Do aeration conditions affect arsenic and phosphate accumulation and phosphate transporter expression in rice (Oryza sativa L.)?

Effects of fresh and field-aged holm-oak biochar on As, Cd and Pb bioaccumulation in different rice growing environments

Arsenic, Cd, and Pb environmental fate is influenced when the traditional permanent flooding rice production systems are replaced by water-saving and soil conservation practices, urging for additional strategies that avoid their bioaccumulation in rice grain. The aim of this two-years field study was to evaluate the effects of fresh and field-aged biochar on As, Cd, and Pb bioaccumulation, and on As speciation, in rice grain produced in different growing environments (flooding versus sprinkler and conventional tillage versus direct seeding). Biochar produced from holm-oak pruning residues (pyrolysis at 550 °C, 48 h), in a single application (28 Mg ha^-1), reduced As bioaccumulation in rice grain in the permanent flooding system to non-quantifiable concentrations (e.g., from 0.178 mg kg^-1 to <0.04 mg kg^-1, for inorganic-As, respectively), an effect which remained under field-aging conditions, increasing rice commercial value. When adopting sprinkler irrigation, the undesirable increase in Cd bioaccumulation in rice, relatively to the anaerobic system, was counteracted by biochar application, reducing its bioaccumulation in kernels between 32 and 80 %, allowing a simultaneous control of metals and metalloids bioaccumulation in rice. The bioaccumulation of Pb was also prevented with biochar application, with a reduction in its concentration four- to 13-times, in all the management systems, relatively to the non-amended plots, under fresh biochar effects. However, Pb immobilization decreased with biochar field-aging, indicating that the biochar application may have to be repeated to maintain the same beneficial effect. Therefore, the present study shows that the implementation of sprinkler irrigation with holm-oak biochar could reduce the risk of heavy metals(loids) bioaccumulation in rice grains and, thereby, ensuring food safety aspects, particularly under fresh biochar effects.

Arsenic biotransformation genes and As transportation in soil-rice system affected by iron-oxidizing strain (Ochrobactrum sp.)

Arsenic (As) biotransformation in soil affects As biogeochemical cycling and is associated with As accumulation in rice. After inoculation with 1% iron-oxidizing bacteria (FeOB) in paddy soil, As speciation, As biotransformation genes in soil, As/Fe in Fe plaques, and As accumulation in rice were characterized. Compared with the control, the available As concentrations in soils decreased while amorphous and poorly crystalline Fe-Al oxidized As and crystalline Fe-Al oxidized As fractions increased of F (FeOB) and RF (rice and FeOB) treatments. Fe concentrations increased and positively correlated with As concentrations in Fe plaques on the rice root surface (***P < 0.001). Compared with R (rice), Monomethyl As (MMA), dimethyl As (DMA), arsenate (As(V)), and arsenite (As(III)) concentrations in rice plants showed a downwards trend of RF treatment. The As concentration in grains was below the National Standard for Food Safety (GB 2762-2017). A total of 16 As biotransformation genes in rhizosphere soils of different treatments (CK, F, R and RF were quantified by high-throughput qPCR (HT-qPCR). Compared with the control, the As(V) reduction and As transport genes abundance in other treatments increased respectively by 54.54%-69.17% and 54.63%-73.71%; the As(III) oxidation and As (de) methylation genes did not change significantly; however, several As(III) oxidation genes (aoxA, aoxB, aoxS, and arsH) increased. These results revealed that FeOB could reduce, transport As, and maybe also oxidize As. In addition, As(III) oxidation gene (aoxC) in rhizosphere soil was more abundant than in non-rhizosphere soil. It indicated that radial oxygen loss (ROL) promoted As(III) oxidation in rhizosphere soils. The results provide evidence for As biotransformation by ROL and FeOB in soil-rice system. ROL affects As oxidation and immobilization, and FeOB affects As reduction, transportation and may also affect As oxidation.

Increased arsenic mobilization in the rice rhizosphere is mediated by iron-reducing bacteria

Rice (Oryza sativa) tends to accumulate elevated levels of arsenic (As) in grain, threatening food safety and human health. The rice rhizosphere has a micro-environment that differs markedly from the bulk soil. Yet, little is known about how this micro-environment influences the mobility of As in the rhizosphere. Using rhizoboxes with two rice cultivars (cv. Shenyou 957 and Yangdao 6) differing in their radial oxygen loss (ROL), we investigated the in situ transformation of As in the rhizosphere associated with changes in microbial communities and As-related functional genes. Contrary to expectation, dissolved (porewater) As concentrations within the rhizosphere increased by 1.3-2.4 fold compared to the bulk soil during the seedling stage, with the magnitude of this difference gradually decreasing over time. The increased As mobilization in the rhizosphere was associated with increased soluble Fe. This increasing trend was associated with the increased abundance of both Fe-reducing bacteria (FeRB) and As-related functional genes within the rhizosphere. Furthermore, bacterial 16S rRNA gene sequencing data showed that the abundances of Geobacter and Clostridium were 3.1 times and 12.4 times higher in the rhizosphere, respectively. The importance of FeRB was also suggested by the fact that dissolved As concentrations were highly correlated with dissolved Fe concentrations (r² = 0.83) and also with the relative abundance of genus Clostridium_sensu_stricto_10 (r² = 0.85). This study highlights that although the rice rhizosphere favors a more aerobic condition compared to the bulk soil, As is more mobilized in the rhizosphere, and that Geobacter and some species of Clostridium play a critical role in controlling As mobilization in the rhizosphere.

