Remobilization Of Cd Research Articles

Effects of iron-modified biochar with S-rich and Si-rich feedstocks on Cd immobilization in the soil-rice system

Fe-modified biochar has been shown to have high sorption ability for cadmium (Cd), while Cd immobilization effects of Fe-modified biochars with Si-rich and S-rich feedstocks have been rarely addressed. To explore the effects of Fe-modified Si-rich and S-rich biochars on Cd translocation in the soil-rice system, a pot experiment was carried out with an acidic Cd-contaminated sandy loam paddy from central South China and a late season rice cultivate during July to November 2018. Rice straw and rice husk were chosen as Si-rich feedstocks, and rape straw was applied as S-rich feedstock, these feedstocks were further collected and pyrolyzed at 450 °C. Pristine and Fe-impregnated rice straw (BRS/BRS-Fe), rice husk (BRH/BRH-Fe) and rape straw (BRE/BRE-Fe) biochars were applied at 0 and 10 t/ha, respectively. The reductions in Cd concentrations in rice grains were 23.8%, 22.3% and 46.1% with treatments of BRE, BRS and BRH, respectively, compared to the control. Compared to other pristine biochars, BRH is more effective in Cd remediation in paddy soil. For Fe-modified biochars, BRE-Fe achieved the highest reductions in Cd concentrations in rice grains with 46.7% and 30.1%, compared with the control and BRE, respectively. BRE-Fe decreased Cd remobilization from leaves to grains. Only BRE-Fe enhanced the formation and Cd sorption capacity of iron plaque. BRS-Fe and BRH-Fe enhanced Fe content in rice plants, which might induce the reduction in iron plaque formation. Fe and S-contained complexes contents increased in the contaminated pristine biochar particles, but reduced in the contaminated BRE-Fe particles. Therefore, Fe modification could not enhance Cd immobilization effect of Si-rich biochar, while Fe modified S-rich biochar has promising potential for Cd remediation with enhancement in iron plaque formation and Cd fixation in rice leaves.

Free Radicals Produced from the Oxidation of Ferrous Sulfides Promote the Remobilization of Cadmium in Paddy Soils During Drainage.

Most of the cadmium (Cd) accumulated in rice grains is derived from its remobilization in soils during the grain filling period when paddy water is drained. The factors affecting Cd remobilization upon drainage remain poorly understood. Here, we show that the free radical effect produced from the oxidation of ferrous sulfides is an important mechanism affecting the oxidative remobilization of Cd during soil drainage. When soils were flooded, microbial sulfate reduction results in the formation of various metal sulfides including CdS and FeS. Upon soil drainage, the oxidation of FeS produced considerable amounts of hydroxyl free radicals (OH•), which could oxidize CdS directly and thereby promote the oxidative dissolution of CdS and increase Cd mobilization in soils. FeS and CdS could also form a within-sulfide voltaic cell, with FeS protecting the oxidative dissolution of CdS due to the lower electrochemical potential of the former. However, this voltaic effect was short-lived and was surpassed by the free radical effect. The amounts and composition of metal sulfides formed during soil flooding vary with soils, and the oxidative dissolution of CdS is affected by both the free radical and voltaic effects offered by different metal sulfides. These effects are also applicable to the biogeochemistry of other chalcophile trace elements coupled with sulfur and iron redox cycles during the anoxic-oxic transition in many environments.

Ammonium-nitrogen addition at the seedling stage does not reduce grain cadmium concentration in two common wheat (Triticum aestivum L.) cultivars

