the readily reducible iron and manganese oxide fraction of Cu and Cd were accompanied by a considerable rise in the concentrations of amorphous iron oxide in the soil and colloid after amendment treatment. This suggested that Cu and Cd were immobilized and stabilized in part by amorphous iron oxide.

Nano zero-valent iron enhances the absorption and transport of chromium in rice (Oryza sativa L.): Implication for Cr risks management in paddy fields

Chromium (Cr) accumulating in soil caused serious pollution to cultivated land. At present, nano zero-valent iron (nZVI) is considered to be a promising remediation material for Cr-contaminated soil. However, the nZVI impact on the behavior of Cr in the soil-rice system under high natural geological background value remains unknown. We studied the effects of nZVI on the migration and transformation of Cr in paddy soil-rice by pot experiment. Three different doses of nZVI (0, 0.001 % and 0.1 % (w/w)) treatments and one dose of 0.1 % (w/w) nZVI treatment without plant rice were set up. Under continuous flooding conditions, nZVI significantly increased rice biomass compared with the control. At the same time, nZVI significantly promoted the reduction of Fe in the soil, increased the concentration of oxalate Fe and bioavailable Cr, then facilitated the absorption of Cr in rice roots and the transportation to the aboveground part. In addition, the enrichment of Fe(III)-reducing bacteria and sulfate-reducing bacteria in soil provided electron donors for Cr oxidation, which helps to form bioavailable Cr that is easily absorbed by plants. The results of this study can provide scientific basis and technical support for the remediation of Cr -polluted paddy soil with high geological background.

Organic fertilization integrated with water management to remediate As and Cd contamination in a paddy soil

Soil heavy metal pollution is the main risk for sustainable agriculture, especially the combination of As and Cd pollution in paddy fields which may lead to the superimposed accumulation in rice. There is an urgent need for environmental-friendly and cost-effective strategies to remediate the contamination of As and Cd in soils. In this work, a pot culture experiment was conducted in a As and Cd polluted paddy soil to explore the effects of organic fertilization (OF) and two water managements (continuous flooding, CF; intermittent irrigation, II) on the fractionation of As and Cd in soil, and the uptake of As and Cd by rice. The results showed that OF integrated with intermittent irrigation performed best in reducing the contents of As and Cd in rice grains by 58.9 % and 69.3 %, respectively, under compound pollution. The significant conversion of available As and Cd to stable species (specifically adsorbed and Fe-Mn/Al oxide bound) under OF + II were supported by the changes in an array of soil attributes such as pH, Eh, soluble Fe and dissolved organic carbon (DOC). Intermittent irrigation was more conducive to the accumulation of As outside the roots, and Fe-plaque prevented As uptake by roots and the translocation to shoots. While more accumulation of Fe-plaque along with Cd on root surface induced by continuous flooding is helpful for depressed assimilation of Cd by rice. Considering the combined contamination of As and Cd polluted in paddy soils, a management approach was proposed based on intermittent irrigation and application of organic fertilizer at the rate of 0.1 % (∼ 2.3 t/ha) in two phases (two weeks before planting or drainage). Organic fertilization will hold great promise in restoring polluted soils and maintaining soil health via suppressing the lability of heavy metals and providing nutrients.

Remediation of cadmium-polluted weakly alkaline dryland soils using iron and manganese oxides for immobilized wheat uptake

Cadmium (Cd) contamination of arable soils seriously threatens the safety of food products and sustainable development of agriculture. Iron/manganese oxide-based amendments have been widely applied to remediate heavy metal polluted paddy soils. However, little is known about the performance and mechanism of single iron/manganese oxides and iron-manganese composite oxides in reducing Cd accumulation in wheat plants from dryland soils. Here, iron oxide (α-FeOOH), manganese oxide (δ-MnO2) and iron-manganese composite oxides (birnessite and weakly crystalline iron oxide) were prepared and used the addition of 0.1%, 0.5% and 1.0% to decrease the Cd accumulation in wheat plants cultivated in weakly alkaline soil through pot experiments. The results showed that iron and manganese oxides could reduce the content of available Cd by transforming acid-extractable Cd to reducible Cd and improve the antagonistic effect of wheat rhizosphere Cd with Fe and Mn in weakly alkaline soils, thereby significantly reducing Cd accumulation in wheat plants. The amendment performance followed the order of manganese oxide > iron-manganese composite oxides > iron oxide. The application of 1.0% manganese oxide decreased the available Cd in soils from 0.41 to 0.21 mg kg−1, and correspondingly decreased Cd content in wheat roots, straw and grains by 56.2%, 49.9% and 70.0%, respectively. More importantly, the Cd content in grains was decreased to below the limit set by the China's food safety standard (0.10 mg kg−1). The present work reveals the effect of Cd accumulation by iron and manganese oxides in wheat, and develops an efficient approach for amending Cd polluted weakly alkaline soils.

