Cd Speciation In Soil Research Articles

Assessment of the Combination of Aspergillus niger and Fluorapatite for Cadmium Immobilization in Soil and Its Effect on Soil Biological Properties

ABSTRACT This study evaluated the effects of combining Aspergillus niger and fluorapatite (FAp) as a remediation strategy on cadmium (Cd) immobilization in contaminated soil. Soil Cd speciation, immobilization efficiency, soil microbial abundance, and soil enzyme activities were used as parameters to evaluate the effectiveness of the amendment. European community Bureau of reference (BCR) sequential extraction was used to determine fractions or speciation of the heavy metal in the soil. At 50 mg/kg Cd contamination level, the concentration of acid-soluble species was significantly the lowest at 19.54 mg/kg under the combination amendment. The concentrations (mg/kg) of acid-soluble species under separate applications of FAp and the fungus were 22.05 and 25.34, respectively. At 50 mg/kg contamination, residual fraction or species immobilized under the combination amendment was 5.74 mg/kg, while that immobilized under applications of FAp and the fungus separately were 2.79 mg/kg and 1.46 mg/kg, respectively. At 25 mg/kg contamination level, the residual fraction immobilized under the combination amendment was 8.42 mg/kg, while that of applications of FAp and the fungus separately were 4.28 mg/kg and 1.27 mg/kg, respectively. The higher populations of soil microorganisms and the improved enzyme activities observed under the combination amendment also indicate that combining the fungus with FAp reduced Cd bioavailability and toxicity effectively, and restored soil biological activities.

Organic matter addition promotes Cd immobilization in alkaline paddy soils

Straw incorporation into the soil is a common agricultural practice, but its effect on soil cadmium (Cd) mobility is not well understood. We added 0–20 g kg−1 organic matters (OMs) with different C/N ratios to three spiked alkaline paddy soils that contained a realistically low concentration of total Cd (0.94 mg kg−1), and then investigated soil Cd solubility in alternate watering conditions. As current physical and chemical methods have difficulties in accurately determining the distribution and speciation of Cd in soil at a low concentration, we measured multiple soil properties to identify key factors regulating dissolved Cd concentration. For all three soils, pH and dissolved Cd concentration both decreased after flooding and increased after subsequent drying. OM addition significantly reduced soil Cd solubility at both flooding and drying stages. Random forest and linear regressions further confirmed that soil total organic carbon, rather than pH, dissolved organic carbon, or total inorganic carbon as previously suggested, was the primary predictor of Cd solubility. OMs with different C/N ratios had similar effects on soil Cd solubility, whereas the effect of OM addition rate depended on soil type. The results demonstrated the potential of straw incorporation for the remediation of Cd-contaminated alkaline paddy soils, through mechanisms that differ from those reported for acid soils.Graphical

Cd isotope fractionation in a soil-rice system: Roles of pH and mineral transformation during Cd immobilization and migration processes

Cd speciation in soil and its transport to rice roots are influenced by the soil pH, oxidation-reduction potential, and mineral transformation; however, the immobilization and migration of Cd in soil-rice systems with different pH values under distinct water regimes remain unclear. This study used Cd isotope fractionation, soil physical analysis, and root gene quantification to elucidate the immobilization and transport of Cd in different soil-rice systems. In drainage soils, the high soil pH enhanced the transformation and magnitude of negative fractionation of Cd from MgCl2 extract to FeMn oxide-bound pool; however, it favored Cd uptake and root-to-grain transport. Compared with drainage regimes, the flooding regimes shifted fractionation toward heavy isotopes from MgCl2-extracted Cd to FeMn oxide-bound Cd in acidic soils (∆114/110CdMgCl2 extract – FeMn oxide-bound Cd = −0.09 ± 0.03 ‰) and to light isotopes from MgCl2-extracted Cd to carbonate-bound Cd in neutral and alkaline soils (∆114/110CdMgCl2 extract – carbonate-bound Cd = 0.29–0.40 ‰). The submerged soils facilitated the forming of carbonate and poorly crystalline minerals (such as ferrihydrite), which were transformed into highly crystalline forms (such as goethite). These results demonstrated that the dissolution-precipitation process of iron oxides was essential for controlling soil Cd availability under flooding regimes, and the relative contribution of carbonate minerals to Cd immobilization was promoted by a high soil pH. Flooding regimes induced lower expressions of OsNRAMP1 and OsNRAMP5 to limit the uptake of light Cd isotopes from MgCl2-extract pool, whereas a teeter-totter effect on gene expression patterns in roots (including those of OsHMA3 and OsHMA2) limited the transport of heavy Cd isotopes from root to grain. These findings demonstrate that flooding regimes could exert multiple effects on soil Cd immobilization and Cd transport to grain. Moreover, alkaline soil was conducive to forming carbonate minerals to sequester Cd.