High cadmium (Cd) concentration in common wheat (Triticum aestivum L.) grains poses potential health risks. Several management strategies have been used to reduce grain Cd concentration. However, limited information is available on the use of ammonium-nitrogen (NH4+-N) as a strategy to manage Cd concentration in wheat grains. In this study, NH4+-N addition at the seedling stage unchanged the grain Cd concentration in the high-Cd accumulator, Zhoumai 18 (ZM18), but dramatically increased that in the low-Cd accumulator, Yunmai 51 (YM51). Further analysis revealed that the effects of NH4+-N addition on whole-plant Cd absorption, root-to-shoot Cd translocation, and shoot-to-grain Cd remobilization were different between the two wheat cultivars. In ZM18, NH4+-N addition did not change whole-plant Cd absorption, but inhibited root-to-shoot Cd translocation and Cd remobilization from lower internodes, lower leaves, node 1, and internode 1 to grains via the down-regulation of yellow stripe-like transporters (YSL), zinc transporters (ZIP5, ZIP7, and ZIP10), and heavy-metal transporting ATPases (HMA2). This inhibition decreased the grain Cd content by 29.62%, which was consistent with the decrease of the grain dry weight by 23.26%, leading to unchanged grain Cd concentration in ZM18. However, in YM51, NH4+-N addition promoted continuous Cd absorption during grain filling, root-to-shoot Cd translocation and whole-plant Cd absorption. The absorbed Cd was directly transported to internode 1 via the xylem and then re-transported to grains via the phloem by up-regulated YSL, ZIP5, and copper transporters (COPT4). This promotion increased the grain Cd content by 245.35%, which was higher than the increased grain dry weight by 132.89%, leading to increased grain Cd concentration in YM51. Our findings concluded that the addition of NH4+-N fertilizer at the seedling stage is not suitable for reducing grain Cd concentration in common wheat cultivars.

Nitrogen fertilizer affects rhizosphere Cd re-mobilization by mediating gene AmALM2 and AmALMT7 expression in edible amaranth roots

In-situ stabilization of Cd-contaminated farmland is a commonly used remediation technology. Yet, rhizosphere metabolites (e.g., organic acids) during crop cultivation may cause Cd re-mobilization and over-accumulation. Here, we identified four pivotal cytomembrane-localized genes underlying Cd accumulation difference between two contrasting edible amaranth cultivars based on root gene expression profile, studied their subcellular localization and functional characteristics, and then investigated effects of nitrogen fertilizer on their expression and rhizosphere Cd re-mobilization. Results showed that more Cd accumulated by edible amaranth was due to rhizosphere Cd mobilization by mediating high expression of AmALMT2 and AmALMT7 genes, not Cd transporters in roots. This was confirmed by heterologous expression of AmALMT2 and AmALMT7 genes in Arabidopsis thaliana, since they mediated malic, fumaric, succinic, and aspartic acids efflux. Furthermore, nitrogen influencing rhizosphere acidification might be closely associated with organic acids efflux genes. Compared with N-NO3- application, N-NH4+ was massively assimilated into glutamates and oxaloacetates through up-regulating glutamine synthetase and alanine-aspartate-glutamate metabolic pathways, thereby enhancing TCA cycle and organic acids efflux dominated by binary carboxylic acids via up-regulating AmALMT2 and AmALMT7 genes, which finally caused Cd re-mobilization. Therefore, N-NO3--dominated nitrogen retarded rhizosphere Cd re-mobilization via inhibiting organic acids efflux function of AmALMT2 and AmALMT7 proteins.

Biogeochemical Control on the Mobilization of Cd in Soil

Cadmium (Cd) is a toxic element that can easily enter the human body through the food chain. Rice grain is the main contributor to dietary Cd intake, especially for those populations who consume rice as the staple. Therefore, reducing Cd accumulation in rice is of significance for food safety and human health. In this review, we summarized the major factors underlying the biogeochemical processes controlling Cd availability in soils. Soil pH is one of the most important factors affecting soil Cd availability. An increase in soil pH can dramatically decrease soil Cd solubility; thus, liming is often found to be an effective and economical method in acidic contaminated soils to reduce grain Cd accumulation. The voltaic effect between Cd-sulfide and other metal sulfides formed during soil flooding regulates the remobilization of Cd during soil drainage. Therefore, addition of small amounts of ZnSO4 or MnSO4 to soils can form a protective voltaic cell partner against the oxidative dissolution of Cd sulfides, thereby decreasing Cd remobilization during soil drainage and Cd accumulation in rice grain. The voltaic effect is expected to be more important than their direct competition between Zn2+ and Cd2+. Besides, water management practices markedly affect soil Cd availability by altering the redox status of soils; continuous flooding and delaying drainage of paddy soils during the grain filling period can effectively reduce grain Cd accumulation. These effective biogeochemical strategies can be used singly or combinedly to reduce soil Cd availability and subsequent Cd uptake/accumulation in rice to ensure food safety.

Nitrogen fertilizer management affects remobilization of the immobilized cadmium in soil and its accumulation in crop tissues.

