Accumulation In Rice Plants Research Articles

Inconsistent effects of limestone on rice cadmium uptake: Results from multi-scale field trials and large-scale investigation

Adding limestone into acidic paddy soils might reduce cadmium (Cd) accumulation in rice plants and the harvested grains but with inconsistent results in the field practice. We conducted three experiments of different field scales, including small-plots, multi-location trial, and large-scale field samplings of rice grown in a major production region of southern China, to investigate whether liming could sustainably limit the Cd phytoavailability to rice. Forty-eight physical, chemical, and biological attributes associated with paired soils and plants were collectively analyzed. Rice Cd uptake was significantly reduced when moderate dosage (2.25–3 t ha−1) of liming was present in the soils. The limes decreased rice Cd uptake by reducing the Cd concentrations of soil solution phase and regulating Ca2+ and Cd2+ competitions for absorption sites at root surfaces. Soil Zn hardly any effect on rice Cd uptake. Rice Cd uptake was suppressed at the higher rates of liming (4.5–9 t ha−1) due to the heavy loss of soil labile Mn. The tendencies of over compensating were soil-, plant-, and climate-dependent and were estimated by a transfer function and the risks were characterized using probabilistic analysis. The sustainable doses of limestone that reduced grain Cd accumulation, but did not compromise yield, or disrupt the rice rhizosphere was 3 t ha−1 annually incorporated two weeks before the seedlings were transplanted.

Effect of amendment of biochar supplemented with Si on Cd mobility and rice uptake over three rice growing seasons in an acidic Cd-tainted paddy from central South China

To examine the effect of rice straw biochar and the synergism with silicon on Cd immobilization, a Cd-contaminated acidic sandy loam paddy, polluted from emissions from industrial activity, was chosen in central South China. A field trial was conducted over three rice growing seasons during 2016–2017. Rice straw biochar (BR), produced by the pyrolysis of rice straw pellets at 450 °C, was amended at 10 t/ha (BR1), 20 t/ha (BR2), and supplemented with 0.75 t/ha sodium silicate (SS) at 10 t/ha, (BR1 + SS) and 20 t/ha (BR2 + SS), compared to the control without biochar and sodium silicate (BR0). BR supplemented with Si enhanced Cd soil immobilization and decreased Cd accumulation in rice plant within three rice seasons. Compared to BR0, BR + SS reduced Cd concentrations in grains by 19.5–73.7%, higher than that of 8.6–50.2% following BR. Cd bio-concentration factor of the root was reduced by an average of 54.6% from BR + SS and by 19.0% from BR, compared to BR0 in last two rice seasons. BR + SS significantly increased soil pH and available Si, and soil CaCl2-Cd was negatively related to soil available Si (p < 0.05). The synergistic effect of BR and Si induced liming effect and co-precipitation of Cd with Si compounds during the aging process of biochar. Thus, we suggest that an alkaline silicon supplementation is used as an amendment to BR, which could be used as a strategic approach for tackling Cd contamination in South China rice paddies.

The effect of an antimony resistant bacterium on the iron plaque fraction and antimony uptake by rice seedlings

Iron plaque (IP) is crucial in mitigating antimony (Sb) uptake and accumulation in rice plants, while, few studies focused on the effect of the iron plaque-associated Sb resistant bacteria on IP and Sb uptake into rice plants. Here, the effect of a Sb resistant bacterium (GenBank accession No. MH345840, with potential of conversion soluble Sb(III) into insoluble Sb2O3) on IP and Sb(III)/Sb(V) uptake under hydroponic condition was investigated. The results showed that in the presence of Sb(III), a large quantity of bacterial cells consorted with IP on rice roots, the bacterial inoculum altered the IP fraction distribution without enhancing its amount. However, it reduced Sb(III) uptake into rice roots. On contrary, seldom bacterial cells associated with the IP on rice roots in the presence of the Sb(V), the bacterial inoculum increased the IP amount slightly, and did not decline the Sb(V) uptake into rice roots. It also showed that the bacterial inoculum decreased Sb concentrations in rice shoots greatly in both Sb(III) and Sb(V) supplied treatments.

Cd accumulation, biomass and yield of rice are varied with silicon application at different growth phases under high concentration cadmium-contaminated soil.