A field study of nano-FeS loaded lignin hydrogel application for Cd reduction, nutrient enhancement, and microbiological shift in a polluted paddy soil

Cadmium (Cd) pollution in paddy soil has caused serious harm to human health. Nano-ferrous sulfide@lignin hydrogel (FeS@LH) composites could be an ideal material for paddy soil Cd removal due to the excellent adsorption performance and mechanical strength. But the FeS@LH’s performance in a paddy field remains unclear. In this study, FeS@LH was applied to a Cd-contaminated paddy field when planting water spinach (Ipomoea aquatica Forssk). The results showed that FeS@LH effectively removed Cd from the soil (37.6 %) and water spinach (34.5 %) after 30 days. The concentrations of Pb, Zn, Cu, and other metals in soil and water spinach were co-reduced. When the FeS@LH dosage was 30 g·m−2, the effective functional distance was within 30 cm. FeS@LH application had a positive effect on soil nutrients. The soil nitrogen and organic matter contents increased by 16.1 % and 13.8 %, respectively. No significant effect on soil total phosphorus and potassium contents was presented. The survival rate of zebrafish in the acute toxicity test of FeS@LH was 98.9 %, indicating its biosafety. In addition, FeS@LH application decreased soil microbial community diversity probably due to the decreased pH. The microbial metabolism tended to enhance, suggesting a restoration of soil microbial community function after Cd removal.

Bioavailability and speciation of Cadmium in contaminated paddy soil as alleviated by biochar from co-pyrolysis of peanut shells and maize straw

BackgroundBiochar is an important material for remediation of Cd in contaminated paddy soils. However, different biochars have variable effects on bioavailability of Cd while single biochar cannot properly amend immobilized Cd. Co-production of biochar from peanut shells and maize straw at different mass mixing ratios (1:0, 1:1, 1:2, 1:3). The characteristics, properties and effects of co-pyrolysis biochars on amendments of Cd polluted paddy soil was evaluated.ResultsOur research revealed that yield, ash, elemental contents and specific surface area of co-pyrolysis biochars have variable amendment effects compared with single biochar. The co-pyrolysis biochars have produced rich oxygen-containing functional groups and crystal structure, especially 1P3M (co-pyrolysis biochar produced from peanut shell and maize straw in mass ratios of 1:3). The addition of biochar has significantly enhanced pH and EC value, however, content of available Cd during incubation was significantly reduced compared with control treatment. The efficiency of biochars have reduced available Cd in order of 1P3M > M > 1P1M > 1P2M > 2P1M > 3P1M > P after incubation. The 1P3M was most effective in reducing CaCl2-extractable Cd concentration up to 43.97%. The BCR sequential extraction method has produced lowest exchangeable fraction Cd content and highest residual fraction Cd content in 1P3M among all biochar amended treatments.ConclusionsIt is concluded that 1P3M has a much greater potential to decreased the bioavailability of Cd in contaminated paddy soil. And 1P3M was highly effective for transporting Cd from soluble form to less toxic stable forms in polluted paddy soils.Graphical

Elucidating the impact of goethite-modified biochar on arsenic mobility, bioaccumulation in paddy rice (Oryza sativa L.) along with soil enzyme activities

Contamination of paddy soils with arsenic (As) poses imminent threat to the environment and public health. This research work explored the effect of goethite-modified biochar (GMBC) on As immobilization in paddy soil and subsequent accumulation in rice grains. The results showed that the soil supplementation with GMBC significantly improved the biomass of rice plants. In addition, the GMBC application effectively decreased the As content in rice grains (0.72–0.16 mg kg−1). Compared with the control, GMBC 1.5% treatment augmented the iron plaque (Fe-plaque) buildup on rice roots and efficiently sequestered the As by 174%, and reduced its uptake in rice tissues. Soil supplementation with GMBC 1.5% greatly enhanced the activities of soil peroxidase (POD) and catalase (CAT) by 90% and 40%, respectively, compared to the control. Moreover, GMBC amendments improved the relative abundance of the soil bacterial communities and minimized the As mobility in the soil. GMBC 1.5% significantly enhanced the abundance of acidobacteria and Firmicutes by 211% and 95% while that of Chloroflexi decreased by 25%, respectively. The findings of the present investigation demonstrated that GMBC could be used as an environment-friendly approach to remediate As polluted paddy soils and minimize its accumulation in rice grains for mitigation of food security risks and protect public health.

