New Barrier Role of Iron Plaque: Producing Interfacial Hydroxyl Radicals to Degrade Rhizosphere Pollutants.

Iron plaque, as a natural barrier between rice and soil, can reduce the accumulation of pollutants in rice by adsorption, contributing to the safe production of rice in contaminated soil. In this study, we unveiled a new role of iron plaque, i.e., producing hydroxyl radicals (·OH) by activating root-secreted oxygen to degrade pollutants. The ·OH was produced on the iron plaque surface and then diffused to the interfacial layer between the surface and the rhizosphere environment. The iron plaque activated oxygen via a successive three-electron transfer to produce ·OH, involving superoxide and hydrogen peroxide as the intermediates. The structural Fe(II) in iron plaque played a dominant role in activating oxygen rather than the adsorbed Fe(II), since the structural Fe(II) was thermodynamically more favorable for oxygen activation. The oxygen vacancies accompanied by the structural Fe(II) played an important role in oxygen activation to produce ·OH. The interfacial ·OH selectively degraded rhizosphere pollutants that could be adsorbed onto the iron plaque and was less affected by the rhizosphere environments than the free ·OH. This study uncovered the oxidative role of iron plaque mediated by its produced ·OH, reshaping our understanding of the role of iron plaque as a barrier for rice.

Green manuring reduces cadmium accumulation in rice: Roles of iron plaque and dissolved organic matter.

In southern China, winter green manure is widely used in rice cropping systems for improving grain yields and soil fertility. Cd pollution has recently been reported in some of these paddy fields. Research on the in-depth understanding of how green manuring affects Cd absorption in rice is limited. This study aimed to investigate the impacts of different green manures, including single plantation and mixed plantation on the absorption of Cd by rice and explore the underlying mechanisms. Pot experiments demonstrated that compared with winter fallow-rice, green manuring treatments considerably decreased rice Cd content, promoted the conversion of bioavailable Cd fraction into a more stable form, induced the formation of iron plaque, and increased the content of humic-like fraction (HF) in soil dissolved organic matter (DOM). Treatment with mixed plantation resulted in a greater decrease in rice Cd content and an increase in HF and iron plaque contents than single plantation. Hydroponic experiments confirmed that both iron plaque and green manure-derived DOM significantly reduced the Cd content in rice seedlings. In conclusion, green manure incorporation is an efficient measure for the safe utilization of Cd-contaminated soil, and mixed plantation of different green manures exerts stronger effects.

Synergistic effects of silicon and goethite co-application in alleviating cadmium stress in rice (Oryza sativa L.): Insights into plant growth and iron plaque formation mechanisms.

Rice is one of the most important staple food crops; however, it is prone to cadmium (Cd) accumulation, which has negative health effects. Therefore, methods to reduce Cd uptake by rice are necessary. At present, there is limited research on the effects of co-application of silicon (Si) and goethite in mitigating Cd stress in rice. Furthermore, the specific mechanisms underlying the effects of their combined application on iron plaque formation in rice roots remain unclear. Therefore, this study analyzed the effects of the combined application of Si and goethite on the biomass, physiological stress indicators, Cd concentration, and iron plaques of rice using hydroponic experiments. The results revealed that co-treatment with both Si and goethite increased the plant height and dry weight, superoxide dismutase and catalase activities, photosynthetic pigment concentration, and root activity. Moreover, this treatment decreased the malondialdehyde concentration, repaired epidermal cells, reduced the Cd concentration in the roots by 57.2 %, and increased the number of iron plaques and Cd concentration by 150.9 % and 266.2 % in the amorphous and crystalline fractions, respectively. The Cd/Fe ratio in amorphous iron plaques also increased. Our findings suggest that goethite serves as a raw material for iron plaque formation, while Si enhances the oxidation capacity of rice roots. The application of a combination of Si and goethite increases the quantity and quality of iron plaques, enhancing its Cd fixation capacity. This study provides theoretical evidence for the effective inhibition of Cd uptake by iron plaques in rice, providing insights into methods for the remediation of Cd contamination.

Mineralogical characteristics of root iron plaque and its functional mechanism for regulating Cr phytoextraction of hyperaccumulator Leersia hexandra Swartz.