The effects of biochar as the electron shuttle on the ferrihydrite reduction and related arsenic (As) fate

The effects of electron shuttles (biochar/anthraquinone-2,6-disulphonate (AQDS)) on the process of the Shewanella oneidensis MR-1-induced As(V)-adsorbed ferrihydrite reduction were studied. The results showed that biochar could stimulate Fe(Ⅱ) and As release during the ferrihydrite bioreduction. After the addition of biochar, more dissolved organic matter (DOM) can be consumed as an electron donor to promote the metabolism of microorganisms by the fluorescence excitation-emission matrix spectra analysis. After microbial treatment, cyclic voltammetry (CV) showed that a unique cathodic peak and a distinct anodic peak appeared, which may represent the reduction of Fe(OH)₃ to Fe(OH)₂ and the complexed oxidation of Fe²⁺ to Fe³⁺. No characteristic peak was associated with arsenate reduction or arsenite oxidation. The mineralogical characterization of the final products indicated that AQDS can promote solid-state conversion from ferrihydrite to vivianite (Fe₃(PO₄)₂·8H₂O). However, the addition of biochar inhibited solid-state conversion of ferrihydrite. It was shown that after 6 d, the secondary mineral vivianite production in the bacteria alone and AQDS treatments was 8.12% and 15.6% respectively by mössbauer spectroscopy analysis. Moreover, the XPS indicated that As(V) has no species transformation. It provided new data for understanding the iron-reducing bacteria induced mineralization process and related biogeochemical cycles of Fe and As.

Phosphate transporters, PnPht1;1 and PnPht1;2 from Panax notoginseng enhance phosphate and arsenate acquisition

BACKGROUND

Panax notoginseng is a medicinally important Chinese herb with a long history of cultivation and clinical application. The planting area is mainly distributed in Wenshan Prefecture, where the quality and safety of P. notoginseng have been threatened by high concentration of arsenic (As) from the soil. The roles of phosphate (Pi) transporters involved in Pi acquisition and arsenate (AsV) tolerance were still unclear in this species.

RESULTS

In this study, two open reading frames (ORFs) of PnPht1;1 and PnPht1;2 separated from P. notoginseng were cloned based on RNA-seq, which encoded 527 and 541 amino acids, respectively. The results of relative expression levels showed that both genes responded to the Pi deficiency or As exposure, and were highly upregulated. Heterologous expression in Saccharomyces cerevisiae MB192 revealed that PnPht1;1 and PnPht1;2 performed optimally in complementing the yeast Pi-transport defect, particularly in PnPht1;2. Cells expressing PnPht1;2 had a stronger AsV tolerance than PnPht1;1-expressing cells, and accumulated less As in cells under a high-Pi concentration. Combining with the result of plasma membrane localization, these data confirmed that transporters PnPht1;1 and PnPht1;2 were putative high-affinity H+/H2PO4- symporters, mediating the uptake of Pi and AsV.

CONCLUSION

PnPht1;1 and PnPht1;2 encoded functional plasma membrane-localized transporter proteins that mediated a putative high-affinity Pi/H+ symport activity. Expression of PnPht1;1 or PnPht1;2 in mutant strains could enhance the uptake of Pi and AsV, that is probably responsible for the As accumulation in the roots of P. notoginseng.

Low arsenate influx rate and high phosphorus concentration in wheat (Triticum aestivum L.): A mechanism for arsenate tolerance in wheat plants

