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The phyllosphere is a vital yet often neglected habitat hosting diverse microorganisms with various functions. However, studies regarding how the composition and functions of the phyllosphere microbiome respond to agricultural practices, like nitrogen fertilization, are limited. This study investigated the effects of long-term nitrogen fertilization with different levels (CK, N90, N210, N330) on the functional genes and pathogens of the rice phyllosphere microbiome. Results showed that the relative abundance of many microbial functional genes in the rice phyllosphere was significantly affected by nitrogen fertilization, especially those involved in C fixation and denitrification genes. Different nitrogen fertilization levels have greater effects on fungal communities than bacteria communities in the rice phyllosphere, and network analysis and structural equation models further elucidate that fungal communities not only changed bacterial-fungal inter-kingdom interactions in the phyllosphere but also contributed to the variation of biogeochemical cycle potential. Besides, the moderate nitrogen fertilization level (N210) was associated with an enrichment of beneficial microbes in the phyllosphere, while also resulting in the lowest abundance of pathogenic fungi (1.14 %). In contrast, the highest abundance of pathogenic fungi (1.64 %) was observed in the highest nitrogen fertilization level (N330). This enrichment of pathogen due to high nitrogen level was also regulated by the fungal communities, as revealed through SEM analysis. Together, we demonstrated that the phyllosphere fungal communities were more sensitive to the nitrogen fertilization levels and played a crucial role in influencing phyllosphere functional profiles including element cycling potential and pathogen abundance. This study expands our knowledge regarding the role of phyllosphere fungal communities in modulating the element cycling and plant health in sustainable agriculture., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024. Published by Elsevier B.V.)

Global interest grows in blue foods as part of sustainable diets, but little is known about the potential and environmental performance of blue foods from rice-animal coculture systems. Here, we compiled a large experimental database and conducted a comprehensive life cycle assessment to estimate the impacts of scaling up rice-fish and rice-crayfish systems in China. We find that a large amount of protein can be produced from the coculture systems, equivalent to ∼20% of freshwater aquaculture and ∼70% of marine wild capture projected in 2030. Because of the ecological benefits created by the symbiotic relationships, cocultured fish and crayfish are estimated to be carbon-negative (-9.8 and -4.7 kg of CO 2 e per 100 g of protein, respectively). When promoted at scale to displace red meat, they can save up to ∼98 million tons of greenhouse gases and up to ∼13 million hectares of farmland, equivalent to ∼44% of China's total rice acreage. These results suggest that rice-animal coculture systems can be an important source of blue foods and contribute to a sustainable dietary shift, while reducing the environmental footprints of rice production. To harvest these benefits, robust policy supports are required to guide the sustainable development of coculture systems and promote healthy and sustainable dietary change.

Wild rice has been demonstrated to possess enriched genetic diversity and multiple valuable traits involved in disease/pest resistance and abiotic stress tolerance, which provides a potential resource for sustainable agriculture. However, unlike the plant compartments such as rhizosphere, the structure and assembly of phyllosphere microbial communities of wild rice remain largely unexplored. Through amplicon sequencing, this study compared the phyllosphere bacterial and fungal communities of wild rice and its neighboring cultivated rice. The core phyllosphere microbial taxa of both wild and cultivated rice are dominated with Pantoea , Methylobacterium , Nigrospora , and Papiliotrema , which are potentially beneficial to rice growth and health. Compared to the cultivated rice, Methylobacterium , Sphingomonas , Phaeosphaeria , and Khuskia were significantly enriched in the wild rice phyllosphere. The potentially nitrogen-fixing Methylobacterium is the dominated wild-enriched microbe; Sphingomonas is the hub taxon of wild rice networks. In addition, the microbiota of wild rice was more governed by deterministic assembly with a more complicated and stable community network than the cultivated rice. Our study provides a list of the beneficial microbes in the wild rice phyllosphere and reveals the microbial divergence between wild rice and cultivated rice in the original habitats, which highlights the potential selective role of wild rice in recruiting specific microbiomes for enhancing crop performance and promoting sustainable food production. IMPORTANCE Plant microbiota are being considered a lever to increase the sustainability of food production under a changing climate. In particular, the microbiomes associated with ancestors of modern cultivars have the potential to support their domesticated cultivars. However, few efforts have been devoted to studying the biodiversity and functions of microbial communities in the native habitats of ancestors of modern crop species. This study provides a list of the beneficial microbes in the wild rice phyllosphere and explores the microbial interaction patterns and the functional profiles of wild rice. This information could be useful for the future utilization of the plant microbiome to enhance crop performance and sustainability, especially in the framework of sustainable agroecosystems.