Leersia hexandra Swartz (L. hexandra) is a promising hyperaccumulator for Cr pollution remediation, but whether its Cr phytoextraction is subject to the root surface-attached iron plaque (IP) remains unclear. In this research, the natural and artificial IPs were proven to be comprised of small amounts of exchangeable Fe as well as carbonate Fe, and dominantly Fe minerals involving amorphous two-line ferrihydrite (Fh), poorly crystalline lepidocrocite (Le) and highly crystalline goethite (Go). The Fe content in the artificial IPs augmented with increasing induced Fe(II) concentration, and the 50 mg/L Fe(II) led to the identical Fe content and different component proportions of artificial IP (Fe50) and natural IP. Fh was consisted of highly aggregated nanoparticles, and the aging of Fh caused its phase conversion to rod-like Le and Go. The Cr(VI) adsorption results of Fe minerals corroborated the coordination of Cr(VI) onto the Fh surface and the significantly greater equilibrium Cr(VI) adsorption amount of Fh over Le and Go. The greatest Cr(VI) reduction capacity of Fh among three Fe minerals was found to be related to its most abundant surface-adsorbed Fe(II) content. The results of hydroponic experiment of L. hexandra showed that the presence of IP facilitated the Cr(VI) removal by L. hexandra during the cultivation period of 10-45 days, and consequently, compared to the Fe0 group (without IP), around 60% of increase in the Cr accumulation of shoots was achieved by Fe50 group. The findings of this work are conductive to furthering our understanding of IP-regulated Cr phytoextraction of L. hexandra.

Combined application of biochar and sulfur alleviates cadmium toxicity in rice by affecting root gene expression and iron plaque accumulation.

Biochar and sulfur are considered useful amendments for soil cadmium (Cd) contamination remediation. However, there is still a gap in the understanding of how combined biochar and sulfur application affects Cd resistance in rice, and the role of the accumulation of iron plaque and the expression of Cd efflux transporter-related genes are still unclear in this type of treatment. In this study, we screened an effective combination of biochar and sulfur (0.75 % biochar, 60 mg/kg sulfur) that significantly reduced the Cd content of rice roots (32.9 %) and shoots (12.3 %); significantly reduced the accumulation of amino acids and their derivatives, organic acids and their derivatives and flavonoids in rice roots; and altered secondary metabolite production and release. This combined biochar and sulfur application alleviated the toxicity of Cd to rice, in which the enhancement of iron plaque (24.8 %) formation and upregulated expression of heavy metal effector genes (NRAMP3, MTP3, ZIP1) were important factors. These findings show that iron plaque and heavy metal transport genes are involved in the detoxification of rice under the combined application of biochar and sulfur, which provides useful information for the combined treatment of soil Cd pollution.

Mitigation of arsenic accumulation in rice (Oryza sativa L.) seedlings by oxygen nanobubbles in hydroponic cultures.

Arsenic (As) is a toxic non-essential metal. Its accumulation in rice has not only seriously affected the growth of rice, but also poses a significant threat to human health. Many reports have been published to decrease the arsenic accumulation in the rice plant by various additives such as chemicals, fertilizers, adsorbents, microorganisms and analyzing the mechanism. Nanobubble is a new technology widely used in agriculture because of its long existence time and high mass transfer efficiency. However, a few studies have investigated the effect of nanobubbles on arsenic uptake in rice. This study investigated the effect of oxygen nanobubbles on the growth and uptake of As in rice. The oxygen nanobubbles could rupture the salinity of nutrients and produce the hydroxyl radical. The hydroxyl radical caused the oxidation of arsenic As(III) to As (V) and the oxidation of ferrous ions. At the same time, the oxidized iron adsorbing As (V) created the iron plaque on the rice roots to stop arsenic introduction into the rice plant. The results indicated that the treatment of oxygen nanobubbles increased rice biomass under As stress, while they increased the chlorophyll content and promoted plant photosynthesis. Oxygen nanobubbles reduced the As content in rice roots to 12.5% and shoots to 46.4%. In other words, it significantly decreased As accumulation in rice. Overall, oxygen nanobubbles mitigated the toxic effects of arsenic on rice and had the potential to reduce the accumulation of arsenic in rice.

Silicon facilitated the physical barrier and adsorption of cadmium of iron plaque by changing the biochemical composition to reduce cadmium absorption of rice roots.