Many reports suggest that exogenous Si addition could reduce Cd translocation to aerial part and make grain safe for rice production. But it remains unclear whether its supplementation during different growth phases can differentially impact Cd uptake in rice. Here, Positive effects of Si applied at different growth phases on biomass and yield of rice were observed. Dry weight of shoot including stem, leaf and grain was enhanced significantly by 15% when Si added at transplanting stage. Grain-yields supplied with Si at transplanting, jointing and heading stages were increased obviously by 14%, 11% and 12%, respectively. Higher percentage of filled spikelet and lower unfilled spikelet number per panicle were found when Si supplied at jointing stage. The increases of Cd accumulation in rice plants by Si application were mainly manifested in roots. Compared to CK, Cd accumulation in root when Si applied at transplanting and tillering stages were elevated by 75% and 64%, respectively. While Cd accumulation in aboveground tissues were all declined by Si addition. Bioconcentration and translocation factors were decreased significantly when Si added at jointing stage. In addition, higher soil pH, lowest available Cd-concentration in soil and Cd2+ concentration in xylem sap at 15d after flowering were found when the Si was applied at jointing stage. Overall, Si application at transplanting would be more beneficial to the growth of rice and increased Cd content in root, while Si supplied at jointing would be more favorable for grain filling and reducing Cd accumulation in shoot.

Soil attribute regulates assimilation of roxarsone metabolites by rice (Oryza sativa L.)

Roxarsone (ROX), an organoarsenic feed additive, and its metabolites, can be present in animal manure used to fertilize rice. Rice is prone to absorb arsenic, and is subject to straighthead disorder, which reduces rice yield and is linked with organic arsenic compounds. This study aims to elucidate how soil property affect arsenic accumulation in rice plants fertilized with chicken manure containing ROX metabolites. Manures of chickens fed without or with ROX, designated as control manure and ROX treated manure (ROXCM), respectively, were applied in eight paddy soils of different origins, to investigate the assimilation of arsenic species in rice plants. The results show that inorganic arsenic (arsenate and arsenite), monomethylarsonic acid and dimethylarsinic acid (DMA) were detected in all brown rice and husk, trace tetramethylarsonium and trimethylarsine oxide were occasionally found in these both parts, whereas all these arsenic species were determined in straw, irrespective of manure type. ROXCM application specifically and significantly increased brown rice DMA (P = 0.002), which remarkably enhanced the risk of straighthead disease in rice. Although soil total As impacted grain biomass, soil free-iron oxides and pH dominated arsenic accumulation by rice plants. The significantly increased grain DMA suggests manure bearing ROX metabolites is not suitable to be used in soils with abundant free-iron oxides and/or high pH, if straighthead disorder is to be avoided in rice.

Sulfur application modifies cadmium availability and transfer in the soil-rice system under unstable pe+pH conditions

The objective of this study was to investigate the responses of cadmium (Cd) availability and transfer in the soil-rice system to added sulfur (S) under unstable pe + pH conditions. Different water management conditions (flooding and aerobic treatments) cause changes in the soil pe + pH. We conducted a pot experiment to investigate the influence of S supply on soil Cd availability and Cd accumulation in rice plants (Oryza sativa L.), using three water regimes (continuous dryness, alternating dry-wet for one cycle, and continuous flooding) combined with two S concentrations (0 and 300 mg/kg). The results showed that the flooding treatment was more effective in decreasing soil pe + pH, Cd availability, and Cd accumulation in rice tissues than were the aerobic treatments. S-induced reduction in Cd uptake and translocation in rice was attributed to the decreased soil pe + pH values and enhanced biosynthesis of phytochelatins (PCs) and glutathione (GSH) in rice roots. Microscopic examination showed that the flooding treatment with added soil S resulted in better rice root growth. Element dispersive spectrometer (EDS) analysis indicated that S addition and flooding treatment promoted the formation of iron plaques and increases in Fe concentration in rice tissues. Conversely, partial disintegration of the root epidermis was observed in the dry treatment without added S.

