Effect of microbial mediated iron plaque reduction on arsenic mobility in paddy soil

The potential of microbial mediated iron plaque reduction, and associated arsenic （As） mobility were examined by iron reducing bacteria enriched from As contaminated paddy soil. To our knowledge, this is the first time to report the impact of microbial iron plaque reduction on As mobility. Iron reduction occurred during the inoculation of iron reducing enrichment culture in the treatments with iron plaque and ferrihydrite as the electron acceptors, respectively. The Fe（II） concentration with the treatment of anthraquinone-2, 6-disulfonic acid （AQDS） and iron reducing bacteria increased much faster than the control. Arsenic released from iron plaque with the iron reduction, and a significant correlation between Fe（II） and total As in culture was observed. However, compared with control, the increasing rate of As was inhibited by iron reducing bacteria especially in the presence of AQDS. In addition, the concentrations of As（III） and As（V） in abiotic treatments were higher than those in the biotic treatments at day 30. These results indicated that both microbial and chemical reductions of iron plaque caused As release from iron plaque to aqueous phase, however, microbial iron reduction induced the formation of more crystalline iron minerals, leading to As sequestration. In addition, the presence of AQDS in solution can accelerate the iron reduction, the As release from iron plaque and subsequently the As retention in the crystalline iron mineral. Thus, our results suggested that it is possible to remediate As contaminated soils by utilizing iron reducing bacteria and AQDS.

Crucial role of iron plaque on thallium uptake by rice plant

Iron plaque is a Fe-containing oxide film produced by the oxidation of Fe(II)in the rice root system under the combined action of oxygen infiltration and other microorganisms.Owing to its special surface structure and physio-chemical properties,the iron plaque has a strong absorption capacity for a variety of heavy metal ions.This study aimed to first investigate the effects of Fe species on the geochemical fractionation of Tl in typical paddy soil systems affected by industrial activities,followed by pot culture experiments to probe the effects of Fe species on the uptake and translocation of Tl in rice plants.The results of field work preliminarily showed that iron at different valences affected the conversion of the Tl geochemical fraction in the soil.Oxidizable Tl exerted significant positive correlation relationships with Fe2+and negative correlation relationships with Fe3+,while reducible Tl only displayed a positive correlation with Fe3+.Further analysis by pot culture experiments revealed that the contents of Fe were significantly positively correlated with Tl contents in Fe plaque(R2=0.529).In contrast,the water-soluble Tl contents in the soil were significantly negatively correlated with the contents of Fe(R2=–0.90,p<0.05).It suggests that the iron plaque promoted the absorption and fixation of Tl on the root surface of rice plants,causing Tl to accumulate in the iron plaque.Besides,the Tl content in the Fe plaque on the root surface of rice plants was greater than that in the above-ground tissues,which indicates that most Fe plaque exerts a certain degree of inhibition on Tl migration into the above-ground tissues of rice plants.All these findings indicate that Fe film is also an important carrier of Tl transfer in the soil–rice plant system,which provides new scientific support for the remediation of typical Tl-contaminated rice fields.

Phosphorus utilization and microbial community in response to lead/iron addition to a waterlogged soil

Constructed wetlands have emerged as a viable option for helping to solve a wide range of water quality problems. However, heavy metals adsorbed by substrates would decrease the growth of plants, impair the functions of wetlands and eventually result in a failure of contaminant removal. Typha latifolia L., tolerant to heavy metals, has been widely used for phytoremediation of Pb/Zn mine tailings under waterlogged conditions. This study examined effects of iron as ferrous sulfate （100 and 500 mg/kg） and lead as lead nitrate （0, 100, 500 and 1000 mg/kg） on phosphorus utilization and microbial community structure in a constructed wetland. Wetland plants （T. latifolia） were grown for 8 weeks in rhizobags filled with a paddy soil under waterlogged conditions. The results showed that both the amount of iron plaque on the roots and phosphorus adsorbed on the plaque decreased with the amount of lead addition. When the ratio of added iron to lead was 1：1, phosphorus utilized by plants was the maximum. Total amount of phospholipids fatty acids （PLFAs） was 23%-59% higher in the rhizosphere soil than in bulk soil. The relative abundance of Gram-negative bacteria, aerobic bacteria, and methane oxidizing bacteria was also higher in the rhizosphere soil than in bulk soil, but opposite was observed for other bacteria and fungi. Based on cluster analysis, microbial communities were mostly controlled by the addition of ferrous sulfate and lead nitrate in rhizosphere and bulk soil, respectively.

Combined toxicity of copper and cadmium to six rice genotypes (Oryza sativa L.)