Arsenic (As) pollution in paddy fields is a major threat to rice safety. Existing As remediation techniques are costly, require external chemical addition and degrade soil properties. Here, we report the use of plastic tubes as a recyclable tool to precisely extract As from contaminated soils. Following insertion into flooded paddy soils, polyethylene tube walls were covered by thin but massive Fe coatings of 76.9-367 mg Fe m -2 in 2 weeks, which adsorbed significant amounts of As. The formation of tube-wall Fe oxides was driven by local Fe-oxidizing bacteria with oxygen produced by oxygenic phototrophs (e.g., Cyanobacteria) or diffused from air through the tube wall. The tubes with As-bound Fe oxides can be easily separated from soil and then washed and reused. We tested the As removal efficiency in a pot experiment to remove As from ~ 20 cm depth/40 kg soils in a 2-year experiment and achieved an overall removal efficiency of 152 mg As m -2 soil year -1 , comparable to phytoremediation with the As hyperaccumulator Pteris vittata. The cost of Fe hooks was estimated at 8325 RMB ha -1 year -1 , and the profit of growing rice (around 16080 RMB ha -1 year -1 can be still maintained. The As accumulated in rice tissues was markedly decreased in the treatment (>11.1 %). This work provides a low-cost and sustainable soil remediation method for the targeted removal of As from soils and a useful tool for the study and management of the biogeochemical Fe cycle in paddy soils., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier B.V. All rights reserved.)

In straw return fields, nitrogen-fertilizers are added to mitigate microbial competition for nitrogen with plants. However, in arsenic (As)-contaminated paddy fields, the specific effects of different nitrogen fertilizers on As mobility after straw incorporation and the interactions among iron(Fe)/carbon(C)/nitrogen(N)/As are not well understood. In the reported microcosm experiment we monitored As-mobility as a function of different dosages of KNO 3 , NH 4 Cl and rice straw incorporation. Addition of both KNO 3 and NH 4 Cl significantly inhibited the As mobilization induced by straw incorporation. Following the KNO 3 addition, the As concentration in porewater dropped by 51-66% after 2 days of the incubation by restraining Fe reduction and enhancing Fe oxidation. High-dose NH 4 Cl addition reduced As in porewater by 22-43% throughout the incubation by decreasing porewater pH. High-throughput sequencing results demonstrated that KNO 3 addition enriches both the denitrifying and Fe-oxidizing bacteria, while diminishing Fe-reducing bacteria; NH 4 Cl addition has the opposite effect on Fe-reducing bacteria. Network analysis revealed that As and Fe concentrations in porewater were positively correlated with the abundance of denitrifying and Fe-reducing bacteria. This study broadens our insight into the As biogeochemistry associated with the N/C/Fe balance in soil, which are of great significance for agronomic management and mitigation the risk of As-contaminated paddy fields., (Copyright © 2022 Elsevier B.V. All rights reserved.)

International food trade is fundamental to global food security but with often negative consequences in the producing country. We propose a method of quantifying flows of inorganic arsenic (iAs) and embedded increased lifetime cancer risks (EHR) at a global scale, where negative impacts are felt on the importing country. Computations were made for 153 countries. Vietnam exports the most iAs embedded in rice (796 kg/year) followed by India (788 kg/year), Thailand (485 kg/year), and the United States (323 kg/year). We show that continental China, Indonesia, and Malaysia have the highest imports of iAs (292, 174, and 123 kg/year, respectively). Bangladesh ranks highest in EHR followed by Vietnam and Cambodia (150, 141, and 111 per 100,000, respectively). Countries that depend exclusively on imported rice are importing a substantial amount of risk, as, e.g., Kiribati and Solomon Islands (57 and 53 per 100,000, respectively). We discuss the potential policy options for reducing population dietary health risks by well-balanced apportioning of rice sources. This study targets policy design solutions based on health gains, rather than on safe levels of the risk factor alone.