Silicon effectively inhibits cadmium (Cd) uptake in rice, iron plaque on root surface was the primary link and first interface of Cd entering into rice root. To elucidate the mechanism of iron plaque under silicon treatment on root Cd uptake, the morphological characteristics of iron plaque, mechanisms of Cd adsorption of iron plaque and effect of iron plaque on Cd uptake by rice roots of Yuzhenxiang (YZX) and Xiangwanxian (XWX) rice varieties were studied by employing energy spectrum analysis technique, non-invasive micro-test technique, and isothermal-kinetic adsorption method. Scanning electron microscopy-X-ray energy dispersive (SEM-EDX) analysis showed that denser crystal structure of iron plaque was observed at Si treatment, silicon promoted the thickening of iron plaque and strengthened the isolation of iron plaque to Cd, which reduced the Cd content of white roots of YZX and XWX varieties by 30.2% and 20.9% respectively. However, the blocking effect of iron plaque on Cd was weakened under silicon treatment with iron plaque removed, Cd content in iron plaque of YZX and XWX cultivars was significantly decreased by 36.3% and 18.4%, Cd concentrations in white root and shoot was significantly increased, and the influxes of Cd2+ at elongation and maturation zone of root were increased in multiples. The results of adsorption test showed that the adsorption process of iron plaque was mainly a monolayer adsorption completed by boundary diffusion. The X-ray photoelectron spectroscopy (XPS) results demonstrated that silicon changed the biochemical composition of iron plaque and increased the density of the carbon-oxygen bound groups on iron plaque, which is the most likely reasons for the higher affinity of Cd adsorption ability of iron plaque observed in the silicon treated iron plaque. This study suggested the silicon-facilitated iron plaque have played critical effects in controlling the Cd accumulation in rice roots by changing the morphology and chemical composition of iron plaque.

A new insight into the role of iron plaque in arsenic and cadmium accumulation in rice (Oryza sativa L.) roots.

Iron plaque, naturally iron-manganese (hydr)oxides adhered to the surface of rice roots, controls the sequestration and accumulation of arsenic (As) and cadmium (Cd) in the paddy soil-rice system. However, the effects of the paddy rice growth on the iron plaque formation and As and Cd accumulation of rice roots are often neglected. This study explores the distribution characteristics of iron plaques on rice roots and their effects on As and Cd sequestration and uptake via cutting the rice roots into 5 cm segments. Results indicated that the percentages of rice root biomass of 0-5 cm, 5-10 cm, 10-15 cm, 15-20 cm, and 20-25 cm are 57.5 %, 25.2 %, 9.3 %, 4.9 %, and 3.1 %, respectively. Iron (Fe) and manganese (Mn) concentrations in iron plaques on rice roots of various segments are 41.19-81.11 g kg-1 and 0.94-3.20 g kg-1, respectively. Increased tendency of Fe and Mn concentrations from the proximal rice roots to the distal rice roots show that iron plaque is more likely to deposit on the distal rice roots than proximal rice roots. The DCB-extractable As and Cd concentrations of rice roots with various segments are 694.63-1517.23 mg kg-1 and 9.00-37.58 mg kg-1, displaying a similar trend to the distribution characteristics of Fe and Mn. Furthermore, the average transfer factor (TF) of As (0.68 ± 0.26) from iron plaque to rice roots was significantly lower than that of Cd (1.57 ± 0.19) (P < 0.05). There was a significant positive correlation between the Cd sequestration in iron plaque and the Cd accumulation in rice roots (R = 0.97, P < 0.01). Still, a similar correlation wasn't observed between As sequestration in iron plaque and As accumulation in rice roots (R = -0.04, and P > 0.05). These results indicated that the formed iron plaque might act as a barrier to As uptake by rice roots and a facilitator to Cd uptake. This study provides insight into the role of iron plaque in the sequestration and uptake of As and Cd in paddy soil-rice systems.

Visualization and quantification of carbon "rusty sink" by rice root iron plaque: Mechanisms, functions, and global implications.