Accumulations of copper （Cu） and cadmium （Cd） in six rice cultivars （94D-22, 94D-54, 94D-64, Gui630, YY-1, and KY1360） were evaluated through exposure to heavy metal contamination （100 mg/kg Cu, 1.0 mg/kg Cd, and 100 mg/kg Cu ＋ 1.0 mg/kg Cd） in a greenhouse. The dry weights of shoot and root, concentrations of Cu and Cd in plant tissues and the Cu, Cd, P, Fe concentrations in the root surface iron plaques were analyzed eight weeks later after treatment. The results indicated that the plant biomass was mainly determined by rice genotypes, not Cu and Cd content in soil. Separated treatment with Cu/Cd increased each metal level in shoot, root and iron plaques. Soil Cu enhanced Cd accumulation in tissues. In contrast, Cu concentrations in shoot and root was unaffected by soil Cd. Compared to single metal contamination, combined treatment increased Cd content by 110.6%, 77.0%, and 45.2% in shoot, and by 112.7%, 51.2% and 18.4% in root for Gui630, YY-1, and KY1360, respectively. The content level of Cu or Cd in root surface iron plaques was not affected by their soil content. Cu promoted Fe accumulation in iron plaques, while Cd has no effect on P and Fe accumulation in it. The translocation of Cu and Cd from iron plaques to root and shoot was also discussed. These results might be beneficial in selecting cultivars with low heavy metal accumulation and designing strategies for soil bioremediation.

Cultivar-dependent rhizobacteria community and cadmium accumulation in rice: Effects on cadmium availability in soils and iron-plaque formation

The association between the rhizospheric microbial community and Cd accumulation in rice is poorly understood.A field trial was conducted to investigate the different rhizobacterial communities of two rice cultivars with high Cd accumulation(HA)and low Cd accumulation(LA)at four growth stages.Results showed that the Cd content in the roots of the HA cultivar was 1.23-27.53 higher than that of the LA cultivar(0.08-10.5μg/plant)at four stages.The LA cultivar had a significantly lower Cd availability in rhizosphere and a higher quantity of iron plaque(IP)on the root surface than the HA cultivar at four stages.This resulted in the reduction of Cd concentration in IPs and Cd translocation from IP-to-root.Microbial analysis indicated that the LA cultivar formed a distinct rhizobacterial community from the HA cultivar and had lessα-diversity.The rhizosphere of the LA cultivar was enriched in specific bacterial taxa(e.g.,Massilia and Bacillus)involved in Cd immobilization by phosphate precipitation and IP formation by iron oxidization.However,the rhizosphere in the HA cultivar assembled abundant sulfur-oxidizing bacteria(e.g.,Sulfuricurvum)and iron reduction bacteria(Geobacter).They promoted Cd mobilization and reduced IP formation via the metal redox process.This study reveals a potential approach in which specific rhizobacteria decrease or increase Cd accumulation in rice on contaminated soil and provides a new perspective for secure rice production.

Variations between rice cultivars in iron and manganese plaque on roots and the relation with plant cadmium uptake

To understand certain mechanisms causing variations between rice cultivars with regard to cadmium uptake and tolerance, pot soil experiments were conducted with two rice cultivars of different genotypes under different soil Cd levels. The relationships between plant Cd uptake and iron/manganese （Fe/Mn） plaque formation on roots were investigated. The results showed that rice cultivars differed markedly in Cd uptake and tolerance. Under soil Cd treatments, Cd concentrations and accumulations in the cultivar Shanyou 63 （the genotype indica） were significantly higher than those in the cultivar Wuyunjing 7 （the genotype japonica） （P 〈 0.01, or P 〈 0.05）, and Shanyou 63 was more sensitive to Cd toxicity than Wuyunjing 7. The differences between the rice cultivars were the largest at relatively low soil Cd level （i.e., 10 mg/kg）. Fe concentrations in dithionite-citrate-bicarbonate root extracts of Shanyou 63 were generally lower than that of Wuyunjing 7, and the difference was the most significant under the treatment of 10 mg Cd/kg soil. The results indicated that the formation of iron plaque on rice roots could act as a barrier to soil Cd toxicity, and may be a ＂buffer＂ or a ＂reservoir＂ which could reduce Cd uptake into rice roots. And the plaque may contribute, to some extent, to the genotypic differences of rice cultivars in Cd uptake and tolerance.

Effect of P fertilizer reduction regime on soil Olsen-P, root Fe-plaque P, and rice P uptake in rice-wheat rotation paddy fields

