Comparison of Transcriptome Differences between Two Rice Cultivars Differing in Cadmium Translocation from Spike-Neck to Grain.

At present, the mechanism of varietal differences in cadmium (Cd) accumulation in rice is not well understood. Two rice cultivars, ZZY (high translocation-high grain Cd) and SJ18 (low translocation-low grain Cd), were used to analyze transcriptome differences in the spike-neck tissue in field trials. The results showed that, compared with ZZY, 22,367 differentially expressed genes (DEGs) were identified in SJ18, including 2941 upregulated and 19,426 downregulated genes. GO analysis enriched 59 downregulated terms, concerning 24 terms enriched for more than 1000 DEGs, including cellular and metabolic processes, biological regulation, localization, catalytic activity, transporter activity, signaling, etc. KEGG enrichment identified 21 significant downregulated pathways, regarding the ribosome, metabolic pathways, biosynthesis of secondary metabolism, signaling transduction, cell membrane and cytoskeleton synthesis, genetic information transfer, amino acid synthesis, etc. Weighted gene co-expression network analysis (WGCNA) revealed that these DEGs could be clustered into five modules. Among them, the yellow module was significantly related to SJ18 with hub genes related to OsHMA and OsActin, whereas the brown module was significantly related to ZZY with hub genes related to mitogen-activated protein kinase (MAPK), CBS, and glutaredoxin. This suggests that different mechanisms are involved in the process of spike-neck-grain Cd translocation among varieties. This study provides new insights into the mechanisms underlying differences in Cd transport among rice varieties.

Biochar loaded with root exudates of hyperaccumulator Leersia hexandra Swartz facilitated Cr(VI) reduction by shaping soil functional microbial communities.

Cr(VI) contamination is widely recognized as one of the major environmental hazards. To address the problem of remediation of soil Cr(VI) contamination and utilization of waste peanut shells, this study comprehensively investigated the effects of peanut shell-derived biochar loaded with root exudates of hyperaccumulator Leersia hexandra Swartz on Cr(VI) reduction and microbial community succession in soil. This study confirmed that root exudate-loaded peanut shell biochar reduced soil pH while simultaneously increasing DOC, sulfide, and Fe(II) concentrations, thereby facilitating the reduction of Cr(VI), achieving a reduction efficiency of 81.8%. Based on XPS and SEM elemental mapping analyses, Cr(VI) reduction occurred concurrently with the Fe and S redox cycles. Furthermore, the microbial diversity, abundance of the functional genera (Geobacter, Arthrobacter, and Desulfococcus) and the metabolic functions associated with Cr(VI) reduction were enhanced by root exudate-loaded biochar. Root exudate-loaded biochar can promote both direct Cr(VI) reduction mediated by the Cr(VI)-reducing bacteria Arthrobacter, and indirect Cr(VI) reduction through Cr/S/Fe co-transformation mediated by the sulfate-reducing bacteria Desulfococcus and Fe(III)-reducing bacteria Geobacter. This study demonstrates the effectiveness of peanut shell biochar loaded with root exudates of hyperaccumulator Leersia hexandra Swartz to promote soil Cr(VI) reduction, reveals the mechanism how root exudate-loaded biochar shapes functional microbial communities to facilitate Cr(VI) reduction, and proposes a viable strategy for Cr(VI) remediation and utilization of peanut shell.

Innovative accumulative risk assessment of co-exposure to Cd, As, and Pb in contaminated rice based on their in vivo bioavailability and in vitro bioaccessibility.

The consumption of cadmium (Cd), arsenic (As), and lead (Pb) co-contaminated rice exposes humans to multiple heavy metals simultaneously, with relative bioavailability (RBA) and bioaccessibility (BAc) being important determinants of potential health risks. This study evaluated the relationship between in vivo RBA and in vitro BAc of Cd, As, and Pb in rice and their cumulative risk to humans. A total of 110 rice samples were collected in Zhejiang Province, China, and 10 subsamples with varying concentration gradients were randomly selected to measure RBA using a mouse model (liver, kidney, femur, blood, and urine as endpoints) and BAc using four in vitro assays (PBET, UBM, SBRC, and IVG). Our results indicated that Cd-RBA varied from 21.2 % to 67.5 %, As-RBA varied from 23.2 % to 69.3 %, and Pb-RBA varied from 22.2 % to 68.9 % based on mouse liver plus kidneys. The BAc values for Cd, As, and Pb in rice varied according to the assay. Compared to Cd and As, Pb exhibited a lower BAc in the gastric (GP) and intestinal (IP) phases. According to the relationship between the BAc and RBA values, IVG-GP (R2 = 0.92), SBRC-IP (R2 = 0.73), and UBM-GP (R2 = 0.80) could be used as predictors of Cd-, As-, and Pb-RBA in rice, respectively. The health risks associated with co-exposure to Cd, As, and Pb in contaminated rice for both adults and children exceeded the acceptable threshold, with Cd and As being the primary risk factors. The noncarcinogenic and carcinogenic risks were markedly reduced when the RBA and BAc values were incorporated into the risk assessment. Due to the risk overestimation inherent in estimating the risk level based on total metal concentration, our study provides a realistic assessment of the cumulative health risks associated with co-exposure to Cd, As, and Pb in contaminated rice using in vivo RBA and in vitro BAc bioassays.