Paddies contain 78% higher organic carbon (C) stocks than adjacent upland soils, and iron (Fe) plaque formation on rice roots is one of the mechanisms that traps C. The process sequence, extent and global relevance of this C stabilization mechanism under oxic/anoxic conditions remains unclear. We quantified and localized the contribution of Fe plaque to organic matter stabilization in a microoxic area (rice rhizosphere) and evaluated roles of this C trap for global C sequestration in paddy soils. Visualization and localization of pH by imaging with planar optodes, enzyme activities by zymography, and root exudation by 14 C imaging, as well as upscale modeling enabled linkage of three groups of rhizosphere processes that are responsible for C stabilization from the micro- (root) to the macro- (ecosystem) levels. The 14 C activity in soil (reflecting stabilization of rhizodeposits) with Fe2+ addition was 1.4-1.5 times higher than that in the control and phosphate addition soils. Perfect co-localization of the hotspots of ß-glucosidase activity (by zymography) with root exudation (14 C) showed that labile C and high enzyme activities were localized within Fe plaques. Fe2+ addition to soil and its microbial oxidation to Fe3+ by radial oxygen release from rice roots increased Fe plaque (Fe3+ ) formation by 1.7-2.5 times. The C amounts trapped by Fe plaque increased by 1.1 times after Fe2+ addition. Therefore, Fe plaque formed from amorphous and complex Fe (oxyhydr)oxides on the root surface act as a "rusty sink" for organic matter. Considering the area of coverage of paddy soils globally, upscaling by model revealed the radial oxygen loss from roots and bacterial Fe oxidation may trap up to 130 Mg C in Fe plaques per rice season. This represents an important annual surplus of new and stable C to the existing C pool under long-term rice cropping.

Zinc Fertilizers Modified the Formation and Properties of Iron Plaque and Arsenic Accumulation in Rice (Oryza sativa L.) in a Life Cycle Study.

This study examined the effect of three forms of zinc fertilizers on arsenic (As) accumulation and speciation in rice tissues over the life cycle of this cereal crop in a paddy soil. The formation and properties of iron plaque on rice roots at the maximum tillering stage and the mature stage were also determined. Elevated As at 5 mg/kg markedly lowered the rice yield by 86%; however, 100 mg/kg Zn fertilizers significantly increased the rice yield by 354-686%, regardless of the Zn form. Interestingly, only Zn2+ significantly lowered the total As in rice grains by 17% to 3.5 mg/kg and As(III) by 64% to around 0.5 mg/kg. Zinc amendments substantially hindered and, in the case of zinc oxide bulk particles (ZnOBPs), fully prevented the crystallization of iron oxides (Fe3O4 and Fe2O3) and silicon oxide (SiO2) and altered the composition of iron plaques on rice roots. SiO2 was first reported to be a significant component of iron plaque. Overall, ZnOBPs, ZnO nanoparticles, and Zn2+ displayed significant yet distinctive effects on the properties of iron plaque and As accumulation in rice grains, providing a fresh perspective on the potentially unintended consequences of different Zn fertilizers on food safety.

Fe fortification limits rice Cd accumulation by promoting root cell wall chelation and reducing the mobility of Cd in xylem.

Fe biofortification and Cd mitigation in rice is essential for human health, thus the effects of fertilization with Fe on Cd uptake and distribution in rice need to be comprehensively studied. In this study, we investigated the roles of root iron (Fe)/manganese (Mn) plaques, root cell wall, organic acid, and expressions of Cd-transport related genes in restricting Cd uptake and translocation. The rice plants were exposed to 1 µM CdCl2 with or without the addition of three doses of Fe at 5, 50, and 500 µM EDTA-Na2Fe. The results showed that increasing supply of Fe remarkably reduced Cd accumulation in the shoots, mainly because of inhibited translocation of Cd from roots to shoots. As compared to 5 µM Fe treatment, 500 µM Fe significantly increased the ionic soluble pectin (ISP) content and decreased citric acid (CA) in the roots, thereby providing more Cd-binding sites in the cell wall of roots and reducing the mobility of Cd in xylem. Plant Fe status-mediated CA act as the main chelator for Cd mobilization, rather than through decreasing the pH. However, the plants supplied with low Fe or excess Fe facilitated the uptake of Cd in rice roots, as low Fe up-regulated the expression of Cd-transport related genes and excess Fe enhanced Cd enrichment on the root by iron plaque. Importantly, soil fertilization with Fe strongly reduced Cd accumulation in rice grain. Thus, optimizing the soil environmental Fe could effectively reduce Cd accumulation in the shoots by immobilizing Cd in the roots.