Biochar improves the yield and quality of Erigeron breviscapus in heavily cadmium-polluted soil

Simultaneously utilizing and remediating soil contaminated with cadmium (Cd) is propitious and economically beneficial. In this study, Erigeron breviscapus, a raw medicinal material used to extract scutellarin, was utilized to conduct a pot experiment including five treatments. Under a control without Cd and biochar addition (CK), soil exogenously added with high concentration of Cd (0.01%) were used to investigate the effects of different biochar doses (0%, 5%, 10%, and 15%, denoted as CdB0, CdB5, CdB10, and CdB15, respectively) on the yield (shoot biomass) and quality (represented by scutellarin content) of E. breviscapus as well as soil remediation. The results showed that soil with addition of high Cd decreased the shoot biomass and scutellarin content of E. breviscapus by 33.6% and 70.8%, respectively. Compared with CdB0, the shoot biomass and scutellarin content of CdB15 increased by 24.7% and 312.0%, respectively. Biochar application (CdB5, CdB10, and CdB15) changed the soil Cd speciation and reduced the soil available Cd and plant Cd concentrations by 17.4%–121.8% and 26.4%–42.6%, respectively, compared with CdB0. However, the chlorophyll content, catalase activity, and acquisition of N, Fe, and Cu of E. breviscapus were increased in CdB5, CdB10, and CdB15 compared with CdB0. In addition, the Cd toxicity mitigation effect enhanced with increasing biochar application, and the application of 15% biochar resulted in the highest shoot biomass and scutellarin content of E. breviscapus. The results of this study highlight the importance of biochar for improving the Cd adaptability of E. breviscapus and provide a reference for the simultaneous utilization and remediation of heavily Cd-contaminated soil.

Endophytic Bacteria in Ricinus communis L.: Diversity of Bacterial Community, Plant−Growth Promoting Traits of the Isolates and Its Effect on Cu and Cd Speciation in Soil

Ricinus communis L. shows certain tolerance to and good accumulation ability with heavy metals. Endophytic bacteria−enhanced phytoremediation is an effective method to improve heavy metal extraction efficiency. Here, for better application of castor in phytoremediation, the Illumina high−throughput sequencing was carried out to reveal the endophytic bacterial community in the tissues of castor grown in two locations, and traditional microbial cultivation was used to isolate endophytic bacteria from castor. The dominant bacterial phyla were Proteobacteria, Bacteroidetes, Actinobacteria, Firmicutes and Acidobacteria, and Proteobacteria was absolutely dominate in all castor tissues. There were significant differences in the composition of endophytic bacterial communities between castor grown in two sites, with obvious variation in the relative abundance of the dominant phylum. The samples from two sites also had their own unique dominant bacterial genera. The analysis of alpha diversity illustrated that the diversity and species richness of endophytic bacteria community in different parts of castor sampled in Tonglushan mining area were lower than those in Gangxia village, Yangxin county. In total, there were 44 endophytic bacteria strains isolated from the tissues of castor, of which 42 strains possessed three or more growth−promoting properties. Most of these isolates were tolerant to Cu or Cd to varying degrees. Eight isolates were selected for further Cu mobilization and soil incubation experiments. Strains TR8, TR18, TR21, YL1, YS3 and YS5 could well solubilize Cu2(OH)2CO3 in medium. Strain YS3 had the best effect on increasing soil DTPA−Cu and DTPA−Cd contents by 8.4% and 6.9%, respectively. Inoculated endophytic isolates were conducive to the conversion of heavy metal forms from insoluble to relatively unstable, and could increase available phosphate content in soil (10.8–29.2%). Therefore, the plant growth−promoting endophytes screened from castor have great application prospects and can provide important support for the microbial−assisted phytoremediation of heavy metal−contaminated soil.