Citric acid (CA) is the major exudate of rice roots, yet the effects of CA on arsenic (As) transformation and microbial community in flooded paddy soil have not been clearly elucidated. In this study, microcosms were established by amending CA to As contaminated paddy soils, mimicking the rhizosphere environment. Results showed that 0.5% CA addition significantly enhanced As mobilization after one-hour incubation, increased total As in porewater by about 20-fold. CA addition induced arsenate release into porewater, and subsequently formed ternary complex of As, iron and organic matters, inhibiting further As transformation (including arsenate reduction and arsenite methylation). Furthermore, the results of linear discriminant analysis (LDA) effect size (LEfSe) and network analysis revealed that CA addition significantly enriched bacteria associated with arsenic and iron reductions, such as Clostridium (up to 35-fold) and Desulfitobacterium (up to 4-fold). Our results suggest that CA exhibits robust ability to mobilize As through both chemical and microbial processes, increasing the risk of As accumulation by rice. This study sheds light on our understanding of As mobilization and transformation in rhizosphere soil, potentially providing effective strategies to restrict As accumulation in food crops by screening or cultivating varieties with low CA exuding., (Copyright © 2021 Elsevier B.V. All rights reserved.)

Electrotrophs play an important role in biogeochemical cycles, but the effects of long-term fertilization on electrotrophic communities in paddy soils remain unclear. Here, we explored the responses of electrotrophic communities in paddy soil-based microcosms to different long-term fertilization practices using microbial electrosynthesis systems (MESs), high-throughput quantitative PCR, and 16s rRNA gene-based Illumina sequencing techniques. Compared to the case in the unfertilized soil (CK), applications of only manure (M); only chemical nitrogen, phosphorous, and potassium fertilizers (NPK); and M plus NPK (MNPK) clearly changed the electrotrophic bacterial community structure. The Streptomyces genus of the Actinobacteria phylum was the dominant electrotroph in the CK, M, and MNPK soils. The latter two soils also favored Truepera of Deinococcus-Thermus or Arenimonas and Thioalkalispira of Proteobacteria. Furthermore, Pseudomonas of Proteobacteria and Bacillus of Firmicutes were major electrotrophs in the NPK soil. These electrotrophs consumed biocathodic currents coupled with nitrate reduction and recovered 18-38% of electrons via dissimilatory nitrate reduction to ammonium (DNRA). The increased abundances of the nrfA gene for DNRA induced by electrical potential further supported that the electrotrophs enhanced DNRA for all soils. These expand our knowledge about the diversity of electrotrophs and their roles in N cycle in paddy soils and highlight the importance of fertilization in shaping electrotrophic communities.

Straw biochar and straw application to paddy soil dramatically altered arsenic (As) biogeochemical cycling in soil-rice system, but it remains unknown how As biotransformation microbes (ABMs) contribute to these processes. In this study, rice pot experiments combining terminal restriction fragment length polymorphism (T-RFLP) analysis and clone library were performed to characterize ABMs. Through linear discriminant analysis (LDA) effect size (LEfSe) and correlation analysis, results revealed that arrA-harbouring iron-reducing bacteria (e.g., Geobacter and Shewanella) and arsC-harbouring Gammaproteobacteria (e.g., fermentative hydrogen-producing and lignin-degrading microorganisms) potentially mediated arsenate [As(V)] reduction under biochar and straw amendments, respectively. Methanogens and sulfate-reducing bacteria (SRB) carrying arsM gene might regulate methylated As concentration in soil-rice system. Network analysis demonstrated that the association among ABMs in rhizosphere was significantly stronger than that in bulk soil. Arsenite [As(III)] methylators carrying arsM gene exhibited much stronger co-occurrence pattern with arsC-harbouring As(V) reducers than with arrA-harbouring As(V) reducers. This study would broaden our insights for the dramatic variation of As biogeochemical cycling in soil-rice system after straw biochar and straw amendments through the activities of ABMs, which could contribute to the safe rice production and high rice yield in As-contaminated fields., Competing Interests: Declaration of Competing Interest The authors declare no competing financial interest., (Copyright © 2020 Elsevier B.V. All rights reserved.)