An insight into the act of iron to impede arsenic toxicity in paddy agro-system.

Surplus research on the widespread arsenic (As) revealed its disturbing role in obstructing the metabolic function of plants. Also, the predilection of As towards rice has been an interesting topic. Contrary to As, iron (Fe) is an essential micronutrient for all life forms. Past findings propound about the enhanced As-resistance in rice plants during Fe supplementation. Thus, considering the severity of As contamination and resulting exposure through rice crops, as well as the studied cross-talks between As and Fe, we found this topic of relevance. Keeping these in view, we bring this review discussing the presence of As-Fe in the paddy environment, the criticality of Fe plaque in As sequestration, and the effectiveness of various Fe forms to overcome As toxicity in rice. This type of interactive analysis for As and Fe is also crucial in the context of the involvement of Fe in cellular redox activities such as oxidative stress. Also, this piece of work highlights Fe biofortification approaches for better rice varieties with optimum intrinsic Fe and limited As. Though elaborated by others, we lastly present the acquisition and transport mechanisms of both As and Fe in rice tissues. Altogether we suggest that Fe supply and Fe plaque might be a prospective agronomical tool against As poisoning and for phytostabilization, respectively.

Impact of Biochars on the Iron Plaque Formation and the Antimony Accumulation in Rice Seedings.

Biochar was a kind of restoration material for soil pollution. Investigation about biochar amendment on the Sb transformation in rice plants is scarce. The pot experiment was conducted to evaluate the impact of biochar on the iron plaque formation in Sb-contaminated soil, and the translocation and accumulation of Sb in rice seedings. After the straw and husk biochar amendments (5% by weight), the levels increased on average by 20.0% and 16.0% for exchangeable Sb in soil, and by 233.3% and 74.8% for soluble Sb in pore water, respectively; but the residual fractions of Sb decreased by 18.5% and 15.1%. The iron plaque formation on rice root surface was enhanced, but its sequestration capacity for Sb decreased due to increasing competition for binding sites led by the elevated phosphorus and silicon levels in pore water after biochar application. The shoot Sb content sharply increased by 215.8% upon straw biochar application.

Effect of Iron Plaque on Gaseous Elemental Mercury Uptake, Translocation, and Volatilization by Paddy Rice.

Paddy rice is a typical wetland plant species, and mercury (Hg) accumulation in this rice has received much attention over the last two decades. The role of root iron plaque on rice Hg accumulation is not well understood. The effects of iron plaque on Hg0 uptake, translocation, and volatilization in rice seedlings were investigated under hydroponic conditions using different rice genotypes. After induction of iron plaque on rice roots with pretreatment solutions containing 0, 15 and 30 mg Fe2+L-1, rice seedlings were transplanted into specially designed airtight culture chambers, where roots were separated from the aerial parts and exposed to saturated Hg0 vapor. The results showed the following: (1) There were significant differences in the amount of iron plaque formed on the rice roots among the three genotypes. (2) A significant correlation was observed between the concentrations of Hg and Fe in the iron plaque of the root surface for the three genotypes (R2 = 0.933, p < 0.01). (3) Iron plaque may act as a barrier for Hg0 behavior, i.e., inhibiting the process of Hg0 uptake and translocation from the rhizosphere.

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 Na2SO4) 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.

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 - 10.5 µg/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 less α-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.

Water Management Alters Cadmium Isotope Fractionation between Shoots and Nodes/Leaves in a Soil-Rice System.