Effect of Arbuscular Mycorrhiza Fungus Diversispora eburnea Inoculation on Lolium perenne and Amorpha fruticosa Growth, Cadmium Uptake, and Soil Cadmium Speciation in Cadmium-Contaminated Soil

Cadmium (Cd) pollution has become aggravated during the past decades of industrialization, severely endangering human health through its entry into the food chain. While it is well understood that arbuscular mycorrhizal fungi (AMF) have a strong ability to regulate plant growth and Cd uptake, studies investigating how they affect soil Cd speciation and influence Cd uptake are limited. We designed a pot experiment comprising two AMF-inoculant groups (inoculation with Diversispora eburnea or no inoculation), three Cd concentration levels (0, 5, and 15 mg/kg), and two plant species (Lolium perenne and Amorpha fruticosa) to study the effect of AMF Diversispora eburnea on plant growth, Cd uptake, and Cd speciation in the soil. The results revealed that L. perenne exhibited higher productivity and greater Cd uptake than A. fruticosa, regardless of AMF D. eburnea inoculation. However, AMF D. eburnea significantly altered soil Cd speciation by increasing the proportion of exchangeable Cd and decreasing residual Cd, resulting in Cd enrichment in the plant root organs and the elimination of Cd from the polluted soils. Our experiments demonstrate that inoculating plants with AMF D. eburnea is an effective alternative strategy for remediating Cd-contaminated soil.

Effects of Straw Return and Moisture Condition on Temporal Changes of DOM Composition and Cd Speciation in Polluted Farmland Soil.

Straw return can improve soil quality and change the mobility and bioavailability of pollutants in soil. Elevated cadmium (Cd) contents in farmland soils were often reported. However, the impacts of straw-derived dissolved organic matter (DOM) on Cd speciation in soil remain poorly understood. In this study, the effects of straw return and moisture condition on temporal changes of DOM composition and Cd speciation in farmland soils were explored through a laboratory incubation experiment. The humified components of DOM were negatively correlated with exchangeable, carbonate-bound, and Fe-Mn oxide-bound Cd (p < 0.01), while its protein-like component was negatively correlated with residual Cd (p < 0.01). It was found that selected fluorescence parameters could be used to predict temporal changes of Cd geochemical fractions. Straw addition led to increases in soil DOM content during the first three days of the incubation. Flooding should be avoided in the first three days following the straw application to reduce the risk of DOM-facilitated Cd mobilization.

Effects of four endophytic bacteria on cadmium speciation and remediation efficiency of Sedum plumbizincicola in farmland soil.

Cadmium (Cd) pollution in farmland soils severely affects agricultural production safety, thereby threatening human health. Sedum plumbizincicola is a Cd and Zn hyperaccumulator commonly used for the phytoremediation of Cd-contaminated soil. This study was aimed to improve the remediation effect of S. plumbizincicola on Cd-contaminated farmland soil and provide a theoretical basis for the enhancement of endophytic bacteria in the repair of Cd-contaminated soil with S. plumbizincicola. Four kinds of endophytic bacteria, namely Buttiauxella, Pedobacter, Aeromonas eucrenophila, and Ralstonia pickettii, were used, and soil culture experiments and pot experiments were conducted to explore the effects of endophytic bacteria on soil Cd speciation and phytoremediation efficiency of Cd-contaminated farmland soils. Under the experimental conditions, after inoculation with endophytic bacteria, the soil pH was effectively reduced, content of weak acid-extracted Cd and oxidizable Cd increased, and content of reducible Cd and residual Cd decreased. Soil Cd activity was increased, and the availability coefficient of soil Cd increased by 1.15 to 6.41 units compared with that of the control (CK2). Compared with CK2, the biomass of S. plumbizincicola significantly increased by 23.23-55.12%; Cd content in shoots and roots of S. plumbizincicola increased by 29.63-46.01% and 11.42-84.47%, respectively; and bioconcentration factor was 2.13 to 2.72 times that of CK2. The Cd removal rate of S. plumbizincicola monocropping was 48.25%. When S. plumbizincicola was planted with inoculating endophytic bacteria, the Cd removal rate in the soil reached 61.18-71.49%, which was significantly higher than that of CK2 (p < 0.05). The treatment with endophytic bacteria activated soil Cd, promoted the growth of S. plumbizincicola, increased its Cd content, and enhanced the phytoremediation of Cd-contaminated farmland soil. Therefore, endophytic bacteria can be used to improve the remediation efficiency of S. plumbizincicola in Cd-contaminated farmland soils.