Immobilization of soil cadmium (Cd) has been the strategy mostly used in remediation of Cd-contaminated arable soil. However, Cd might be remobilized after the immobilization process through the acid-soluble and complexation effects. Development of agronomic management technologies to prevent soil Cd remobilization after the immobilization process was an important pathway to control the food safety of agricultural products in soils with the immobilized Cd. In this study, the ammonia (NH4+-N) and nitrate (NO3--N) forms with concentrations of 60, 90, and 150 mg-N kg-1 soil were performed for evaluating their effects on Cd remobilization with planted or unplanted treatments and Cd accumulation in tissues of edible amaranth (Liuye). With an initial soil palygorskite-bound fraction Cd concentration of 0.6 mg kg-1, bioavailable Cd in rhizosphere soils and Cd in crop shoots respectively increased from 11.4 to 20.6 μg kg-1 (dry soil weight) and 6.92 to 14.92 mg kg-1 (dry plant weight) in planted NH4+-N treatments, while significantly lower concentrations of bioavailable Cd in rhizosphere soils and Cd in crop tissues were observed with planted NO3--N treatments. Compared with that of planted NO3--N treatments, decreasing pH value (i.e., 7.64 to 7.18) induced by root proton efflux during the absorption of NH4+-N, enhancive organic/amino acid (oxalic acid, lactic acid, L-proline, and so on) secretion from roots, and increasing abundance of bacteria distributed in phyla Proteobacteria, Cyanobacteria, and Bacteroidetes with Cd mobilization ability in rhizosphere soils were the main reasons found in this study for the higher Cd remobilization in soils and Cd accumulation in crop under NH4+-N treatments. Moreover, the direct effect of NH4+-N on remobilization of immobilized Cd by upregulating the expression abundances of genes associated with pyruvate metabolism and amino acids metabolism was more significant than that of NO3--N. In summary, the use of NO3--N as preferred N fertilizer was more efficient to ensure the food safety of agricultural products than that of NH4+-N in Cd-contaminated arable soil after immobilization process.

The Voltaic Effect as a Novel Mechanism Controlling the Remobilization of Cadmium in Paddy Soils during Drainage.

Excessive cadmium (Cd) accumulation in rice grain is a global issue that affects human health. The drainage of paddy soils during the grain filling period leads to the remobilization of Cd in soils, resulting in most of the Cd accumulated in rice grain. The rate of Cd remobilization during drainage differs markedly among soils, but the mechanisms underlying these differences remain largely unknown. Using microcosm soil incubation, electrochemical experiments, isotope labeling, and microscopic and spectroscopic analyses, here, we discover the voltaic effect as a novel mechanism controlling the remobilization of Cd during soil drainage. During soil flooding, microbial sulfate reduction results in the formation of various metal sulfides. When the soils are subsequently drained, the various metal sulfides can form within sulfide voltaic cells. The metal sulfides with a lower electrochemical potential act as anodes and are prone to oxidative dissolution, whereas the metal sulfides with a higher potential act as cathodes and are protected from oxidation. This voltaic effect explains why the presence of ZnS (with a low potential) suppresses the oxidative dissolution of Cd sulfides, whereas the presence of CuS (with a high potential) promotes the oxidative dissolution of Cd sulfides. The voltaic effect is applicable to all chalcophile trace metals coupled with the sulfur redox cycle in periodically anoxic-oxic environments, thus playing an important role in the biogeochemistry of trace metals.

Long-term effects of low-molecular-weight organic acids on remobilization of Cd, Cr, Pb, and As in alkaline coastal wetland soil

ABSTRACT Low-molecular-weight organic acids (LMWOAs) that have a wide variety of biological activities can affect the mobilization of heavy metals in soil. In this study, the effects of three kinds of LMWOAs on the remobilization of Cd, Cr, Pb, and As were investigated. The results showed that the concentrations of Cr, Pb, and As reached the maximum values at 7 d, and then decreased at 58 d, indicating that the LMWOAs induced the release of these elements from soil. The ability of LMWOAs to affect the mobility of Cr and As followed the order citric acid (1.32 and 2.51mg/L) > malic acid > oxalic acid, while the order for Pb was malic acid (4.49mg/L) > citric acid > oxalic acid. In addition, the sequential extraction indicated that the LMWOAs advanced the reactivation of Pb, Cd, and As, but led to the immobilization of Cr after a long-term reaction.

Crucial roles of cadmium retention in nodeⅡ for restraining cadmium transport from straw to ear at reproductive period in a grain low-cadmium rice line (Oryza sativa L.)