Increasing CO 2 levels are speculated to change the effects of engineered nanomaterials in soil and on plant growth. How plants will respond to a combination of elevated CO 2 and nanomaterials stress has rarely been investigated, and the underlying mechanism remains largely unknown. Here, we conducted a field experiment to investigate the rice ( Oryza sativa L. cv. IIyou) response to TiO 2 nanoparticles (nano-TiO 2 , 0 and 200 mg kg -1 ) using a free-air CO 2 enrichment system with different CO 2 levels (ambient ∼370 μmol mol -1 and elevated ∼570 μmol mol -1 ). The results showed that elevated CO 2 or nano-TiO 2 alone did not significantly affect rice chlorophyll content and antioxidant enzyme activities. However, in the presence of nano-TiO 2 , elevated CO 2 significantly enhanced the rice height, shoot biomass, and panicle biomass (by 9.4%, 12.8%, and 15.8%, respectively). Furthermore, the Kyoto Encyclopedia of Genes and Genomes pathway enrichment analysis revealed that genes involved in photosynthesis were up-regulated while most genes associated with secondary metabolite biosynthesis were down-regulated in combination-treated rice. This indicated that elevated CO 2 and nano-TiO 2 might stimulate rice growth by adjusting resource allocation between photosynthesis and metabolism. This study provides novel insights into rice responses to increasing contamination under climate change.

In China, the common use of antibiotics in agriculture is recognized as a potential public health risk through the increasing use of livestock derived manure as a means of fertilization. By doing so this may increase the transfer of antibiotic resistance genes (ARGs) from animals, to soils and plants. In this study two staple crops (rice and wheat) were investigated for ARG enrichment under differing fertilization regimes. Here, we applied 4 treatments, no fertilizer, mineral fertilizer, clean (reduced antibiotic practice) and dirty (current antibiotic practice) pig manure, to soil microcosms planted with either rice or wheat, to investigate fertilization effects on the abundance of ARGs in the respective phyllospheres. For both rice and wheat, samples were collected after two separate fertilization periods. In total, 162 unique ARGs and 5 mobile genetic elements (MGEs) were detected from all rice and wheat samples. The addition of both clean and dirty manure, enhanced ARG abundance significantly when compared to no fertilizer treatments (P < 0.001), though clean manure enriched ARGs to a lesser extent than dirty manure, in all rice and wheat samples (P < 0.001). The classes of ARGs recorded were different between crops, with wheat samples having a higher ARG diversity than rice. These results revealed that staple crops in China such as rice and wheat may be a reservoir for ARGs when clean and dirty pig manure is used for fertilization., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

Global paddy soil is the primary source of methane, a potent greenhouse gas. It is therefore highly important to understand the carbon cycling in paddy soil. Microbial reduction of iron, which is widely found in paddy soil, is likely coupled with the oxidation of dissolved organic matter (DOM) and suppresses methanogenesis. However, little is known about the biotransformation of small molecular DOM accumulated under flooded conditions and the effect of iron reduction on the biotransformation pathway. Here, we carried out anaerobic incubation experiments using field-collected samples amended with ferrihydrite and different short-chain fatty acids. Our results showed that less than 20% of short-chain fatty acids were mineralized and released to the atmosphere. Using Fourier transform ion cyclotron resonance mass spectrometry, we further found that a large number of recalcitrant molecules were produced during microbial consumption of these short-chain fatty acids. Moreover, the biotransformation efficiency of short-chain fatty acids decreased with the increasing length of carbon chains. Ferrihydrite addition promoted microbial assimilation of short-chain fatty acids as well as enhanced the activation and biotransformation of indigenous stable carbon in the soil replenished with formate. This study demonstrates the significance of ferrihydrite in the biotransformation of labile DOM and promotes a more comprehensive understanding of the coupling of iron reduction and carbon cycling in paddy soils.