Laboratory versus field soil aging: Impacts on cadmium distribution, release, and bioavailability.

To date, most studies about the aging of metals in soil were based on the controlled laboratory experiments, and few works have attempted to investigate how aging process influences the distribution and bioavailability of metals in soil under the field condition. The purpose of this study was to compare the aging of cadmium (Cd) in soils under the controlled laboratory and the field by monitoring time-dependent soil Cd speciation changes, Cd release kinetics, and Cd bioavailability to plant through the 438-day aging experiments. During the aging process, the proportions of Cd associated with the most weakly bound fraction tended to decrease, with corresponding increases in the more stable binding fractions. After aging, a higher concentration of available Cd was found in the field aging soil (0.74mgkg-1) than the laboratory aging soil (0.65mgkg-1). The Elovich equation was the best model to describe the soil available Cd aging process. The constant b in the Elovich equation, which was defined as the transformation rate, was in the order of laboratory aging soil > field aging soil. Moreover, higher Cd release amounts were found for the field aging soil (2.74mgkg-1) than the laboratory aging soil (2.57mgkg-1) at the end of aging. Additionally, higher body Cd concentrations were found for the vegetables grown in the field aging soils (1.49mgkg-1, fresh weight) than those grown in the laboratory aging soils (1.32mgkg-1, fresh weight). Therefore, this study indicated that the metal distribution process and its bioavailability may be overestimated or underestimated if research data from the laboratory experiments are used to derive soil quality criteria or investigate soil metal bioavailability.

Alleviation of Cadmium Toxicity to Medicago Truncatula by AMF Involves the Changes of Cd Speciation in Rhizosphere Soil and Subcellular Distribution

Arbuscular mycorrhizal fungi (AMF) can improve plant tolerance to several abiotic stresses, including heavy metals, drought or salinity exposure. However, the role of AMF in alleviation of soil cadmium (Cd)-induced toxicity to plants is still largely unknown. In this study, Cd speciation in soil and subcellular distribution of Cd were used to characterize the roles of application AM fungi in the alleviation of Cd toxicity in alfalfa plants. Our results showed that the addition of <i>Glomus mosseae</i> in Cd contaminated soil (10 mg/Kg) significantly increased soil pH, cation exchange capacity (CEC) and organic matter in rhizosphere soil with <i>Medicago truncatula</i> L., and then account for significantly decreased contents of exchangeable and carbonate-bounded Cd speciation in rhizosphere soil, indicating alleviation of plant toxicity by reduction of bioavailable fractions of Cd. Although there is no significant difference found in Cd accumulation by roots and shoots respectively between Cd and AM-Cd treatments, more portion of Cd was recorded compartmentalization in cell wall fraction of both root and shoot in treatment of Cd with AM application, indicating alleviation of Cd toxicity to plant cell. Herein, application of AM fungi in Cd treatments performed to inhibit the appearance of Cd toxicity symptoms, including the improvement of leaf electrolyte leakage, root elongation, seedling growth and biomass. This information provides a clearer understanding of detoxification strategy of AM fungi on Cd behavior with development and stabilization of soil structure and subcellular distribution of plant.

Increased biomass and reduced tissue cadmium accumulation in rice via indigenous Citrobacter sp. XT1-2-2 and its mechanisms

Microbial remediation is a promising technique to remediate heavy metals contaminated soils. In this study, the cadmium (Cd)- resistant Citrobacter sp. XT1-2-2, isolated from heavy metals contaminated paddy soils, was investigated to evaluate the effect of this strain on soil Cd speciation, cellular Cd distribution, tissue Cd accumulation and rice biomass. The percentage of Cd2+ removal by Citrobacter sp. XT1-2-2 was up to 82.3 ± 2.1% within 240 min in the solution. The average content of soil soluble plus exchangeable and carbonate-bound fractions of Cd decreased, whereas Fe/Mn oxide-bound, organic matter-bound and residual fractions increased with bacteria inoculation. For the paddy soil inoculated with the XT1-2-2 strain, Cd concentrations of roots, culms, leaves and grains were significantly reduced by 24.1%, 46.9%, 41.5% and 66.7%, respectively. In addition, inoculation bacteria significantly increased the biomass of the roots, above-ground tissues and the rice grains. All results indicated that the XT1-2-2 strain had the ability to immobilize soil Cd and decrease Cd accumulation in rice grains. Therefore, the XT1-2-2 strain has potential for application to remediate Cd-contaminated paddy soils. It is possible to exploit a new bacterial-assisted technique for the remediation in Cd-contaminated paddy soils.