Penicillium oxalicum SL2 as a sustainable option to mitigate the accumulation of Pb in rice (Oryza sativa L.)

Heavy metal contamination in agricultural soil and its associated risk of food safety are of great concern globally. It is therefore an urgent need to develop sustainable option to mitigate the accumulation of metals in crop plants. Here we investigated the potential of phosphorus-solubilizing fungus, Penicillium oxalicum SL2, on regulating the bioavailability of Pb in a lead (Pb) polluted soil-rice system. Our results showed that the content of Pb in rice grain was significantly decreased by ~80% with the application of P. oxalicum SL2. The competition between oxalate and phosphate for the complexation of Pb showed to be effective in mediating the bioavailability of Pb, and such impact varied with water fluctuation in paddy soil. The solubilization of phosphorus also played an important role in alleviating the dissolution of iron plaque caused by oxalic acid, which helped maintaining the biomass of iron plaque as a barrier to the uptake of Pb by root. The predominant indigenous microbial community was not affected by the inoculation with P. oxalicum SL2, suggesting it as an eco-friendly strain. Therefore, we suggest P. oxalicum SL2 as a promising fungus in enhancing the safe use of moderately Pb polluted paddy soil for safe rice.

Combined amendment improves soil health and Brown rice quality in paddy soils moderately and highly Co-contaminated with Cd and As

In situ remediation technology applied aims to not only decrease cadmium (Cd) and arsenic (As) uptake by rice but also improve soil health and rice quality in contaminated paddy soils. Here the effects of a combined amendment, consisting of limestone, iron powder, silicon fertilizer, and calcium-magnesium-phosphate fertilizer, with three application rates (0, 450, and 900 g m−2) on soil health, rice root system, and brown rice quality were compared in moderately versus highly Cd and As co-contaminated paddy fields. After the amendment application, soil pH, cation exchange capacity, four kinds of soil enzyme activities increased (sucrase, urease, acid phosphatase, and catalase), and concentrations of leached Cd/As decreased, as measured by the DTPA (diethylene triamine pentaacetic acid) and TCLP (toxicity characteristic leaching procedure). Changes in the above soil indicators promoted soil health. In both fields, the dithionite-citrate-bicarbonate (DCB)-Fe and DCB-Mn concentration in iron plaque increased and root length became longer. Changes in the above root system indicators reduced the root system's absorption of Cd and As but increased that of nutrients. Under 900 g m−2 treatment, the Cd concentration in brown rice of two sites decreased by 55.8% and 28.9%, likewise inorganic As (iAs) decreased by 50.0% and 21.1%, whereas essential amino acids increased by 20.4% and 20.0%, respectively. Furthermore, the Cd and iAs concentrations in brown rice were <0.2 mg kg−1 (maximum contaminant level of Cd and iAs in the Chinese National Food Safety Standards GB2762-2017 for brown rice) under the 900 g m−2 in the moderately contaminated field. These results suggest the combined amendment can improve soil health and brown rice quality in the moderately and highly Cd- and As-co-contaminated paddy soils, offering potential eco-friendly and efficient remediation material for applications in such polluted paddy soils.

Goethite modified biochar simultaneously mitigates the arsenic and cadmium accumulation in paddy rice (Oryza sativa) L

Cadmium (Cd) and arsenic (As) contamination of paddy soils is a serious global issue because of the opposite geochemical behavior of Cd and As in paddy soils. Rice plant (Oryza sativa L.) cultivation in Cd- and As- contaminated paddy soil is regarded as one of the main dietary cause of Cd and As entry in human beings. This study aimed to determine the impact of goethite-modified biochar (GB) on bioavailability of both Cd and As in Cd- and As- polluted paddy soil. Contrary to control and biochar (BC) amendments, the application of GB amendments significantly impeded the accumulation of both Cd and As in rice plants. The results confirmed an obvious reduction in Cd and As content of rice grains by 85% and 77%, respectively after soil supplementation with GB 2% amendment. BC 3% application minimized the Cd uptake by 59% in the rice grains as compared to the control but exhibited a little impact on As accumulation in rice grains. Sequential extraction results displayed an increase in immobile Cd and As fractions of the soil by decreasing the bioavailable fractions of both elements after GB treatments. Fe-plaque formation on the root surfaces was significantly variable (P ˂ 0.05) among all the amendments. GB 2% treatment significantly increased the Fe content (10 g kg−1) of root Fe-plaque by 48%, which ultimately enhanced the sequestration of Cd and As by Fe-plaque and minimized the transport of Cd and As in rice plants. Moreover, GB treatments significantly changed the relative abundance of the microbial community in the rice rhizosphere and minimized the metal(loid)s mobility in the soil. The relative abundance of Acidobacteria, Firmicutes and Verrucomicrobia increased with GB 2% treatment while those of Bacteroidetes and Choloroflexi decreased. Our findings confirmed improvement in the rice grains quality regarding enhanced amino acid contents with GB application. Overall, the results of this study demonstrated that GB amendment simultaneously alleviated the Cd and As concentrations in edible parts of rice plant and provided a new valuable method to protect the public health by effectively remediating the co-occurrence of Cd and As in paddy soils.