Land plants directly contact soil through their roots. An enormous diversity of microbes dwelling in root-associated zones, including endosphere (inside root), rhizoplane (root surface) and rhizosphere (soil surrounding the root surface), play essential roles in ecosystem functioning and plant health. Rice is a staple food that feeds over 50% of the global population. Its root is a unique niche, which is often characterized by an oxic region (e.g. the rhizosphere) surrounded by anoxic bulk soil. This oxic-anoxic interface has been recognized as a pronounced hotspot that supports dynamic biogeochemical cycles mediated by various functional microbial groups. Considering the significance of rice production upon global food security and the methane budget, novel insights into how the overall microbial community (i.e. the microbiome) of the rice root system influences ecosystem functioning is the key to improving crop health and sustainable productivity of paddy ecosystems, and alleviating methane emissions. This mini-review summarizes the current understanding of microbial diversity of rice root-associated compartments to some extent, especially the rhizosphere, and makes a comparison of rhizosphere microbial community structures between rice and other crops/plants. Moreover, this paper describes the interactions between root-related microbiomes and rice plants, and further discusses the key factors shaping the rice root-related microbiomes., (© FEMS 2019.)

Rice plants are the habitat for large and diverse populations of microbes, which play important roles on rice health and productivity. However, the response of microbiome on rice culm to water flooding is poorly understood. In this study, the bacterial community on non-flooded (RSA) and flooded (RSB) rice culms was investigated through 16S rRNA gene sequencing. The results showed that RSA and RSB had significantly distinct bacterial communities. In RSA, Gammaproteobacteria and Pantoea were the most abundant class (57%), genus (37.06%), respectively, while in RSB, the most abundant phylum and genus was Firmicutes (54%) and Bacillus (52.63%), respectively. Compared with RSA, the abundance of 27 genera significantly increased and 21 genera significantly decreased in RSB, and some remarkably changed species, such as Aeromonas, Bacillus were identified, which are sensitive to non-flooded or flooded conditions. In addition, rare operational taxonomic units (OTUs) was much more than abundant OTUs in all samples, and RSB had significantly higher bacterial richness than RSA due to having more rare taxa. Our study would advance the insights into the microbiome of rice culms and its response to flooding, which would help to identify potential beneficial bacteria for improving crop health and sustainable productivity in agroecosystems., (© FEMS 2019.)

High throughput genomics technologies are applied widely to microbiomes in humans, animals, soil and water, to detect changes in bacterial communities or the genes they carry, between different environments or treatments. We describe a method to test the statistical significance of differences in bacterial population or gene composition, applicable to metagenomic or quantitative polymerase chain reaction data. Our method goes beyond previous published work in being universally most powerful, thus better able to detect statistically significant differences, and through being more reliable for smaller sample sizes. It can also be used for experimental design, to estimate how many samples to use in future experiments, again with the advantage of being universally most powerful. We present three example analyses in the area of antimicrobial resistance. The first is to published data on bacterial communities and antimicrobial resistance genes (ARGs) in the environment; we show that there are significant changes in both ARG and community composition. The second is to new data on seasonality in bacterial communities and ARGs in hooves from four sheep. While the observed differences are not significant, we show that a minimum group size of eight sheep would provide sufficient power to observe significance of similar changes in further experiments. The third is to published data on bacterial communities surrounding rice crops. This is a much larger data set and is used to verify the new method. Our method has broad uses for statistical testing and experimental design in research on changing microbiomes, including studies on antimicrobial resistance.