Gene identification and transcriptome analysis of low cadmium accumulation rice mutant (lcd1) in response to cadmium stress using MutMap and RNA-seq

BackgroundCadmium (Cd) is a widespread toxic heavy metal pollutant in agricultural soil, and Cd accumulation in rice grains is a major intake source of Cd for Asian populations that adversely affect human health. However, the molecular mechanism underlying Cd uptake, translocation and accumulation has not been fully understood in rice plants.ResultsIn this study, a mutant displaying extremely low Cd accumulation (lcd1) in rice plant and grain was generated by EMS mutagenesis from indica rice cultivar 9311 seeds. The candidate SNPs associated with low Cd accumulation phenotype in the lcd1 mutant were identified by MutMap and the transcriptome changes between lcd1 and WT under Cd exposure were analyzed by RNA-seq. The lcd1 mutant had lower Cd uptake and accumulation in rice root and shoot, as well as less growth inhibition compared with WT in the presence of 5 μM Cd. Genetic analysis showed that lcd1 was a single locus recessive mutation. The SNP responsible for low Cd accumulation in the lcd1 mutant located at position 8,887,787 on chromosome 7, corresponding to the seventh exon of OsNRAMP5. This SNP led to a Pro236Leu amino acid substitution in the highly conserved region of OsNRAMP5 in the lcd1 mutant. A total of 1208 genes were differentially expressed between lcd1 and WT roots under Cd exposure, and DEGs were enriched in transmembrane transport process GO term. Increased OsHMA3 expression probably adds to the effect of OsNRAMP5 mutation to account for the significant decreases in Cd accumulation in rice plant and grain of the lcd1 mutant.ConclusionsAn extremely low Cd mutant lcd1 was isolated and identified using MutMap and RNA-seq. A Pro236Leu amino acid substitution in the highly conserved region of OsNRAMP5 is likely responsible for low Cd accumulation in the lcd1 mutant. This work provides more insight into the mechanism of Cd uptake and accumulation in rice, and will be helpful for developing low Cd accumulation rice by marker-assisted breeding.

Use of soil amendments to reduce cadmium accumulation in rice by changing Cd distribution in soil aggregates

The objectives of this study were to investigate the response of cadmium (Cd) distribution and stability in soil aggregates as affected by applying different amendments and to understand the relationship between changes in soil aggregates and alleviation of Cd phytotoxicity to rice after amendment application. In the present study, rice (Oryza sativa L.) was cultivated on a Cd-polluted soil. Five soil amendments were applied, which are as follows: rice husk biochar (BC), Fe-added rice husk biochar (Fe-BC), attapulgite-based mixture (AM), zeolite-based mixture (ZM), and cow manure-based mixture (MM). The effect on Cd redistribution in soil and Cd accumulation in rice plant was evaluated. The results showed that the five amendments applied at the rate of 3% (w/w) significantly increased soil pH and decreased Cd mobility in soil and Cd accumulation in rice plants. The reduction rate of Cd content in rice grains ranged from 41 to 62% after amendment application. The remediation efficiency of the different amendments for decreasing Cd accumulation in rice tissues followed the order of Fe-BC > MM > BC > ZM > AM. Adding amendments promoted the formation of large aggregates (0.2-2.0mm) with more mass loading of Cd and enhanced aggregate stability. Comparatively, Fe-BC was more effective than others for remediation of acid Cd-polluted paddy soil, as a significantly decreased Cd concentration in rice grain after its application was observed. Structural equation modeling (SEM) analysis revealed that DTPA-extractable Cd in small aggregates was the main factor affecting Cd accumulation in rice grain; soil pH directly affected aggregate stability; and aggregate stability was closely related to Cd availability in different size soil particles. These results indicated that the applied amendments were effective in reducing Cd bioavailability, most likely through raising the soil pH, improving aggregate stability, and re-distributing Cd from smaller soil aggregates to larger ones.

Dark septate endophytic fungi increase the activity of proton pumps, efficiency of 15N recovery from ammonium sulphate, N content, and micronutrient levels in rice plants.