Cd uptake by Phytolacca americana L. promoted by cornstalk biochar amendments in Cd-contaminated soil

Cadmium (Cd) contamination is the most extensive pollution in China farmland. A greenhouse pot trial was conducted to investigate the effects of cornstalk biochar on Cd accumulation by Phytolacca americana L. (pokeweed) in Cd-contaminated soil. The Cd concentration increased in leaves, shoots, and roots of plants amended with 5% biochar by 79%, 113%, and 32%, respectively, compared with the pokeweed without biochar. The Cd availability, soil Cd speciation, soil fertility, root biomass, and Cd chemical forms in root were investigated to explore the mechanism by which Cd uptake increased in presence of biochar. The extractability of Cd by DTPA decreased in presence of biochar by 30% compared with that of soil without biochar. The increases occurred with dose of biochar increased in available phosphorus, labile organic carbon, and C/N atom ratio. Although, the dry weight of the aboveground part of the pokeweed decreased by 38.5%, however, the weight of roots increased by 20.8%. Root biomass and microbial activity reached maximum in the treatment that recieved 5% biochar. Cd forms extracted by NaCl and acetic acid (HAc) were predominant in root. When 5% biochar applied to soil, HAc-extracted Cd took up maximum of the increase in root.

Distribution, transfer, and time-dependent variation of Cd in soil-rice system: A case study in the Chengdu plain, Southwest China

Heavy metal contamination in soil has been a major environmental concern and affected agricultural safety, and it is generally believed that the risk of heavy metals in soil is associated critically with their forms. This study was carried out to identify the distribution of cadmium (Cd) speciation in paddy soil of Chengdu Plain and its time-dependent variation during rice growing process and to determine the controls of soil Cd fractions on their accumulation in rice. The speciation of Cd in soil was analysed by Community Bureau of Reference (BCR) sequential extraction method. The result showed that the Cd concentration range in study fields was 0.54 mg/kg ˜ 1.65 mg/kg and the distribution order of Cd in soil was basically: residua > exchangeable ≈ reducible > oxidizable. Cd accumulation and distribution in rice largely depended on non-residual fraction Cd, rather than the total Cd, especially exchangeable fraction Cd having a linear relationship with total Cd in grain within a use limit value of Cd concentration in soil. In the process of rice growth, the distribution of Cd in rhizosphere soil was changed, and the relatively high percentage of non-residual fraction Cd was reached before heading, which suggested that the amount of Cd took up by paddy roots during the time of tillering was more than at any other period. Overall, this study provided a basis for exploring more appropriate risk assessment and heavy metal contaminated soil remediation.

Effects of organic-inorganic amendments on the cadmium fraction in soil and its accumulation in rice (Oryza sativa L.).

Cadmium (Cd) stress is a serious concern in agricultural soils worldwide, and increasing accumulation and subsequent transfer to humans via the food chain can have potentially harmful effects. In this study, field experiments were conducted to examine the uptake and translocation of Cd in rice, changes in the soil Cd speciation, and the subsequent effect on Cd accumulation in rice under combined organic (farmyard manure and crop straw) and inorganic (sepiolite, lime, and calcium-magnesium phosphate) soil amendments. The results showed that farmyard manure combined with sepiolite or lime and straw combined with lime or calcium-magnesium phosphate reduced the Cd translocation from the rice roots to the straw and the grains, significantly decreasing the Cd accumulation in brown rice. In addition, straw combined with sepiolite, lime, or calcium-magnesium phosphate reduced the Cd accumulation in brown rice but increased the Cd translocation from the roots to the straw by 37.8-279.3% compared with the control. Organic-inorganic amendments also decreased the soil exchangeable Cd and increased the organic-bound Cd by more than 40%. Fe-Mn oxide-bound Cd also increased but varied with growth. Cd accumulation in brown rice showed a significant positive relationship with soil exchangeable Cd at 90days after transplantation, while at 30days, the increase in Fe-Mn oxide- and organic-bound Cd was found to be the key factor in reducing the Cd accumulation in rice. These findings suggest that straw (under rice-rape rotation) and farmyard manure (under rice-wheat rotation) combined with sepiolite or lime are widely applicable as agronomic control techniques aimed at lowering Cd pollution.