Comparative transcriptome analysis reveals key genes and coordinated mechanisms in two rice cultivars differing in cadmium accumulation.

Although Cd accumulation varies among rice varieties is recognized, the underlying mechanisms are not well clarified. In this study, comparative transcriptome analysis were performed by hydroponic culture system with two rice varieties, Y1540 (high Cd accumulator) and Y15 (low Cd accumulator) under 20 µM Cd stress. Results revealed 17,320 differentially expressed genes (DEGs) in roots of Y15 (7,655 upregulated and 9,665 downregulated) and 17,386 DEGs in roots of Y1540 (8,823 upregulated and 8,563 downregulated) expose to 20 µM Cd stress. Gene ontology (GO) analysis enriched 24 and 26 terms in Y15 and Y1540 respectively, including 23 common terms. Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment showed 27 and 28 significant pathways in Y15 and Y1540 respectively, with 19 common pathways. Different responses to Cd stress between cultivars were not only reflected in differently enriched GO terms and KEGG pathways but also in different DEGs of 23 common GO terms and significant sequences represented by p-values of 19 common KEGG pathways. Both cultivars resist Cd through common processes with different weights; hence glutathione metabolism, mineral absorption, biosynthesis of secondary metabolites, and degradation of aromatic compounds could be playing a more important role in Y1540, whereas ribosome biogenesis in eukaryotes, mismatch repair, aminoacyl-tRNA biosynthesis, and the cell cycle maybe playing a more important role in Y15. Weighted gene co-expression network analysis (WGCNA) showed that five and three modules were clustered in Y15 and Y1540, respectively, with yellow and brown modules in Y15 and brown modules in Y1540 being significantly related to Cd stress. Further analysis showed that most of hub genes in Y15 were related to signal transduction or transcription factors, while most of hub genes in Y1540 were related to binding, metabolic, and secondary metabolic processes, which demonstrated their different response patterns at transcriptomic level to Cd stress.

Comparative transcriptomic analysis reveals the important process in two rice cultivars with differences in cadmium accumulation.

To date, Cd remains a major contaminant in rice production. An in-depth exploration of the mechanism that causes genotypic differences in Cd enrichment in rice is necessary to develop strategies to regulate Cd enrichment in rice. Here, two rice cultivars (low grain Cd, ZZ143; and high grain Cd, YX409) displayed different transcriptomic profile patterns when subjected to 100µmol/L Cd stress. In fact, 18,721(9833 upregulated and 8888 downregulated) and 16,403 (8366 upregulated and 8037 downregulated) differentially expressed genes (DEGs) were found in ZZ143 and YX409, respectively. Gene ontology (GO) classification revealed 28 and 26 terms enriched in ZZ143 and YX409, respectively. ZZ143 had more enriched DEGs than YX409, primarily in cellular processes, metabolic processes, binding terms, catalytic activity, etc. Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis revealed that 21 and 24 pathways were significantly enriched in ZZ143 and YX409, respectively. Based on the DEGs, ZZ143 had a stronger ability for sulfur assimilation and Cys synthesis, whereas YX409 had a stronger ability to maintain cell wall stability. A series of DEGs involved in metabolic pathways, biosynthesis of secondary metabolites, plant hormone signal transduction pathways, and mitogen-activated protein kinase signaling pathways were identified, which maybe closely related to Cd resistance and the different Cd concentrations between cultivars. The above pathways and the greater number of identified DEGs in more than half of the GO terms and KEGG pathways suggest a higher absorption and stronger tolerance of the roots of ZZ143 than YX409 to Cd.