Plants colonised by dark septate endophytic (DSE) fungi show increased uptake of nutrients available in the environment. The objective of the present study was to evaluate the impact of DSE fungi on the activity of proton pumps, nitrogen (N) recovery from ammonium sulphate, and nutrient accumulation in rice plants. Treatments consisted of non-inoculated plants and plants inoculated with two isolates of DSE fungi, A101 and A103. To determine N recovery from the soil, ammonium sulphate enriched with 15N was added to a non-sterile substrate while parameters associated with the activity of proton pumps and with NO3- uptake were determined in a sterile environment. The A101 and A103 fungal isolates colonised the roots of rice plants, promoting 15N uptake, growth, and accumulation of nutrients as compared with the mock control. A103 induced the expression of the plasma membrane H+-ATPase (PM H+-ATPase) isoforms OsA5 and OsA8, the activity of the PM H+-ATPase and H+-pyrophosphatase. Our results suggest that the inoculation of rice plants with DSE fungi represents a strategy to improve the N recovery from ammonium sulphate and rice plant growth through the induction of OsA5 and OsA8 isoforms and stimulation of the PM H+-ATPase and H+-pyrophosphatase.

Silicon Fertilizer Application Promotes Phytolith Accumulation in Rice Plants.

In this study, a pot experiment was designed to elucidate the effect of varying dosages of silicon (Si) fertilizer application in Si-deficient and enriched paddy soils on rice phytolith and carbon (C) bio-sequestration within phytoliths (PhytOC). The maximum Si fertilizer dosage treatment (XG3) in the Si-deficit paddy soil resulted in an increase in the rice phytolith content by 100.77% in the stem, 29.46% in the sheath and 36.84% in the leaf compared to treatment without Si fertilizer treatment (CK). However, the maximum Si fertilizer dosage treatment (WG3) in the Si -enriched soil increased the rice phytolith content by only 32.83% in the stem, 27.01% in the sheath and 32.06% in the leaf. Overall, Si fertilizer application significantly (p < 0.05) increased the content of the rice phytoliths in the stem, leaf and sheath in both the Si-deficient and enriched paddy soils, and the statistical results showed a positive correlation between the amount of Si fertilizer applied and the rice phytolith content, with correlation coefficients of 0.998 (p < 0.01) in the Si-deficient soil and 0.952 (p < 0.05) in the Si-enriched soil. In addition, the existence of phytoliths in the stem, leaf, and sheath of rice and its content in the Si-enriched soil were markedly higher than that in the Si-deficient soil. Therefore, Si fertilizer application helped to improve the phytolith content of the rice plant.

Biochar significantly alters rhizobacterial communities and reduces Cd concentration in rice grains grown on Cd-contaminated soils

Cadmium (Cd) contamination poses a serious problem in paddy soils. Biochar is frequently reported to deactivate Cd in soils and reduce Cd accumulation in rice plants, but few studies have addressed whether and how biochar affected the microbial communities in rice rhizosphere, which was an important factor determining the metal bioavailability and plant growth. In this study, biochar was pyrolyzed from bamboo (Phyllostachys heterocycla) chips at 350 °C. By using ICP-MS analysis and 16S rRNA gene sequencing, the impact of the biochar on Cd uptake by rice and on rhizospheric bacterial communities was investigated in both high-accumulating (HA) and low-accumulating (LA) rice cultivars grown in soils artificially contaminated with different Cd levels. Applied biochar significantly reduced Cd contents in rice plants of both cultivars, with substantially lower grain Cd contents for LA grown in highly contaminated soil. Soil pH was slightly increased by the applied biochar. Cd bioavailability was somehow reduced in soils, but not as significant as the reduction of Cd contents in rice plants. More interestingly, biochar application significantly altered the rhizobacterial community: it stimulated growth-promoting bacteria, such as Kaistobacter, Sphingobium (order Sphingomonadales), and Rhizobiaceae (order Rhizobiales); improved natural barrier formation and the transformation of metal mobilization around the rhizosphere mediated by, e.g., Rhodocyclaceae (class Betaproteobacteria) and Geobacter (class Deltaproteobacteria); and enhanced colonization of the LA rhizosphere possibly by taxa involved in Cd immobilization (Desulfovibrionales and Desulfobacterales). These results indicate that biochar application significantly reduces Cd uptake and accumulation by altering the rhizosphere bacterial community in rice grown on Cd-contaminated soils. The baseline data generated in this study provide insights that pave the way toward safer rice production.