Inoculation of soil with cadmium-resistant bacterium Delftia sp. B9 reduces cadmium accumulation in rice (Oryza sativa L.) grains

Bioremediation of heavy metal polluted soil using metal-resistant bacteria has received increasing attentions. In the present study, we isolated a heavy metal-resistant bacterial strain from a Cd-contaminated soil, and conducted pot experiments to evaluate the effect of bacterial inoculation in soil on soil Cd speciation, rice grain biomass and Cd accumulation. We find that the isolated bacterial strain is a Gram-negative bacterium, and named as Delftia sp. B9 based on the 16S rDNA gene sequence analysis. TEM–EDS manifests that Cd can be bioaccumulated inside cell, resulting in intracellular dissolution. The Cd contents of rice grain in the two rice cultivars (early and late rice) are all below the standard limit for Food Safety of People's Republic of China (0.2 mg/kg) after the treatment of both living and non-living cells. Non-living cells are more applicable than the use of living cells for the short time bioremediation. The average content of soil exchangeable fraction of Cd decreases whereas the residual fraction increases with bacterial inoculation. All our results suggest Delftia sp. B9 is able to the stabilization of Cd in soil and reduce Cd accumulation in rice grain, therefore, this strain is potentially suitable for the bioremediation of Cd-contaminated paddy soils.

Effect of Nano Zeolite on the Transformation of Cadmium Speciation and Its Uptake by Tobacco in Cadmium-contaminated Soil

Abstract The study was aimed at exploring the effects of applying ordinary and nano zeolite on the soil pH, soil available cadmium (Cd) content, soil Cd speciation and Cd uptakes by tobacco using pot experiment with simulated Cd contaminated soil indoors. The results showed that soil pH increased and available Cd content reduced with the amount of ordinary and nano zeolite increasing. Compared with the control, the application of ordinary and nano zeolite increased soil pH at 0.47 - 1.05 and 0.73 - 1.57, respectively, and reduced the available Cd contents at 19.3% - 32.7% and 23.2% - 40.5%, respectively. In addition, soil pH had significantly negative correlation with available Cd content in each treatment (p&lt;0.05). Nano zeolite could more effectively reduce Cd contents of all parts of tobacco than ordinary zeolite with the same amount treatments, and Cd content in all parts of tobacco plants was positively correlated with soil available Cd content. The content of exchangeable Cd (EXE) in soil decreased to some extent with different zeolite treatments, application of nano zeolite was better than that of application of ordinary zeolite in reducing Cd bioavailability and transferability. Overall, application of nano zeolite has an advantage over ordinary zeolite in reducing available Cd content in soil and Cd content in tobacco.

Effects of arbuscular mycorrhizal inoculation and biochar amendment on maize growth, cadmium uptake and soil cadmium speciation in Cd-contaminated soil

Experiments conducted to understand how arbuscular mycorrhizal (AM) inoculation or biochar application affect plant growth and heavy metal uptake have thus far looked at single applications of either soil amendment. There is little evidence of their synergistic effects, in particular for plants grown in cadmium (Cd) contaminated soil. We conducted a mesocosm experiment to investigate the effect of AM inoculation (Glomus intraradices BEG 141) and/or wheat-straw biochar amendment on maize (Zea mays L. cv. Hongdan No. 897) growth, antioxidant enzymatic activities, and Cd uptake, as well as soil Cd speciation under applications of 0, 3, 6 mg Cd per kg soil. Applying either AM inoculant or biochar alone significantly increased maize growth and reduced Cd uptake. Furthermore, solo AM inoculation alleviating Cd stress more fully than biochar, in turn facilitating maize growth and decreasing soil Cd translocation into plant tissue. Still, solo biochar amendment was more effective at inducing soil alkalinization and contributing to Cd immobilization. Adding biochar together with AM inoculant significantly promoted fungal populations compared to a control. Amending soil with AM inoculant and biochar together produced the largest increase in maize growth and decrease in tissue Cd concentrations. This effect was additive, with 79.1% greater biomass, 51.42%, 82.91%, 43.96% higher activities of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and 50.06%, 67.19%, 58.04% and 76.19% lower Cd concentrations in roots, stems, leaves, and ears, respectively, at a 6 mg kg−1 Cd contamination rate. The combined treatment also had a synergistic effect on inducing soil alkalinization and causing Cd immobilization, and decreasing Cd phytoavailability and post-harvest transfer risks. These results suggest that AM inoculation in combination with biochar application may be applicable not only for maize production but also for phytostabilization of Cd-contaminated soil.