In situ formation of Ca(OH)2 coating shell to extend the longevity of zero-valent iron biochar composite derived from Fe-rich sludge for aqueous phosphorus removal.

Despite being an effective and attractive functional strategy for aqueous phosphorus (P) removal, the use of zero valent iron (ZVI) biochar composites has been severely impeded by rapid self-erosion. We describe a new approach for extending the lifespan of Fe-rich sludge-derived ZVI biochar composites via CaCl2 modification. Preliminary results showed that composites obtained at 900 °C without modification (MBC900) and at 900 °C with 100 g Cl/kg addition (MBC900100) had the highest P removal efficiency. In subsequent batch experiments, MBC900100 exhibited more stable P adsorption capacities than MBC900 over a wide pH range (4-10) and at various dosages, which was enhanced by the presence of HCO3-. The theoretical maximum P adsorption capacities of MBC900 and MBC900100 were 227.14 and 224.15 mg g-1, respectively. Kinetic analysis indicated that chemisorption dominated the removal process. Continuous experimental data using the Yoon-Nelson model indicated that MBC900100 had a considerably longer half-penetration time. The primary mechanism of P removal by MBC900 was Fe/C micro-electrolysis. As the embedded CaO formed a dissolvable Ca(OH)2 shell in situ on the surface of MBC900100, the phosphate formed a precipitate with free Ca2+ before being removed via micro-electrolysis. Overall, CaCl2 modification successfully enhanced the longevity of the ZVI biochar composites.

Citric acid secretion from rice roots contributes to reduction and immobilization of Cr(VI) by driving microbial sulfur and iron cycle in paddy soil.

Root exudates released by plants can promote microbial growth and activity, thereby affecting the transformation and availability of soil pollutants. However, the effects of the root exudates of rice plants on chromium (Cr) transformation in paddy soils and the underlying mechanisms are yet to be elucidated properly. The present study investigated how rice root exudates interact with rhizosphere microorganisms to influence the transformation of Cr and explored the key components in root exudates that affect Cr(VI) reduction. The results showed that the addition of root exudate and citric acid markedly decreased soil pH and increased dissolved organic carbon content that created favorable conditions and provided sufficient electron donors for Cr(VI) reduction, thereby greatly facilitating the reduction of Cr(VI) and the transformation of HOAc-extractable Cr into more stable oxidizable and residual Cr. Additionally, Desulfovibrio-related sulfate-reducing bacteria, Thiobacillus-related sulfide-oxidizing bacteria, and Geobacter-related Fe(III)-reducing bacteria were enriched with the addition of root exudate and citric acid. Among them, sulfate would be reduced by Desulfovibrio to sulfide, which would be further utilized by Thiobacillus to reduce Cr(VI), thereby enabling the continuous reduction of Cr(VI); simultaneously, Geobacter would sustain the reduction of Cr(VI) by reducing Fe(III) to Fe(II). Furthermore, based on the high-level secretion of citric acid in response to Cr(VI) exposure and the similar effects of root exudates and citric acid on Cr(VI) reduction, it is proposed that citric acid is the key component in rice root exudates that affects Cr(VI) reduction. These results suggest that root exudates (citric acid as the key component) contribute to the reduction and immobilization of Cr(VI) by driving microbial S and Fe cycles, with Desulfovibrio, Thiobacillus, and Geobacter being the keystone genera. The study provides a novel insight into the Fe/S/Cr co-transformation processes with microbial involvement, and the artificial root exudate mixtures designed to reduce Cr(VI).

Continuously applying compost for three years alleviated soil acidity and heavy metal bioavailability in a soil-asparagus lettuce system.