Using an ideal parental line to breed hybrid rice with low cadmium (Cd) accumulation in grain is an environmental-friendly approach to reduce the risk of Cd contamination in field. A grain low-Cd rice line YaHui2816 has stably low Cd in grain but strong Cd accumulation in straw, revealing specific pattern of its straw-grain Cd remobilization is beneficial to effectively breed hybrid rice for safe production as well as phytoremediation. In this study, a pot experiment was conducted to investigate Cd partitioning of YaHui2816 at different stages by comparison with a common rice C268A. The Cd from lower nodes and leaves was restricted in nodeⅡ, this Cd retention led to weak Cd transport from straw to ear in YaHui2816. Cd concentration in nodeⅡ of YaHui2816 was 1.56-fold and 7.36-fold higher than C268A at filling and mature stages. Thus, glutathione (GSH) and phytochelatin (PC) concentration, PC synthase (OsPCS1), GSH S-Transferase (OsGST) and Cd-remobilization associated genes were measured at filling stage. OsHMA2 and OsCCX2 were up-regulated in nodeⅡ of YaHui2816, relative expression of them were significantly higher than C268A. PCs participated in Cd remobilization process, remarkable PC increase in nodeⅡ of YaHui2816 was found in response to Cd treatment under regulation of OsPCS1 and OsGST of which PC2 was dominant form. Taken as a whole, the Cd retention in nodeⅡ of YaHui2816 acts as a ‘firewall’ to restrain Cd transport to grain. This work provides more insight to understand possible function of alleles for reducing Cd concentration in grain as well as strengthening Cd accumulation in straw of YaHui2816.

Cadmium allocation to grains in durum wheat exposed to low Cd concentrations in hydroponics

This study aims to characterize the response of durum wheat to different concentrations of Cd found in agricultural soils. One French durum wheat cultivar (i.e. Sculptur) was exposed to low concentrations of Cd (5 nM or 100 nM) in hydroponics. After anthesis, the plants were fed with a solution enriched with the stable isotope 111Cd to trace the newly absorbed Cd. Plants were sampled at anthesis and grain maturity to assess how plant growth, Cd uptake and partitioning among organs, as well as Cd remobilization, differed between the two Cd exposure levels. Durum wheat did not show any visual symptoms of Cd toxicity, regardless of which Cd treatment was applied. However, post-anthesis durum wheat growth was 14% penalized at 100 nM due to the large transpiration-based accumulation of Cd in leaves at this stage. The allocation of Cd to the grains was not restricted but enhanced at 100 nM compared to 5 nM. Both the root-to-shoot Cd translocation and the fraction of aboveground Cd allocated to grains were higher in plants exposed to 100 nM. Cadmium was remobilized exclusively from roots and stems, and remobilized Cd contributed on average to 40–45% of the Cd accumulated in mature grains, regardless of which Cd treatment was applied. The relevance of these results to decreasing the concentration of Cd in durum wheat grains is discussed.

Interactive effects between cadmium stabilized by palygorskite and mobilized by siderophores from Pseudomonas fluorescens

The application of palygorskite (PAL) for potentially toxic trace elements (Cd2+, Ni2+, etc.) remediation in polluted soil can substantially reduce the bioavailability and toxicity of these hazard materials. However, the secretion of organic acids and siderophores by microorganisms might result in the re-mobilization of cadmium (Cd) in PAL-bound forms (PAL-Cd). In this study, the interactive effects between Cd stabilized by PAL and mobilized by siderophores from Pseudomonas fluorescens were performed with four flask-shaking experimental treatments, namely, strain with or without an ability of siderophores production respectively associated with or without PAL-Cd. The GC-MS and UHPLC-MS test methods were used to analyze the concentrations of metabolites. Results showed that the Cd mobilized by strain with siderophores production was 22.1% higher than that of strain without the ability of siderophores production (p < 0.05). The mobilization of Cd in PAL in turn significantly reduced the siderophores production of Pseudomonas fluorescens by 25.1% (p < 0.05). The numbers of metabolites significantly up-regulated and down-regulated were 9 and 22 in strain groups with PAL-Cd addition compared with the groups without PAL-Cd, respectively. Metabolomics analysis revealed that the mobilized Cd affects the signal transduction pathway and primary metabolic processes, reduces the metabolic capacity of pentose phosphate pathway, glycolysis and tricarboxylic acid cycle pathway. These changes inhibit the ability of strain to biosynthesize amino acids during the mobilization processes, further reducing the capacity of Pseudomonas fluorescens to produce siderophores. This study provides a useful information on how to select soil Cd-stabilizing materials in a targeted manner and how to avoid Cd re-mobilization by siderophores.