In agricultural systems, it is vital to use limited yet optimal phosphorus(P) resources, because excessive P fertilizer application leads to the accumulation of P in soil, increasing the risk of environmental pollution and causing the waste and exhaustion of P resources. In a rice-wheat rotation system, omitting P fertilizer application in the rice-growing season is a good alternative;however, how this P fertilization reduction influences changes in P in the soil-root-aboveground system is unclear. In this study, after a seven-year rice-wheat rotation at the Yixing(YX) and Changshu(CS) sampling sites, China, compared with P fertilization in rice-and wheat-growing seasons(PR+W), reduced P fertilization(no P fertilizer application in either season, P0;P fertilization only in wheat-growing seasons, PW;and P fertilization only in rice-growing seasons, PR) did not result in substantial variation in crop biomass. The PW treatment did not reduce crop total P, root iron(Fe)-plaque P, and soil Olsen-P at three stages of rice growth(seedling, booting, and harvesting stages) at the YX and CS sites. In contrast, concentrations of soil Olsen-P, aboveground crop total P, and root Fe-plaque P decreased in the P0 treatment by 45.8%–81.0%,24.6%–30.9%, and 45.6%–73.4%, respectively. In addition, a significant negative correlation was observed between the root Fe-plaque P and crop biomass at the two sites. Significant positive correlations were also observed between root Fe-plaque P and root total P, crop total P, and soil Olsen-P. In addition, the results of a redundancy analysis revealed that soil alkaline phosphatase(ALP) played a major role in the supply of P in soil, and was closely associated with root Fe-plaque P. The results of this study will enhance the understanding of the changes in P in the soil-root-aboveground system, particularly under P fertilizer reduction regimes.

Rapid evaluation of arsenic contamination in paddy soils using field portable X-ray fluorescence spectrometry

Arsenic（As） in paddy fields is deteriorating food security and human health through rice ingestion. Rice is the dominant food source of arsenic exposure to half of the world’s population. Therefore, an in situ effective method for As risk evaluation in paddy soil is strongly needed to avoid As exposure through rice ingestion. Herein, we developed a rapid analytical methodology for determination of As in plant tissues using field portable X-ray fluorescence spectrometry（FP-XRF）. This method was applied to rice roots in order to evaluate the As contamination in paddy soils. The results showed that rice roots with iron plaques were superior to rhizosphere soils for generating FP-XRF signals, especially for field sites with As concentrations lower than the soil detection limit of FP-XRF（30.0 mg/kg）.Moreover, the strong linear relationships of As concentrations between the rice roots and corresponding leaves and grains proved that the rice root, rather than the soil, is a better predictor of As concentrations in rice grains. The research provides an efficient As monitoring method for As contaminated paddy fields by using wetland plant roots with iron plaques and XRF-based analytical techniques.

Arsenic dynamics in the rhizosphere and its sequestration on rice roots as affected by root oxidation

A pot experiment was conducted to investigate the effects of root oxidation on arsenic （As） dynamics in the rhizosphere and As sequestration on rice roots. There were significant differences （P 〈 0.05） in pH values between rhizosphere and non-rhizosphere soils, with pH 5.68-6.16 in the rhizosphere and 6.30-6.37 in non-rhizosphere soils as well as differences in redox potentials （P 〈 0.05）. Percentage arsenite was lower （4%-16%） in rhizosphere soil solutions from rice genotypes with higher radial oxygen loss （ROL） compared with genotypes with lower ROL （P 〈 0.05）. Arsenic concentrations in iron plaque and rice straw were significantly negatively correlated （R = -0.60, P 〈 0.05）. Genotypes with higher ROL （TD71 and Yinjingmanzhau） had significantly （P 〈 0.001） lower total As in rice grains （1.35 and 0.96 mg/kg, respectively） compared with genotypes with lower ROL （IAPAR9, 1.68 mg/kg; Nanyangzhan 2.24 mg/kg） in the As treatment, as well as lower inorganic As （P 〈 0.05）. The present study showed that genotypes with higher ROL could oxidize more arsenite in rhizosphere soils, and induce more Fe plaque formation, which subsequently sequestered more As. This reduced As uptake in aboveground plant tissues and also reduced inorganic As accumulation in rice grains. The study has contributed to further understanding the mechanisms whereby ROL influences As uptake and accumulation in rice.

The risks of sulfur addition on cadmium accumulation in paddy rice under different water-management conditions

Recently, the application of sulfur(S) has been recommended to control the accumulation of cadmium(Cd) in rice in contaminated paddy soil. However, the effects of exogenous S on Cd transfer in paddy rice systems under different water-management practices have not been systematically investigated. Pot experiments were performed to monitor the composition of soil pore water and the Cd accumulation in iron plaque and rice tissue were compared under different S(0 and 200 mg/kg Na_(2)SO_(4)) and water(continuous and discontinuous flooding) treatments. Sulfur application significantly increased Cd concentrations in soil pore water under discontinuous flooding conditions, but slightly reduced them under continuous flooding. Moreover, the oxidation/reduction potential(Eh) was the most critical factor that affected the Cd levels. When the Eh exceeded-42.5 mV, S became the second critical factor, and excessive S application promoted Cd dissolution. In addition, S addition elevated the Cd levels in iron plaque and reduced the Cd transfer from the iron plaque to rice roots. In rice, S addition inhibited Cd transfer from the rice roots to the straw;thus, more Cd was stored in the rice roots. Nevertheless, additional S application increased the Cd content in the rice grains by 72% under discontinuous flooding, although this effect was mitigated by continued flooding. Under simulated practical water management conditions, S addition increased the risk of Cd contamination in rice, suggesting that S application should be reconsidered as a paddy fertilization strategy.