Soil acidification and heavy metal pollution are two common barrier factors threatening plant growth and agro-product quality. Applying manure compost is promising to alleviate soil acidity, while it may increase heavy metal accumulation in soil. In a 3-year field experiment, compost was applied for 12 consecutive harvest seasons at 15, 30, and 45 t ha-1 in a slightly acidic soil. Samples were taken at the twelfth season to examine the changes of soil properties, vegetable productivity, heavy metal accumulation and bioavailability in the soil-asparagus lettuce system. The results showed that the pH values of the topsoil were increased by 0.49-0.75 units in compost added soils compared with no compost control, soil organic matter (SOM) contents and cation exchange capacity (CEC) were increased by 34-101% and 43-44%, respectively. The soil nutrient contents were also increased in compost treatments. Continuously applying compost increased Cd, Cu, and Zn concentrations in topsoil by up to 32, 20, and 22% and decreased Pb by 10%, while soil available Cd and Zn concentrations were reduced by up to 54 and 86%, and available Cu was increased by 19-63%. The biomass of asparagus lettuce was increased by 30-59% in compost treatments, with Cd and Zn concentrations in the plant tissues reduced by 28-50% and 14-67%. Cu concentrations in the lettuce shoots were increased by 20-39%. The concentration factor and total uptake of Cd and Zn in lettuce were effectively reduced in compost treatments. Cd was more prone to be taken up, translocated and accumulated from soil to the lettuce plant than the other heavy metals. Continuously applying compost over 3 years increased soil pH, SOM, CEC, nutrient contents, and lettuce productivity, decreased Cd and Zn bioavailability in the soil-lettuce system, while posing a risk of increasing heavy metal accumulation in topsoil.

Multi-Component Passivators Regulate Heavy Metal Accumulation in Paddy Soil and Rice: A Three-Site Field Experiment in South China.

To fulfill sustainability principles, a three-site field experiment was conducted to screen suitably mixed passivators from lime + biochar (L + C, 9000 kgha-1 with a rate of 1:1) and lime + biochar + sepiolite (L + C + S, 9000 kg ha-1 with a rate of 1:1:1), in Yuecheng (YC), Zhuji (ZJ), and Fuyang (FY), where there are typical contaminated soils, in South China. Treated with passivators in soil, DTPA-extractable Cd, Crand Pb in soil were decreased by 9.87-26.3%, 37.2-67.5%, and 19.0-54.2%, respectively; Cd, Cr, and Pb in rice were decreased by 85.9-91.5%, 40.0-76.5%, and 16.4-45.4%, respectively; and these were followed by slightly higher efficacy of L + C + S than L + C. The differences between L + C and L + C + S mainly lie in soil microbial communities, enzymes, and fertility. In YC, treatment with L + C + S increased microbial carbon and activities of urease (EC3.5.1.5) and phosphatase (EC3.1.3.1) by 21.0%, 85.5%, and 22.3%; while treatment with L + C decreased microbial carbon and activities of phosphatase and sucrose (EC3.2.1.26) by 1.31%, 34.9%, and 43.4%, respectively. Moreover, the treatment of FY soils with L + C + S increased microbial carbon and activities of urease, phosphatase, and sucrase by 35.4%, 41.6%, 27.9%, and 7.37%; and L + C treatment only increased the microbial carbon and the activity of phosphatase by 3.14% and 30.3%, respectively. Furthermore, the organic matter and available nitrogen were also increased by 8.8-19.0% and 7.4-14.6% with L + C + S treatments, respectively. These suggested that the combination of L + C + S stimulated the growth of soil microbial communities and increased the activity of soil enzymes. Therefore, the L + C + S strategy can be a practical and effective measure for safe rice production as it was more suitable for the remediation of heavy metals in our experimental sites.

Responses of microbial community composition and function to biochar and irrigation management and the linkage to Cr transformation in paddy soil.

Combining biochar with irrigation management to alter the microbial community is a sustainable method for remediating soils contaminated by heavy metals. However, studies on how these treatments promote Cr(VI) reduction are limited, and the corresponding microbial mechanisms are unclear. Therefore, we conducted a pot experiment to explore the responses of soil microbial communities to combined biochar amendment and irrigation management strategies and their involvement in Cr transformation in paddy soils. Six treatments were established using varying concentrations of biochar (0, 1, and 2% [w/w]) combined with two irrigation management strategies (continuous flooding [CF] and dry-wet alternation [DWA]). The results showed that the combined biochar addition and irrigation management strategy significantly altered soil pH, redox potential, organic matter content, and Fe(II) and sulfide concentrations. In addition, the Cr(VI) concentration under CF irrigation management was conspicuously lower (48.2-54.4%) than that under DWA irrigation management. Biochar amendment also resulted in a substantial reduction (8.8-27.4%) in Cr(VI) concentration. Moreover, the changes in soil physicochemical properties remarkably affected the soil microbial community. The microbial diversity and abundance significantly increased with biochar amendment. Furthermore, the combined biochar amendment and CF strategy stimulated the growth of Geobacter- and Anaeromyxobacter-related Fe(III)-reducing bacteria, Gallionella-related Fe(II)-oxidizing bacteria, and Desulfovibro- and Clostridium-related sulfate-reducing bacteria, which simultaneously facilitated the generation of Fe(II) and sulfide, thereby enhancing Cr(VI) reduction. Consequently, our results suggest that the effectively increased abundance of Fe-reducing/oxidizing bacteria and sulfate-reducing bacteria via combined CF irrigation management and biochar addition may be a key factor in reducing Cr(VI) in paddy soil. The keystone genera responsible for Cr(VI) reduction were Geobacter, Anaeromyxobacter, Gallionella, Desulfovibro, and Clostridium. This study provides novel insights into the coupling mechanism of the Fe/S/Cr transformation mediated by Fe-reducing/oxidizing bacteria and sulfate-reducing bacteria.