Cadmium retention and distribution in contaminated soil: effects and interactions of soil properties, contamination level, aging time and in situ immobilization agents

As soil cadmium (Cd) contamination becomes a serious concern and one of the significant environmental pollution issues all over the world, knowledge of the basic chemistry, origin, inputs, sources, quantity, chemical forms, reactions, as well as the fate and transport of Cd in different types of soil is crucial for better understanding Cd bioavailability, health risks and remedial options. This study aimed to increase the current knowledge on the complex interdependence between the factors affecting behavior, transport and fate of Cd in the soil and to test and compare the performance of the stabilization agents in different soil types. Soils demonstrated various sorption affinity and capacity for Cd accumulation, which proved to be positively correlated with soil pH and the cation exchange capacity (CEC). With increasing levels of contamination, sequential extraction analysis showed the highest increase of relative Cd amounts in the exchangeable fraction regardless of the soil properties, suggesting that added Cd is principally associated with the easily accessible and mobile fraction. For different initial Cd concentrations and soil types, Cd sorption reached the quasi-equilibrium within 24 h of contact. Prolonged aging (two months) influenced the natural stabilization of Cd in all types of soil, but only at low contamination level. The application of both, conventional (slaked lime Ca(OH)2) and alternative phosphate-rich (annealed bovine bones B400) amendments, resulted in Cd relocation and reduction of exchangeable Cd content. Although the effect was smaller when apatite amendment was utilized, observed re-distribution of Cd to more stable soil fractions is preferable for achieving long-term stabilization. Cd concentrations extracted in exchangeable and acid soluble fractions after the treatments of contaminated soil samples suggest that the practical applicability of in situ immobilization depends on the soil properties and the level of contamination, as well as that effect, should be monitored for the possible re-mobilization of Cd.

Accumulation and distribution of arsenic and cadmium in winter wheat (Triticum aestivum L.) at different developmental stages

Arsenic (As) and cadmium (Cd) are known to be toxic to humans, and elevated concentrations have been documented in food crops worldwide. However, little is known regarding their uptake, translocation, and distribution in wheat plants during plant development. A series of experiments were conducted to investigate the spatial distribution and dynamics of As and Cd in two wheat cultivars (cv. JN12 and JM85; the latter is a low grain Cd accumulator) at different developmental stages. Root concentrations of As decreased by 84%, and those of Cd by 67%, from tillering to maturity. In contrast, As concentrations in the stems increased 3.1-fold. A significant decrease in root As accumulation was observed at the mature stage, whereas root Cd accumulation decreased largely at the elongation stage. The concentrations of Cd in all leaves and As in new leaves increased as plant growth advanced. However, As concentrations in old leaves decreased significantly from grain filling to maturity. In both cultivars, the upward transfer toward younger parts of shoots was greater in the case of Cd than of As. The remobilization of As and Cd from stems and roots differed between the two cultivars. Arsenic concentrations in rachis, glumes, and grain in JM85 were significantly higher than those in JN12, whereas As concentrations in roots and stems did not differ between the cultivars. Grain Cd was significantly higher in JN12 than in JM85, but Cd concentrations in rachis and glumes were similar between the cultivars. The difference in grain Cd concentration between the two cultivars depended on root and stem Cd remobilization and redistribution from rachis to glumes and grain; in contrast, accumulation of As in grain was influenced by As remobilization from the leaves and stem to the spike.

Characteristics of Cd accumulation and distribution in two sweet potato cultivars

In this study, we compared the chemical forms and subcellular distribution of Cd in high-Cd (X16) and low-Cd (N88) sweet potato cultivars through hydroponic experiments and examined the Cd distribution in their roots by histochemical staining. The results showed that inorganic and pectate/protein-integrated Cd predominated in the leaves, and Cd concentrations were significantly higher in X16 than in N88. However, in the roots, Cd was mostly integrated with pectate and protein, and Cd concentration was higher in N88 than in X16. It was mainly stored through vacuolar sequestration and cell wall binding. In the leaves and stems, Cd concentrations in all subcellular fractions were higher in X16 than in N88; the opposite was observed in the roots. In X16, Cd was mostly accumulated in the root stele, and its Cd translocation factor was higher than that of N88. Overall, the subcellular fractions of X16 roots retained less Cd than N88 roots, and more Cd entered the root stele of X16 and subsequently moved to the shoots. The higher amounts of inorganic, water-soluble, and pectate/protein-integrated Cd with high mobility in the shoots of X16 than in N88 might facilitate Cd remobilization to other tissues, but this needs to be further studied.