Immobilization of Cd in soil and changes of soil microbial community by bioaugmentation of UV-mutated Bacillus subtilis 38 assisted by biostimulation

To develop an efficient bio-immobilization technology for the remediation of heavy metals in soil, a mutant species of Bacillus subtilis (B38) was acquired by UV irradiation and selection under high concentration of cadmium (Cd) in a previous study. Immobilization of Cd in soil has been achieved and reported earlier by a technology of bioaugmentation of B38 assisted by biostimulation (BAB) using a bio-waste (NovoGro) from an enzyme company. In the present study, B38 was characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Fourier-transform infrared spectroscopy (FT-IR). The results proved that the cell morphology and functional groups on the cell surface of B. subtilis changed after the mutation and interaction with Cd. The applicability of BAB as a practicable bioremediation technology was evaluated by checking the inhibition on Cd bioaccumulation by carrots (Daucus carota), the enhancement on carrot biomass, and the change of Cd speciation in soil. The BAB technology reduced Cd contents in carrot shoots and roots by 16% and 55%, respectively. Simultaneously, the exchangeable fraction of Cd in soil under BAB treatment decreased significantly. Furthermore, a PCR-DGGE-based experiment was performed to evaluate the effect of BAB technology on soil microbial communities. The BAB technology could enhance the proliferation of both exotic B38 and native microbes.

Speciation and phytoavailability of cadmium in soil treated with cadmium-contaminated rice straw.

When grown on Cd-contaminated soil, rice typically accumulates considerable Cd in straw, and which may return to the soil after harvest. This work was undertaken to assess the pollution risk of Cd associated to the Cd-contaminated rice straw after incorporating into an uncontaminated soil. With the Cd-contaminated rice straw added at 0, 1, 2, 3, 4 and 5 % (w/w), an incubation experiment (28 days) with non-planting and a followed pot experiment sequent with two planting (rice and Chinese cabbage, transplanted after 28-day incubation) were carried out to investigate the changes of soil Cd speciation and phytoavailability. The results indicated that the Cd-contaminated rice straw addition significantly increased soil pH and dissolved organic carbon during the 28-day incubation. For the high availability of Cd in contaminated rice straw, diethylenetriaminepentaacetic acid (DTPA) extractable Cd significantly increased, and the percentages of acetic acid extractable and reducible Cd in soil significantly enhanced after the addition of Cd-contaminated rice straw. However, the Cd-contaminated rice straw addition inhibited the rice growth and induced the decrease of Cd in rice grain and straw by 12.8 to 70.2 % and 39.3 to 57.3 %, respectively, whereas the Cd contents increased by 13.9 to 84.1 % in Chinese cabbage that planted after rice harvest. In conclusion, Cd associated with Cd-contaminated rice straw was highly available after incorporating into the soil, and thus the Cd pollution risk via the Cd-contaminated rice straw incorporation should be evaluated in the Cd-contaminated paddy region.

Phytoavailability and geospeciation of cadmium in contaminated soil remediated by Rhodobacter sphaeroides

A microorganism was isolated from oil field injection water and identified as Rhodobacter sphaeroides. It was used for the remediation of simulated cadmium-contaminated soil. The phytoavailability of Cd was investigated through wheat seedling method to determine the efficiency of remediation. It was found that after remediation, the accumulation of Cd in wheat roots and leaves decreased by 67% and 53%, respectively. The Cd speciation in soil was determined with Tessier extraction procedure. It was found that the total Cd content in soil did not change during the experiments, but the geo-speciation of Cd changed remarkably. Among the five fractions, the concentration of exchangeable phases decreased by 27–46% and that of the phases bound to Fe–Mn oxides increased by 22–44%. The decrease of Cd accumulation in wheat showed significant positive correlation with the decrease of exchangeable phases. It could be concluded that the remediation of R. sphaeroides was carried out through the conversion of Cd to more stable forms. The decrease of sulfate concentration in supernatant indicated that the R. sphaeroides consumed sulfate.