Continuous flooding stimulates root iron plaque formation and reduces chromium accumulation in rice (Oryza sativa L.).

Chromium (Cr) contamination in rice poses a serious threat to human health. Therefore, we conducted pot experiments to investigate the influence of water management regimes on the formation of iron plaque on rice roots, and its effect on the accumulation and translocation of Cr in rice grown on contaminated soil. The results showed that water management regimes, including continuous and intermittent flooding, exerted notable effects on soil solution concentrations of Cr(VI) and Cr(III) through changes in redox potential, pH, and dissolved Fe(II) concentrations. In particular, 69.2%-71.8% of Cr(VI) was reduced to Cr(III) under continuous flooding, whereas only 33.3%-38.6% was reduced under intermittent flooding conditions. Additionally, continuous flooding created a rhizosphere environment favorable to the formation of iron plaque. The amount of iron plaque formed increased by 28.2%-47.2% under continuous flooding conditions as compared with that under intermittent flooding conditions. Moreover, compared with intermittent flooding, under continuous flooding, more Cr (18.0%-23.9%) was adsorbed in the iron plaque, thereby sequestering Cr and reducing its mobility. The Cr concentrations in rice root, straw, husk, and grain under continuous flooding conditions were, respectively, 32.0%-36.5%, 32.7%-36.3%, 34.2%-46.9%, and 25.4%-37.7% lower than those under intermittent flooding conditions. Therefore, continuous flooding caused a substantial decrease in the Cr concentrations in rice tissues, as well as an increased distribution of Cr in the iron plaque that acted as a barrier to reduce Cr transfer to the rice roots. These results indicate that continuous flooding irrigation was effective in minimizing the accumulation of Cr in rice plants, as it not only enhanced Cr(VI) reduction in the soil but also improved the blocking capacity of the iron plaque.

Combined effects of rice straw-derived biochar and water management on transformation of chromium and its uptake by rice in contaminated soils.

Chromium (Cr) pollution in soil is a global problem owing to its wide industrial use. The mobility, toxicity, and crop uptake of Cr depends on its valence state. Cr(VI) is highly mobile and toxic whereas Cr(III) is generally considered immobile and less toxic. We performed a pot experiment to investigate the combined effects of rice straw-derived biochar and water management on transformation of Cr and its uptake by rice in contaminated soils. The main plots had water management treatments of alternating wetting and drying (AWD) and continuous flooding (CF), and the subplots had three levels of straw biochar (0, 5, and 10 g kg-1). The results showed that water management and the addition of biochar had a significant effect on the dynamics of soil redox potential (Eh), pH, dissolved organic carbon (DOC), and Fe(II) concentration. As these parameters are important factors affecting Cr transformation in paddy soils, the dynamics of the Cr(III) and Cr(VI) concentrations were clearly different under different treatments. The highest reduction of Cr(VI) was observed in the treatment with CF water management in combination with 10 g kg-1 of biochar amendment, which resulted in a 62% reduction of Cr(VI) to Cr(III) in soil. The alterations in the oxidation state of Cr greatly affected its accumulation in the rice grains. The CF combined with 10 g kg-1 of biochar treatment, caused the Cr concentration in rice grains to be 66.2% lower compared with that of the unamended control under AWD water management. Possibly owing to the reduction in phytotoxic effects of Cr(VI), the combined treatment showed an improvement in rice grain weight. In conclusion, the combination of 10 g kg-1 of biochar amendment and CF water management may potentially be used in Cr-contaminated soil to mitigate the impacts of Cr contamination on rice production.

Evaluation of cadmium (Cd) transfer from paddy soil to rice (Oryza sativa L.) using DGT in comparison with conventional chemical methods: derivation of models to predict Cd accumulation in rice grains.