Effects and mechanisms on the reduction of lead accumulation in rice grains through lime amendment

The influence of lime amendment on the absorption and accumulation of lead (Pb) in the grains of three rice cultivars ((Dong Lian 5 (DL-5, Indica), TeYou 009 (TY-009, Hybrid Indica), and YiYou 673 (YY-673, Hybrid Indica)) was investigated and the associated mechanisms were explored. Pot experiment was conducted in heavily Pb-polluted paddy soil. The transformation of Pb species in soil and roots under different levels of lime application was studied by Pb L3-edge X-ray absorption spectroscopy (Pb L3-XAS) and BCR sequential extraction. The results showed that lime amendment significantly increased soil pH and decreased DTPA-extractable Pb in soil. When the additional ratio of Liming was lower than 32 g kg–1, the Pb accumulation in rice plants, particularly in the grains was remarkably reduced. Moreover the biomass of rice was not significantly affected. Among the three cultivars, the lowest Pb content in rice grains occurred in YY-673. The cultivation of rice cultivar with low Pb accumulation will reduce the Pb content in the grains. Liming significantly inhibited the translocation of Pb from the roots to the aboveground parts, and from the stems and leaves to grains. Pb L3-XAS and BCR s sequential extraction method analysis revealed that lime amendment increased Pb2+ association with Fe oxides (Pb-Ferr), the precipitation of 2PbCO3·Pb(OH)2, the residual fraction of paddy soil, and also promoted the transformation of Pb(NO3)2 and Pb-pectin to 2PbCO3·Pb(OH)2 and Pb-Ferr in rice roots. The suitable lime addition (> 4 g kg–1, under this pot experiment) could effectively reduce Pb accumulation in rice grains.

ERECT LEAF1 suppresses jasmonic acid response in rice by decreasing OsWRKY4 stability

ABSTRACTERECT LEAF 1 (ELF1), which was identified as a component of brassinosteroid signaling in rice, is involved in brassinosteroid-mediated suppression of jasmonic acid response. Here, by conducting yeast two‐hybrid assay and in vitro ubiquitination experiments, we demonstrate that ELF1 interacts with the OsWRKY4 transcription factor, a positive regulator of defense responses to rice sheath blight. ELF1 decreased the stability of OsWRKY4, whereas exogenous jasmonic acid treatment suppressed this effect of ELF1, resulting in OsWRKY4 accumulation in rice plants. In wild-type rice, OsWRKY4 expression was up-regulated by jasmonic acid treatment but down-regulated by brassinosteroid treatment, suggesting that jasmonic acid–induced OsWRKY4 accumulation was caused by a combination of increased production and suppressed degradation. The expression levels of the OsWRKY4 target genes, PR1b and PR5, seemed to be correlated with the OsWRKY4 level. These results suggest that ELF1 indirectly controls the expression of PR1b and PR5 genes by regulating the OsWRKY4 protein level, and support a hypothesis that brassinosteroid and jasmonic acid cooperate to maintain the balance between growth and defense responses. We conclude that ELF1 participates in the antagonistic interaction between these two phytohormones by suppressing the jasmonic acid response through the down-regulation of OsWRKY4 protein level in rice.

The effect of straw mulch on nitrogen, phosphorus and potassium uptake and use in hybrid rice

To optimize straw application and understand the effects of straw mulch and straw nutrient release on rice nutrient uptake, we investigated the effects of two types of straw mulch (S1—wheat straw and S2—rapeseed straw) and no straw mulch (S0) on straw decomposition and nutrient release; rice growth and nutrient accumulation; and the nitrogen, phosphorus and potassium stimulation ratio and recovery efficiency of straw on rice in 2013 and 2014. Results showed that wheat straw mulch was associated with a higher decomposition ratio and total nutrient release and frequent rainfall could further increase decomposition and nutrient release in the 0–30 days after transplanting growth stage. Remarkably, the later stages were associated with decreased decomposition and nutrient release, especially on rapeseed straw mulch. Wheat and rapeseed straw mulch had obvious effects on rice growth and nutrient uptake. Compared with S0, S1 could distinctly promote dry matter accumulation and increase the levels of nitrogen, phosphorus and potassium in the rice plant at each growth stage, as well as increase grain yield in 2013 and 2014. By contrast, S2 inhibited rice growth; decreased nitrogen, phosphorus and potassium uptake and dry matter accumulation in rice plants during the early growth stage, while significantly improving that in the following growth stage; but decreased grain yield under rainy and cold weather conditions (2014). Compared with S0, wheat and rapeseed straw mulch increased total nitrogen, phosphorus and potassium accumulation in rice plants by 1.81–10.79%, 2.70–42.21% and 16.41–17.92%, respectively, thus improving nitrogen, phosphorus and potassium utilization.