P.2.22 Finding new secretomes for Parkinson's disease regenerative medicine applications

The application of palygorskite (PAL) for potentially toxic trace elements (Cd2+, Ni2+, etc.) remediation in polluted soil can substantially reduce the bioavailability and toxicity of these hazard materials. However, the secretion of organic acids and siderophores by microorganisms might result in the re-mobilization of cadmium (Cd) in PAL-bound forms (PAL-Cd). In this study, the interactive effects between Cd stabilized by PAL and mobilized by siderophores from Pseudomonas fluorescens were performed with four flask-shaking experimental treatments, namely, strain with or without an ability of siderophores production respectively associated with or without PAL-Cd. The GC-MS and UHPLC-MS test methods were used to analyze the concentrations of metabolites. Results showed that the Cd mobilized by strain with siderophores production was 22.1% higher than that of strain without the ability of siderophores production (p < 0.05). The mobilization of Cd in PAL in turn significantly reduced the siderophores production of Pseudomonas fluorescens by 25.1% (p < 0.05). The numbers of metabolites significantly up-regulated and down-regulated were 9 and 22 in strain groups with PAL-Cd addition compared with the groups without PAL-Cd, respectively. Metabolomics analysis revealed that the mobilized Cd affects the signal transduction pathway and primary metabolic processes, reduces the metabolic capacity of pentose phosphate pathway, glycolysis and tricarboxylic acid cycle pathway. These changes inhibit the ability of strain to biosynthesize amino acids during the mobilization processes, further reducing the capacity of Pseudomonas fluorescens to produce siderophores. This study provides a useful information on how to select soil Cd-stabilizing materials in a targeted manner and how to avoid Cd re-mobilization by siderophores.

Effect of Acidic Industrial Effluent Release on Microbial Diversity and Trace Metal Dynamics During Resuspension of Coastal Sediment.

Both industrial effluent discharge and the resuspension of contaminated marine sediments are important sources of trace metals in seawater which potentially affect marine ecosystems. The aim of this study was to evaluate the impact of the industrial wastewaters having acidic pH (2–3) and containing trace metals on microbial diversity in the coastal ecosystem of the Gulf of Gabès (Tunisia, southern Mediterranean Sea) subjected to resuspension events of marine sediments. Four trace elements (As, Cd, U, and V) were monitored during 10-day sediment resuspension experiments. The highest enrichment in the seawater dissolved phase was observed for Cd followed by U, V, and As. Cd remobilization was improved by indigenous microbial community, while U release was mainly abiotic. Acidic effluent addition impacted both trace metal distribution and microbial diversity, particularly that of the abundant phylum Bacteroidetes. Members of the order Saprospirales were enriched from sediment in natural seawater (initial pH > 8), while the family Flavobacteriaceae was favored by acidified seawater (initial pH < 8). Some Flavobacteriaceae members were identified as dominant species in both initial sediment and experiments with acidic wastewater, in which their relative abundance increased with increasing dissolved Cd levels. It could be therefore possible to consider them as bioindicators of metal pollution and/or acidification in marine ecosystems.

Stability of immobilization remediation of several amendments on cadmium contaminated soils as affected by simulated soil acidification

Chemical immobilization is a practical approach to remediate heavy metal contamination in agricultural soils. However, the potential remobilization risks of immobilized metals are a major environmental concern, especially in acid rain zones. In the present study, changes in the immobilization efficiency of several amendments as affected by simulated soil acidification were investigated to evaluate the immobilization remediation stability of several amendments on two cadmium (Cd) contaminated soils. Amendments (hydrated lime, hydroxyapatite and biochar) effectively immobilized Cd, except for organic fertilizer, and their immobilizations were strongly decreased by the simulated soil acidification. The ratio of changes in CaCl2-extractable Cd: pH (△CaCl2-Cd/△pH) can represent the Cd remobilization risk of different amended soils. Hydroxyapatite and biochar had a stronger durable immobilizing effect than did hydrated lime, particularly in soil with a lower pH buffering capacity, which was further confirmed by the Cd concentration and accumulation in lettuce. These results can be attributed to that hydroxyapatite and biochar transformed greater proportions of exchangeable Cd to other more stable fractions than lime. After 48 weeks of incubation, in soil with a lower pH buffering capacity, the immobilization efficiencies of lime, hydroxyapatite, biochar and organic fertilizer in the deionized water group (pH 6.5) were 71.7%, 52.7%, 38.6% and 23.9%, respectively, and changed to 19.1%, 33.6%, 26.5% and 5.0%, respectively, in the simulated acid rain group (pH 2.5). The present study provides a simple method to preliminarily estimate the immobilization efficiency of amendments and predict their stability in acid rain regions before large-scale field application. In addition, hydrated lime is recommended to be combined with other acid-stable amendments (such as hydroxyapatite or biochar) to remediate heavy metal-contaminated agricultural soils in acid precipitation zones.