The consumption of rice contaminated with soil cadmium (Cd) threatens human health. It is essential to ensure the production of rice that meets food quality standards. Therefore, a large-scale field survey was conducted in Zhejiang province, southeastern China, to investigate the relationship between Cd accumulation in rice grains and Cd bioavailability in soil, and thus to establish a model to predict Cd contents in rice grains based on soil properties. For this purpose, a total of 156 paired rice and soil samples were collected. Pearson's correlation analysis revealed that Cd measurements obtained by diffusive gradient in thin films (DGT) had a higher correlation (r = 0.818, p < 0.001) with the Cd in rice grains as compared to the Cd measured by the DTPA, CaCl2, EDTA, and HCl extraction methods, which indicated that the DGT technique was a reliable method for the assessment of Cd bioavailability in soils. In addition, among the four extraction methods, the DTPA-extractable Cd showed the highest correlation with the Cd contents in rice grains. Therefore, we developed two predictive models (modelDGT and modelDTPA) to predict Cd levels in rice grains via Cubist multivariate mixed linear regression, using "soil DGT-measured Cd, pH, and oxide contents of Ca, Si, and Fe" or "soil DTPA-extractable Cd, pH, OM, and oxide contents of Ca and Fe" as explanatory variables, respectively. The overall modelDGT and modelDTPA had R2 values of 0.95 and 0.93, respectively, and relative error values of 0.30 and 0.33, respectively. Simple correlation analysis showed direct and close relationships between the measured Cd in rice grains and the Cd concentrations predicted by the Cubist modelDGT and modelDTPA, with R2 values of 0.979 and 0.922, respectively. Therefore, Cd levels in rice grains could be predicted very well based on the two prediction models, and thus, the two models derived in this study are effective in identifying soils in which the Cd in rice grains will exceed food safety standards, thereby helping to ensure safe rice production.

The effect of sepiolite application on rice Cd uptake - A two-year field study in Southern China.

Sepiolite (SEP) is a clay mineral with great potential to stabilize soil heavy metals. A two-year field experiment was conducted to explore the optimum use of SEP to immobilize soil Cd and to promote the consumption safety of rice grown in a typical paddy field in Southern China. SEP was applied once or twice over the two-year study at three levels (0.1, 0.5, and 1%, w/w) before rice planting. The results showed that SEP effectively reduced rice grain Cd concentrations by 47-49% in the first year and by 44-50% in the second year due to the residue effect. Application of SEP for two consecutive years reduced the rice grain Cd concentration by up to 75%, achieving a safe level (<0.2 mg kg-1). SEP also reduced Zn concentrations in rice grains (by 6-10%), while the Cd/Zn ratios of rice grains were decreased by 24-72% over the two years, implying it was much safer for consumption. SEP significantly increased the soil pH (0.9-1.8 units) and available phosphorus, and it reduced the soil available Cd (by 20-95%) and Zn concentrations (by 30-99%). In brief, SEP effectively stabilized soil Cd and reduced uptake by rice; the effect was dose-dependent and 0.5% (w/w) was optimum in the present study. The main mechanism of SEP to stabilize soil Cd is the increase in soil pH analogous to liming. This study shows that SEP application can be an efficient way to remediate Cd contaminated rice paddies and fulfill the goal of safe production of rice and thereby reduce the health risks associated with consuming rice.

Evaluation of cadmium transfer from soil to leafy vegetables: Influencing factors, transfer models, and indication of soil threshold contents.

Food chain contamination by soil cadmium (Cd) through leafy vegetable consumption poses a threat to human health. It is imperative to understand the relationship between Cd phytoavailability in soils and its uptake in common leafy vegetables. A large-scale field survey in Zhejiang Province, southeast China, was conducted to develop models to evaluate the Cd phytoavailability to leafy vegetables based on soil properties and to establish soil Cd thresholds based on food safety. The empirical models developed in this study explained the combined effects of soil properties and diethylenetriaminepentaacetic acid (DTPA)-extractable Cd content on Cd phytoavailability to leafy vegetables. The Cd accumulation in celery, pak choi, and amaranth was quantitatively predicted by measurement of DTPA-extractable soil Cd and soil pH, organic matter, cation exchange capacity and clay content. For predicting Cd accumulation, the DTPA-extractable Cd, pH and clay content had a major influence in lettuce; and for water spinach, the DTPA-extractable Cd, pH, and cation exchange capacity had a major influence. Soil DTPA-extractable Cd was suitable to be used as Cd thresholds in soils cultivating celery, amaranth, pak choi, lettuce, and water spinach, with values of 0.24, 0.13, 0.23, 0.32, and 0.37 mg kg-1, respectively. However, the threshold values of soil total Cd were 0.26, 0.34, and 0.83 mg kg-1 for amaranth, celery, and pak choi fields, indicating that the current soil quality standard (GB 15618-1995) for soils cultivating different types of vegetables could be overestimated or underestimated for Cd contamination and the associated risk. This study will provide a useful reference for controlling Cd contamination in common leafy vegetables and developing sustainable production of leafy vegetables.