Selenite mitigates cadmium-induced oxidative stress and affects Cd uptake in rice seedlings under different water management systems

Cadmium (Cd) is toxic throughout the food chain. Selenium (Se) can mitigate Cd accumulation in rice plants, although it is unclear why it has such effect. The objectives of this study were to investigate the effects of Se application on Cd-induced oxidative stress and antioxidant activities, and the combined effects of Se application and water management on the formation of iron plaque on the rice surface and Fe, Cd, and Se accumulation in rice plants. Rice seedlings were grown in Cd-contaminated soil with or without the addition of Se, and in aerobic or flooded conditions. Exogenous Se reduced Cd-induced oxidative stress. In the flooded treatment, exogenous selenite significantly decreased Cd concentrations in rice tissues, whereas it noticeably enhanced Cd concentrations in rice tissues in the aerobic treatment. Furthermore, selenite addition and flooding promote the formation of iron plaque and increase Fe concentrations in rice tissues. Pearson correlation analysis shows that plant Cd was significantly negatively correlated with Fe concentrations in rice tissues, and plant Fe was significantly positively correlated with Se concentrations in rice tissues, but no significant correlation was found between Cd and Se concentrations Thus, exogenous selenite may indirectly affect Cd uptake by influencing the formation of iron plaque on rice root surface, Fe uptake and Fe levels in rice.

Influence of Nitrogen Fertilization on Herbicide Selectivity in Rice

ABSTRACT: Chemical weed control and nitrogen fertilization are common practices in irrigated rice crops. Nitrogen is an essential element for growth and development of plants and may influence herbicide selectivity. In this study, herbicide selectivity was evaluated according to nitrogen application time in irrigated rice crops. The experiment was conducted in the field in a split-plot design with four replications. Treatments were arranged in a factorial scheme: factor A consisted of different nitrogen application times (without nitrogen, all nitrogen applied before flooding, half of the nitrogen rate applied before flooding and the other half after flooding, all nitrogen applied after flooding); factor B consisted of the application of the herbicides quinclorac, bentazon, bispyribac-sodium and carfentrazone-ethyl. Nitrogen application increased plant height and dry matter accumulation in rice plants, while nitrogen application before flooding and split N applications were favorable to the crop. The herbicide bispyribac-sodium causes the greatest injury and the greatest reduction in rice plant height, followed by bentazon and carfentrazone-ethyl, while the lowest injury and the highest grain yield were caused by the application of quinclorac.

Selenium application alters soil cadmium bioavailability and reduces its accumulation in rice grown in Cd-contaminated soil.

Selenium (Se) alleviates cadmium (Cd) accumulation in several plants. Nevertheless, it is still unclear why it has such effect. Thus, this study aimed to investigate the effects of Se on soil Cd bioavailability, and Cd accumulation in flooded rice plants, and to determine the mechanisms underlying these effects. Concentration of Cd and Se in different rice tissues was determined along Cd and Se concentrations in the soil solution and soil Cd fractions. Results showed that exogenous selenite and selenate treatments significantly increased rice grain Se by 4.25- and 2.39-fold and decreased Cd by 36.5% and 25.3% relative to control treatment, respectively. The addition of Se to Cd-contaminated soil significantly decreased total Cd concentration in the soil solution by 11.2-13.0%, increased soil pH by 0.06-0.32units, and enhanced soil Cd immobilization in relation to control. Exogenous Se also reduced diethylenetriaminepentaacetic acid-Cd, exchangeable, and residual Cd but increased the levels of Cd bound to carbonate and iron and manganese oxides. Thus, amending Cd-contaminated soil with Se may help decrease Cd content as well as increase Se levels in rice grain, as Se may mitigate Cd accumulation in rice plants by increasing soil pH, reducing Cd bioavailability, and inhibiting Cd translocation from roots to shoots.

Thiosulfate amendment reduces mercury accumulation in rice (Oryza sativa L.)