Two wheat (Triticum aestivum L.) cultivars differing in arsenic (As)-tolerance were used to investigate the effects of phosphorus (P) concentration and nutrient solution pH on As(V) toxicity and As(V) uptake kinetics, and to illustrate the mechanism of As(V) tolerance in wheat seedlings. Low pH and low phosphate concentration enhanced wheat uptake of As, resulting in high As toxicity. The As(V)-tolerant cultivar MM45 exhibited higher relative root elongation than non-tolerant cultivar HM29 in all treatments, except that no genotypic difference was recorded for the solution P at 100 μmol L^-1 or greater. Wheat seedling As(V) tolerance was positively correlated with P concentration in roots and shoots. In short-term (30 min) As(V)-uptake kinetics experiments, the maximum influx rate (V_max) of As(V) increased with decreasing solution pH (from 7.0 to 6.0). Compared with HM29, although MM45 had lower V_max, its Michaelis-Menten constant (K_m) did not exceed that of HM29 in all treatments. The V_max values of both cultivars were not significantly affected by phosphate treatments, except for HM29 which had significantly higher V_max value in the presence of phosphate at pH 7.0. The K_m values of the two cultivars increased by 9- to 20-fold when phosphate was present as opposed to absent from the uptake solution. This study showed that the V_max values are mainly increased by high pH and As(V) uptake K_m is mainly increased by phosphate presence. Decreased As(V) influx rates during early stages and increased P concentration in plant tissues are associated with increased As tolerance in wheat seedlings.

Aromatic organoarsenic compounds (AOCs) occurrence and remediation methods

Many researchers at home and abroad have made a body of researches and have gained great achievements on the environmental occurrence, fate, and toxicity of inorganic arsenic. But there is less research on the use of aromatic organoarsenic compounds (AOCs), which are common feed additives for livestock in the poultry industry. In this review, we outline the current state of knowledge acquired on the occurrence and remediation of AOCs, respectively. We also identify knowledge gaps and research needs, including the elucidation of the environmental fate of AOCs, metabolic pathway, the impact of metabolic modification on toxicity, and advanced analytical or repaired methods that allows for monitoring, identification or removal of the degradation products.

Plant species diversity for vegetation restoration in manganese tailing wasteland

Vegetation restoration is one of the most effective measures to restore degraded ecosystem in mining wasteland. A field experiment was conducted to study the effects of some site treatments' three different approaches on the benefits of selective vegetation in the manganese mine. Three different approaches included (1) exposed tailings, the control treatment (tailing site); (2) soil covering of 10-cm thickness (external-soil site), and (3) soil covering of 10-cm thickness, soil ameliorating (adding fowl dung), and seeding propagation of Cynodon dactylon (Linn.) Pers. (rehabilitation site). The results indicated that 18 herb species were taken from 8 families and 4 woody plants in three sites after 1 year. After 3 years, 29 species from 14 families were observed in 3 sites. Meanwhile, compared with tailing site, the plant species of rehabilitation site was more than tailing site, and the plant abundance of external-soil site was similar to rehabilitation site. It was worthy to be mentioned that the plant species of external-soil site and rehabilitation site had a better effect on the plant community coverage of herb layer as compared with tailing site. In summary, the plant species of rehabilitation site had the most species diversity and could be recommended as the ve-restoration modes in manganese tail wasteland.

Pollution characteristics of surface runoff under different restoration types in manganese tailing wasteland

A great deal of manganese and associated heavy metals (such as Ni, Cu, Zn, Cd, Pb, etc.) was produced in manganese mining, smelting, and other processes and weathering and leaching of waste slag, which entered rainwater runoff by different means under the action of rainfall runoff. It caused heavy metal pollution in water environment to surrounding areas, and then environmental and human health risks were becoming increasingly serious. In the Xiangtan manganese mine, we studied the characteristics of nutritional pollutants and heavy metals by using the method of bounded runoff plots on the manganese tailing wasteland after carrying out some site treatments using three different approaches, such as (1) exposed tailings, the control treatment (ET), (2) external-soil amelioration and colonization of Cynodon dactylon (Linn.) Pers. turf (EC), and (3) external-soil amelioration and seedling seeding propagation of Cynodon dactylon (Linn.) Pers. (ES). The research showed that the maximum runoff occurred in 20,140,712 rainfall events, and the basic law of runoff was EC area > ET area > ES area in the same rainfall event. The concentration of total suspended solids (TSS) and chemical oxygen demand (COD) of three ecological restoration areas adopted the following rule: ET area > EC area > ES area. Nitrogen (N) existed mainly in the form of water soluble while phosphorus (P) was particulate. The highest concentrations of total nitrogen (TN) and total phosphorus (TP) were 11.57 ± 2.99 mg/L in the EC area and 1.42 ± 0.56 mg/L in the ET area, respectively. Cr, Ni, Pb, Zn, Mn, and Cu in surface runoff from three restoration types all exceeded the class V level of the environmental quality standard for surface water except Cu in EC and ES areas. Pollution levels of heavy metals in surface runoff from three restoration areas are shown as follows: ET area > EC area > ES area. There was a significant positive correlation between TSS and runoff, COD, and TP. And this correlation was significant between total dissolved nitrogen (TDN), TN, total dissolved phosphorus (TDP), and TP. The six heavy metals (Cu, Ni, Pb, Zn, Mn, and Cr) in surface runoff of different ecological restoration areas were strongly related to each other, and were significantly related to the TSS.