Enhancement of Cd phytoextraction by hyperaccumulator Sedum alfredii using electrical field and organic amendments.

The combined use of organic amendment-assisted phytoextraction and electrokinetic remediation to decontaminate Cd-polluted soil was demonstrated in a laboratory-scale experiment. The plant species selected was the hyperaccumulator Sedum alfredii. Prior to the pot experiment, the loamy soil was treated with 15 g kg-1 of pig manure compost, 10 g kg-1 of humic acid, or 5 mmol kg-1 of EDTA, and untreated soil without application of any amendment was the control. Two conditions were applied to each treatment: no voltage (without an electrical field) and a direct current (DC) electrical field (1 V cm-1 with switching polarity every day). Results indicated that Cd concentrations in S. alfredii were significantly (p < 0.05) increased by application of the electrical field and soil amendments (pig manure compost, humic acid, and EDTA). By switching the polarity of the DC electrical field, significant pH variation from anode to cathode can be avoided, and no significant impact was observed on shoot biomass production. Electrical field application increased DTPA-extractable Cd in soils and the Cd accumulation in shoots by 6.06-15.64 and 24.53-52.31%, respectively. The addition of pig manure compost and humic acid enhanced shoot Cd accumulation by 1.54- to 1.92- and 1.38- to 1.64-fold because of their simultaneous enhancement of Cd concentration in shoots and biomass production. However, no enhancement of Cd accumulation was found in the EDTA treatment, which can be ascribed to the inhibition of plant growth caused by EDTA. In conclusion, pig manure compost or humic acid addition in combination with the application of a switched-polarity DC electrical field could significantly enhance Cd phytoextraction by hyperaccumulator S. alfredii.

The effect of biochar and crop straws on heavy metal bioavailability and plant accumulation in a Cd and Pb polluted soil.

Biochar derived from various materials has been investigated with regard to its ability to decrease the bioavailability of heavy metals in contaminated soils, and thus reduce their potential to enter the food chain. However, little attention has been given to the adsorption capacity of untreated crop straws, which are commonly used as a biochar feedstock, especially in soils. Hence, this study was conducted to investigate the effect of crop straws on heavy metal immobilization and subsequent heavy metal uptake by maize and ryegrass in a soil artificially polluted by Cd and Pb. Bamboo biochar, rice straw, and wheat straw were mixed into soil four weeks before the experiment, enabling them to reach equilibrium at 2% (w/w), 1% (w/w), and 1% (w/w), respectively. The results showed that soil pH for both species was significantly increased by all treatments, except when wheat straw was used for ryegrass cultivation. Soil organic carbon was only improved in the rice straw treatment and the soil alkali-hydrolyzable N content was significantly decreased with all of the amendments, which may have contributed to the lack of an effect on plant biomass. Soil available Cd was significantly lower in the rice straw treatment than in the control soil, while Pb levels clearly decreased in wheat straw treatment. The Cd concentration in shoots of maize was reduced by 50.9%, 69.5%, and 66.9% with biochar, rice straw, and wheat straw, respectively. In addition, shoot Cd accumulation was decreased by 47.3%, 67.1%, and 66.4%, respectively. Shoot Pb concentration and accumulation were only reduced with the rice straw treatment for both species. However, metal uptake in plant roots was more complex, with increased metal concentrations also detected. Overall, the direct application of crop straw could be considered a feasible way to immobilize selected metals in soil, once the long-term effects are confirmed.

Assessment of heavy metal pollution in vegetables and relationships with soil heavy metal distribution in Zhejiang province, China.