Biochar-based fertilizer enhanced Cd immobilization and soil quality in soil-rice system

Cadmium (Cd) contamination poses a serious problem in paddy soils because of its high health risk through soil-food chain transfer. To evaluate the effect of biochar-based fertilizer on Cd uptake, soil and rice quality, biochar amendment at rates from 0 to 15 t/hm2 was conducted in Cd polluted paddy soils. For successive two rice seasons, biochar treatments greatly reduced rice Cd and soil bioavailable Cd content. Furthermore, the concentration of bioavailable Cd decreased accordingly with the increase of biochar content. When soil properties, such as pH and soil organic carbon (SOC) were significantly improved, grow indexes of rice, especially for the late rice, would be partly improved. Most of the bioavailable Cd was immobilized in root, only a small partition of Cd was transferred to brown rice. The main stage for Cd accumulation was from heading stage to harvest stage.

Microplastics influence on Hg methylation in diverse paddy soils

Microplastics are widespread in estuarine, coastal, and deep sea sediments. The influence of microplastics on mercury (Hg) methylation in paddy soils with different characteristics, however, has not been well reported. In this research, we conducted a microcosmic experiment using red soil and alkaline soil with 2%, 7% and 10% polyvinyl chloride microplastics (PVC-MPs). Diffusive gradients in thin film (DGT) were used to test bioavailable Hg2+ and bioavailable methylmercury (MeHg) in soils. Results showed that PVC-MPs could decrease bioavailable MeHg concentrations both in red soil and alkaline soil. We demonstrated that these decreases could be due to three possible mechanisms: (1) PVC-MPs affected DOM composition, which resulted in a difference in combining capacity for bioavailable Hg2+; (2) PVC-MPs decreased MeHg via changing soil properties (including sulfate and dissolved Fe); (3) PVC-MPs affected the abundance of Proteobacteria, Firmicutes, and hgcA gene in soils. Our results emphasized the significance of investigating effects of microplastics on specific contaminants to implement effective environmental remediation strategies in polluted paddy soils.

Immobilization Mechanism of Four Types of Amendments on Cu and Cd in Polluted Paddy Soil

Cu and Cd are common pollutants in the soil surrounding copper smelting enterprises. The regional characteristics of southern China results in a high risk of Cu and Cd reactivation in soil after immobilization with soil amendment. To clarify the degree of risk of secondary activation of Cu and Cd, four types of amendments, namely limestone (LS), maifanite (MF), biochar (BC), and iron modified biochar (Fe-BC), were used to study Cu and Cd fraction distribution in soil and soil colloids and the type and fractional changes of in-situ iron oxides. The results showed that the soil amendments were ranked by their immobilizing effect on soil Cu and Cd in the order LS, MF, Fe-BC, and BC; the exchangeable and carbonate-bound fractions of Cu in the soil reduced by 8.19% and 2.33%, and the readily reducible iron- and manganese-bound fractions of Cu increased by 8.00% and 2.69%, respectively, when treated with LS and MF. The risk of secondary activation of heavy metals was higher in soil treated with LS and MF than in soil treated with other amendments. The readily reducible iron- and manganese-bound fractions of Cu reduced by 2.21% and 5.90% and the organic-bound fractions of Cu increased by 4.75% and 3.48% when treated with BC and Fe-BC, respectively. This indicated that the immobilization effect tends to be stable. The exchangeable and carbonate-bound fractions of Cd in the soil decreased by 7.64%, 8.34%, 2.37%, and 6.73%, and the residual fractions increased by 8.27%, 9.18%, 5.73%, and 9.60% respectively, indicating that the amendment treatments resulted in better stability of Cd than Cu. The Cu and Cd contents of soil colloids were 489.92 mg ·kg-1 and 2.57 mg ·kg-1, respectively, which were considerably higher than those in soil (239.98 mg ·kg-1 and 1.93 mg ·kg-1, respectively). The amorphous iron oxide-bound Cu and Cd contents of soil colloids increased significantly with the application of each of the four amendment, which was the main reason and mechanism for the decrease in heavy metal bioavailability. With the extension of aging time, long-term immobilization can be achieved if the heavy metals are further transformed into crystalline iron oxide-bound and residual fractions.