The drainage of rice soils increases Cd solubility and results in high Cd concentrations in rice grains. However, plant Cd uptake is limited by sorption to iron plaques, and Cd redistribution in the plant is regulated by the nodes. To better understand the interplay of Cd uptake and redistribution in rice under drained and flooded conditions, we determined stable Cd isotope ratios and the expression of genes coding transporters that can transport Cd into the plant cells in a pot experiment. In soil, both water management practices showed similar patterns of isotope variation: the soil solution was enriched in heavy isotopes, and the root Fe plaque was enriched in light isotopes. In rice, the leaves were heavier (Δ114/110Cdleaf-shoot = 0.17 to 0.96‰) and the nodes were moderately lighter (Δ114/110Cdnode-shoot = -0.26 to 0.00‰) relative to the shoots under flooded conditions, indicating preferential retention of light isotopes in nodes and export of heavy isotopes toward leaves. This is generally reversed under drained conditions (Δ114/110Cdleaf-shoot = -0.25 to -0.04‰, Δ114/110Cdnode-shoot = 0.10 to 0.19‰). The drained treatment resulted in significantly higher expression of OsHMA2 and OsLCT1 (phloem loading) but lower expression of OsHMA3 (vacuolar sequestration) in nodes and flag leaves relative to the flooded treatment. It appeared that OsHMA2 and OsLCT1 might preferentially transport isotopically heavier Cd, and the excess Cd was purposefully retranslocated via the phloem under drained conditions when the vacuoles could not retain more Cd. Cd in seeds was isotopically heavier than that in stems under both water management practices, indicating that heavy isotopes were preferentially transferred toward seeds via the phloem, leaving light isotopes retained in stems. These findings demonstrate that the Fe plaque preferentially adsorbs and occludes light Cd isotopes on the root surface, and distinct water management practices alter the gene expression of key transporters in the nodes, which corresponds to a change in isotope fractionation between shoots and nodes/leaves.

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.

Constructed wetland, an eco-technology for wastewater treatment: A review on types of wastewater treated and components of the technology (macrophyte, biolfilm and substrate).

Constructed wetland (CW) represents an efficient eco-technological conglomerate interweaving water security, energy possibility and environmental protection. In the context of wastewater treatment technologies requiring substantial efficiency at reduced cost, chemical input and low environmental impact, applications of CW is being demonstrated at laboratory and field level with reasonably high contaminant removal efficiency and ecological benefits. However, along with the scope of applications, role of individual wetland component has to be re-emphasized through related research interventions. Hence, this review distinctively explores the concerns for extracting maximum benefit of macrophyte (focusing on interface of pollutant removal, root radial oxygen loss, root iron plaque, endophyte-macrophyte assisted treatment in CW, and prospects of energy harvesting from macrophyte) and role of biofilm (effect on treatment efficiency, composition and factors affecting) in a CW. Another focus of the review is on recent advances and developments in alternative low-cost substrate materials (including conventional type, industrial by-products, organic waste, mineral based and hybrid type) and their effect on target pollutants. The remainder of this review is organized to discuss the concerns of CW with respect to wastewater type (municipal, industrial, agricultural and farm wastewater). Attempt is made to analyze the practical relevance and significance of these aspects incorporating all recent developments in the areas to help making informed decisions about future directions for research and development related to CW.

Rhizosphere iron and manganese-oxidizing bacteria stimulate root iron plaque formation and regulate Cd uptake of rice plants (Oryza sativa L.).

Iron plaque is the amorphous and/or crystalline layer of Fe and Mn (hydr)oxides formed on the root surface of wetland plants. It could adsorb and co-precipitate metal(loid)s at the rhizosphere, thus modulating the uptake and accumulation of metal elements in plants. In this study, the Fe(II)/Mn(II)-oxidizing bacteria Burkholderia sp. D416 (D416) and Pseudomonas sp. YGL (YGL) were isolated from Cd-contaminated rice field, both hydroponic experiment and pot experiment were performed to assess the impact of bacterial inoculation on iron plaque formation, elemental content of the plaque, plant dry mass, antioxidant enzyme activity and Cd content in rice plants. The results revealed that inoculation with D416, YGL, and D416+YGL stimulated iron plaque formation on the root surface of the hydroponic rice. The content of C, N, O, Na, Mg, Al, Si, P, S, Cl, K, Fe and Ca in the root plaque were affected by the bacterial inoculation and varied among different plant growth stages. The pot experiment indicated that inoculation with D416 increased the root dry biomass by 58.89%, and the combined inoculation of D416 and YGL increased the dry biomass of root, shoot and grain by 16.89%, 21.66% and 23.26%, respectively. Importantly, YGL inoculation decreased the Cd translocation from root to shoot and from glume to brown rice grain by 50.00% and 50.27%, respectively, and the Cd content in shoot and brown rice grain were decreased by 20.00% and 34.48%, respectively. Taken together, the elemental content of the iron plaque and Cd content in rice plants varied among different plant growth stages and when plants were inoculated with different bacterial strains. YGL dramatically reduced the Cd content in brown rice grain, thus it could potentially be used to reduce Cd content in rice crop grown in Cd-contaminated soils.