There are increasing concerns on heavy metal contaminant in soils and vegetables. In this study, we investigated heavy metal pollution in vegetables and the corresponding soils in the main vegetable production regions of Zhejiang province, China. A total of 97 vegetable samples and 202 agricultural soil samples were analyzed for the concentrations of Cd, Pb, As, Hg, and Cr. The average levels of Cd, Pb, and Cr in vegetable samples [Chinese cabbage (Brassica campestris spp. Pekinensis), pakchoi (Brassica chinensis L.), celery (Apium graveolens), tomato (Lycopersicon esculentum), cucumber (Colletotrichum lagenarium), cowpea (Vigna unguiculata), pumpkin (Cucurbita pepo L.), and eggplant (Solanum melongena)] were 0.020, 0.048, and 0.043 mg kg(-1), respectively. The Pb and Cr concentrations in all vegetable samples were below the threshold levels of the Food Quality Standard (0.3 and 0.5 mg kg(-1), respectively), except that two eggplant samples exceeded the threshold levels for Cd concentrations (0.05 mg kg(-1)). As and Hg contents in vegetables were below the detection level (0.005 and 0.002 mg kg(-1), respectively). Soil pollution conditions were assessed in accordance with the Chinese Soil Quality Criterion (GB15618-1995, Grade II); 50 and 68 soil samples from the investigated area exceeded the maximum allowable contents for Cd and Hg, respectively. Simple correlation analysis revealed that there were significantly positive correlations between the metal concentrations in vegetables and the corresponding soils, especially for the leafy and stem vegetables such as pakchoi, cabbage, and celery. Bio-concentration factor values for Cd are higher than those for Pb and Cr, which indicates that Cd is more readily absorbed by vegetables than Pb and Cr. Therefore, more attention should be paid to the possible pollution of heavy metals in vegetables, especially Cd.

Effects of alternating wetting and drying versus continuous flooding on chromium fate in paddy soils.

Anthropogenic chromium (Cr) pollution in soils poses a great threat to human health through the food chain. It is imperative to understand Cr fate under the range of conditions suitable for rice growth. In this study, the effects of irrigation managements on dynamics of porewater Cr(VI) concentrations in rice paddies and Cr distribution in rice were investigated with pot experiments under greenhouse conditions. Soil redox potential in continuous flooding (CF) treatments showed that reducing conditions remained for the whole duration of rice growing period, while soil redox potential in alternating wetting and drying (AWD) treatments showed that soil conditions alternately changed between reducing and oxic. As soil redox potential is an important factor affecting Cr(VI) reduction in paddy soils, dynamics of Cr(VI) concentration were clearly different under different irrigation managements. In CF treatments, porewater Cr(VI) concentrations decreased with time after planting, while in AWD treatments porewater Cr(VI) concentrations were increased and decreased alternately response to the irrigation cycles. Chromium(VI) concentrations in the CF treatments were lower than those in AWD treatments for most part of rice-growing season. Moreover, Cr concentrations in rice tissues were significantly influenced by irrigation with relatively higher values in the AWD treatments, which might be attributed to the higher porewater Cr(VI) concentrations in AWD treatments. Therefore, it would be better to use CF than AWD management in Cr-contaminated paddy soils to reduce Cr accumulation in rice, and thus to reduce the potential risk to human health.

[Effect of DMPP on inorganic nitrogen runoff loss from vegetable soil].

The effect of urea with 1% 3,4-dimethyl pyrazole phosphate (DMPP) on inorganic nitrogen runoff loss from agriculture field was determined in an undisturbed vegetable soil by using the simulated artificial rainfall method. The results show that, during the three simulated artificial rainfall period, the ammonium nitrogen content in the runoff water is increased 1.42, 2.82 and 1.95 times with the DMPP application treatment compared to regular urea treatment, respectively. In the urea with DMPP addition treatment, the nitrate nitrogen content is decreased 70.2%, 59.7% and 52.1% in the three simulated artificial rainfall runoff water, respectively. The nitrite nitrogen content is also decreased 98.7%, 90.6% and 85.6% in the three simulated artificial rainfall runoff water, respectively. The nitrate nitrogen and nitrite nitrogen runoff loss are greatly declined with the DMPP addition in the urea. Especially the nitrite nitrogen is in a significant low level and is near to the treatment with no fertilizer application. The inorganic nitrogen runoff loss is declined by 39.0% to 44.8% in the urea with DMPP addition treatment. So DMPP could be used as an effective nitrification inhibitor to control the soil ammonium oxidation, decline the nitrogen runoff loss, lower the nitrogen transformation risk to the waterbody and be beneficial for the ecological environment.