Comparative physiological and transcriptomic analyses illuminate common mechanisms by which silicon alleviates cadmium and arsenic toxicity in rice seedlings

The inessential heavy metal/loids cadmium (Cd) and arsenic (As), which often co-occur in polluted paddy soils, are toxic to rice. Silicon (Si) treatment is known to reduce Cd and As toxicity in rice plants. To better understand the shared mechanisms by which Si alleviates Cd and As stress, rice seedlings were hydroponically exposed to Cd or As, then treated with Si. The addition of Si significantly ameliorated the inhibitory effects of Cd and As on rice seedling growth. Si supplementation decreased Cd and As translocation from roots to shoots, and significantly reduced Cd- and As-induced reactive oxygen species generation in rice seedlings. Transcriptomics analyses were conducted to elucidate molecular mechanisms underlying the Si-mediated response to Cd or As stress in rice. The expression patterns of the differentially expressed genes in Cd- or As-stressed rice roots with and without Si application were compared. The transcriptomes of the Cd- and As-stressed rice roots were similarly and profoundly reshaped by Si application, suggesting that Si may play a fundamental, active role in plant defense against heavy metal/loid stresses by modulating whole genome expression. We also identified two novel genes, Os01g0524500 and Os06g0514800, encoding a myeloblastosis (MYB) transcription factor and a thionin, respectively, which may be candidate targets for Si to alleviate Cd and As stress in rice, as well as for the generation of Cd- and/or As-resistant plants. This study provides valuable resources for further clarification of the shared molecular mechanisms underlying the Si-mediated alleviation of Cd and As toxicity in rice.

Effect of Water Regimes on Pb and Cd Immobilization by Biochar in Contaminated Paddy Soil

An incubation experiment was conducted to explore the influence of 30% water holding capacity (WHC), flooding, and alternate dry-wet conditions on changes in heavy metal fractions with 1% rice straw biochar in Pb and Cd co-contaminated paddy soils, to provide a scientific basis for a water regime of biochar remediation on heavy metal contaminated paddy soil. Results showed that flooding and alternating wet-dry conditions could significantly increase soil pH, the contents of dissolved organic carbon (DOC), and amorphous iron oxide (Feo) after adding biochar. Compared with a 30% WHC treatment, when the soil is flooded and alternating wet-dry conditions, the content of the TCLP extractable Pb decreased by 31.87% and 20.33%, respectively, and the content of the TCLP extraction Cd decreased by 25.29% and 16.07%, respectively. Under flooding, the acid soluble Pb and Cd content decreased by 24.78% and 20.14%, respectively, and the acid soluble Cd content decreased over time. The decreasing order of available Pb and Cd content was flooding > alternating dry-wet > 30% WHC. Correlation analysis results showed that soil pH and Feo have significant negative correlation with available heavy metals, which means flooding with biochar could effectively immobilize Pb and Cd by increasing soil pH and Feo content. Flooding and biochar have a synergistic interaction on promoting the transformation of Pb and Cd to more stable fractions in acidic co-contaminated heavy metal polluted paddy soil.

Effectiveness of rice husk biochar in controlling heavy metals at polluted paddy soil

Polluted irrigation can degrade soil chemical properties and then decrease land productivity. This research aimed to compare the effectiveness of biochar against manure in reducing the concentration of some heavy metals(HM) in the polluted rice field. This pot trial was conducted at Glasshouse using two types of amelioration (biochar and manure). Biochar and manure were incubated with paddy soil under field capacity at the glasshouse for one month. The results showed that increasing the dosage of either biochar or manure applied decreased the availability of HM, especially Fe (r=-0.87, r=-0.89) and Mn (r=-0.82, r=-0.91), respectively. However, there was no effect of biochar (r=0.25) but increased by manure (r=0.95) application on Zn concentration. Furthermore, some other heavy metals concentration also decreased, such as Ag (r=-0.89), Pb (r=-0.54), Cu (r=-0.51), Cr (=- 0.50), and Ni (r=-0.64) by applying biochar but tended to increase by applying manure. Both biochar and manure could increase some plant nutrients, especially K (r=0.74, r=0.82), P (r=0.85, r=-0.44), Mg (r=0.98, r=0.91) and Ca (r=0.98, r=0.84), respectively.