Thiosulfate addition increases the solubility of mercury (Hg) in soil and Hg uptake by plants under oxic conditions. However, anoxic conditions could dominate the biogeochemical processes of Hg cycling during rice cultivation. The present study aimed to determine whether thiosulfate, a sulfur-containing fertilizer, could be used for Hg immobilization in paddy soil. A pot experiment was conducted using soil newly spiked with Hg and different doses of thiosulfate. Total Hg concentrations in rice tissues, Hg speciation in roots, and geochemical fraction of Hg in soils were investigated. Hydroponic cultivation was conducted to determine the subcellular distribution of Hg in root tissues. Thiosulfate application significantly reduced Hg concentration in rice plants. It increased the percentage of organic-bound Hg, but decreased the percentage of iron/manganese oxide-bound Hg. Thiosulfate enhanced iron plaque formation and Hg adsorption on the iron plaque. Its application increased the percentage of Hg forms similar to HgS and decreased those similar to Hg-glutathione [Hg(GS)2]. Thiosulfate amendments had a remarkable inhibitory effect on Hg accumulation in rice plants in newly Hg-spiked soil. This occurred because thiosulfate reduced Hg mobility in the rhizosphere and root tissues, promoted the formation of iron plaque, and facilitated more Hg adsorption by the iron plaque. Our findings suggest that appropriate thiosulfate treatment could be used as Hg-immobilizing agents in paddy soil.

Characteristics of Mercury Transformation in Soil and Accumulation in Rice Plants in an Acidic Purple Paddy Soil Area

A pot experiment was conducted to analyze the characteristics of mercury (Hg) accumulation during the rice growth period in acidic purple paddy soil. Results showed that the soil total Hg concentration was stable with time during the rice growth period, while methylmercury (MeHg) increased greatly. Therefore, the ratio of MeHg/THg was mainly affected by MeHg concentration. THg concentration in rice plant parts followed the trend:root > grain > stalk > leaf > husk at the mature grain stage, while MeHg concentration followed grain > root > stalk > leaf > husk. The average of MeHg/THg in rice (24.03%) far exceeded that in soil (3.05%), suggesting that rice plants have a strong affinity for MeHg accumulation. This conclusion was supported also by the great difference in average of bio-concentration factors between MeHg (8.16) and THg (2.31) or the transfer factor in the grain, which was more than 1. Moreover, the bio-concentration factor for grain was higher than that for roots, stalks, and leaves, suggesting MeHg was prone to accumulation in grain compared with other part of rice plants.

Distribution and availability of cadmium in profile and aggregates of a paddy soil with 30-year fertilization and its impact on Cd accumulation in rice plant

The research was conducted to investigate the accumulation, distribution and availability of Cd in paddy soil and their relation to Cd in rice plants under 30-year fertilization regimes. Six treatments were involved in the study: control without fertilization (CK), chemical fertilizer (NPK), high nitrogen chemical fertilizer (HN), rice straw incorporation (ST), low and high dosage of manure fertilizer (LM and HM). Total and DTPA extractable concentration of Cd (T-Cd and DTPA-Cd) in bulk soils (20 cm topsoil), profiles (0–60 cm) and aggregates (>2, 1–2, 0.5–1, 0.25–0.5, 0.053–0.25 and < 0.053 mm) were investigated. The Cd concentration in relevant rice plant (roots, stems, leaves, husks and grains) were also analyzed. Manure fertilizers caused T-Cd accumulation in bulk soil with a significant increase of 36.2% in LM and 81.2% in HM. Similar impacts of manure fertilizers were observed in DTPA-Cd in the bulk soil. Further, the HM generated a further accumulation in deeper soil layers, presenting a remarkable increase of T-Cd (28.3%–225%) in 10–40 cm and DTPA-Cd (116%–158%) in 10–30 cm profiles. Moreover, the continuous application of manure fertilizers enhanced the availability of Cd in all aggregate size classes with an increase of 17.3%–87.8% in DTPA-Cd. Organic fertilizers (LM, HM and ST) heightened the content of Cd (38.0%–152%) in all parts of rice plant. The accumulation of Cd in rice plants was directly affected by fertilization regimes and Cd availability in the 10–20 cm soil layers and 0.25–0.5 mm aggregates. In conclusion, long-term application of manures resulted in increasing availability of Cd in aggregates and in topsoil and subsoil layers, which accordingly enhanced the accumulation of Cd in rice plants.