Contribution of remobilization to the loading of cadmium in durum wheat grains: impact of post-anthesis nitrogen supply

This study focuses on quantifying the contribution of remobilization to the amount of cadmium accumulated in durum wheat grains. The impact of post-anthesis N supply was tested in two cultivars that differ in their shoot biomass partitioning. Two French durum wheat cultivars were grown hydroponically and exposed to 100 nM Cd. After anthesis, the plants were fed with a solution enriched in the stable isotope 111Cd to trace the Cd newly absorbed, and subjected or not to nitrogen deprivation. Plants were sampled at anthesis and grain maturity to assess the post-anthesis fluxes of Cd and N among organs. Cd remobilized from pre-anthesis stores contributed to more than half of the Cd accumulated in mature grains. Cd was mainly remobilized from stem and poorly remobilized from leaves. Stopping N supply during grain filling enhanced N remobilization but had no impact on post-anthesis uptake and remobilization of Cd, and thereby, on Cd concentration in grains. No difference was observed between the two cultivars in the contribution of Cd remobilization and its dependence toward post-anthesis N supply. Cd remobilization significantly contributes to the accumulation of Cd in durum wheat grains. Cd remobilization is not tightly linked with N remobilization and behaves like a senescent-independent process in durum wheat.

Cadmium uptake and partitioning in potato (Solanum tuberosum L.) cultivars with different tuber-Cd concentration.

Potatoes grown in soil with high Cd concentrations can accumulate high levels of Cd in the tubers. Although there is significant environmental variation involved in the trait of crop uptake of Cd, there are also distinctive cultivar differences. In order to understand this differential Cd accumulation mechanism, two potato cultivars were chosen that accumulate high and low levels of Cd in tubers. The patterns of Cd concentration, Cd content and dry weight accumulation of the two cultivars were examined at different stages of plant growth. The data suggest that differences in total Cd uptake and in Cd partitioning among organs are the mechanisms governing differential Cd-tuber accumulation in the two cultivars. The low tuber-Cd accumulator exhibited lower root-to-shoot and shoot-to-tuber translocation driven by higher root and shoot biomass that retained more Cd in roots and shoots, respectively, reducing its movement to the tubers. Higher remobilization and more efficient tuber loading was observed in the high tuber-Cd accumulator, indicating that remobilization of Cd from leaves to tubers was a major factor, not only in tuber-Cd loading, but also in the establishment of differential tuber-Cd levels. Regardless of cultivar differences, the concentration of Cd in the tuber was very low compared to that in other organs suggesting that, despite its high phloem mobility, Cd tends to be sequestered in the shoots.

Cadmium remobilization from shoot to grain is related to pH of vascular bundle in rice

The remobilization of cadmium (Cd) from shoots to grain is the key process to determine the Cd accumulation in grain. The apoplastic pH of plants is an important factor and signal in influencing on plant responding to environmental variation and inorganic elements uptake. It is proposed that pH of rice plants responds and influences on Cd remobilization from shoots to grain when rice is exposed to Cd stress. The results of hydroponic experiment showed that: pH of the rice leaf vascular bundles among 3 cultivars was almost increased, pH value of 1 cultivar was slightly increasing when rice plants were treated with Cd. The decrease degree of H+ concentration in leaf vascular bundles was different among cultivars. The cultivar with higher decreasing in H+ concentration, showed higher Cd transfer efficiency from shoots to grain. The H+ concentration of leaf vascular bundles under normal condition was negatively correlated to cadmium accumulation in leaf. Moreover, pH change was related to Cd accumulation in shots and remobilization from shoots to grain. Uncovering the role of pH response is a key component for the understanding Cd uptake and remobilization mechanism for rice production.