Rhizosphere microbes are critical for plant health and biogeochemical cycles. Understanding the diversity of active microorganisms in the rhizosphere is key to enhancing plant growth and productivity. We examined rhizosphere bacterial communities of rice by comparison of the 16S ribosomal subunit amplicons generated from both the total (DNA-based, 16S rRNA gene) and the active (RNA-based, 16S rRNA) soil microbiota. Analysis based on the 16S rRNA gene showed a higher microbial diversity, but with little change in bacterial populations across the growth stages of the plant. Analysis of 16S rRNA recovered much less diversity, demonstrating that much of the 16S signal was derived from free DNA, dead or inactive cells. The rRNA analysis showed a stable microbial population present in the rhizosphere, and this was distinct from that in the bulk soil, which was also stable across the growth period. Root exudates (e.g., acetate, lactate, oxalate and succinate), which are major components contributing to the rhizosphere effect, appeared to shape the bacterial community, with some taxa (e.g., Oxobacter, Lachnospiraceae, Coprococcus and α-Proteobacteria) being enhanced in the rhizosphere. Soil compartments (rhizosphere vs. bulk) had a greater effect on the bacterial communities than did the plant phenological stages, especially at the rRNA level. These results suggest that the rhizosphere effect plays a key role in structuring the bacterial communities in rhizosphere soils with a distinct effect on active and total bacterial communities., (Copyright © 2018 Elsevier B.V. All rights reserved.)

Background: Paddy soil dissolved organic matter (DOM) represents a major hotspot for soil biogeochemistry, yet we know little about its chemodiversity let alone the microbial community that shapes it. Here, we leveraged ultrahigh-resolution mass spectrometry, amplicon, and metagenomic sequencing to characterize the molecular distribution of DOM and the taxonomic and functional microbial diversity in paddy soils across China. We hypothesized that variances in microbial community significantly associate with changes in soil DOM molecular composition., Results: We report that both microbial and DOM profiles revealed geographic patterns that were associated with variation in mean monthly precipitation, mean annual temperature, and pH. DOM molecular diversity was significantly correlated with microbial taxonomic diversity. An increase in DOM molecules categorized as peptides, carbohydrates, and unsaturated aliphatics, and a decrease in those belonging to polyphenolics and polycyclic aromatics, significantly correlated with proportional changes in some of the microbial taxa, such as Syntrophobacterales, Thermoleophilia, Geobacter, Spirochaeta, Gaiella, and Defluviicoccus. DOM composition was also associated with the relative abundances of the microbial metabolic pathways, such as anaerobic carbon fixation, glycolysis, lignolysis, fermentation, and methanogenesis., Conclusions: Our study demonstrates the continental-scale distribution of DOM is significantly correlated with the taxonomic profile and metabolic potential of the rice paddy microbiome. Abiotic factors that have a distinct effect on community structure can also influence the chemodiversity of DOM and vice versa. Deciphering these associations and the underlying mechanisms can precipitate understanding of the complex ecology of paddy soils, as well as help assess the effects of human activities on biogeochemistry and greenhouse gas emissions in paddy soils.

Previous studies suggested that microbial photosynthesis plays a potential role in paddy fields, but little is known about chemoautotrophic carbon fixers in drained paddy soils. We conducted a microcosm study using soil samples from five paddy fields to determine the environmental factors and quantify key functional microbial taxa involved in chemoautotrophic carbon fixation. We used stable isotope probing in combination with phospholipid fatty acid (PLFA) and molecular approaches. The amount of microbial (13)CO2 fixation was determined by quantification of (13)C-enriched fatty acid methyl esters and ranged from 21.28 to 72.48 ng of (13)C (g of dry soil)(-1), and the corresponding ratio (labeled PLFA-C:total PLFA-C) ranged from 0.06 to 0.49%. The amount of incorporationof (13)CO2 into PLFAs significantly increased with soil pH except at pH 7.8. PLFA and high-throughput sequencing results indicated a dominant role of Gram-negative bacteria or proteobacteria in (13)CO2 fixation. Correlation analysis indicated a significant association between microbial community structure and carbon fixation. We provide direct evidence of chemoautotrophic C fixation in soils with statistical evidence of microbial community structure regulation of inorganic carbon fixation in the paddy soil ecosystem.

Anaerobic ammonium oxidation coupled to iron(III) reduction (termed Feammox) with dinitrogen, nitrite, or nitrate as the end-product is a recently discovered process of nitrogen cycling. However, Feammox has not been described in paddy soils, which are rich in iron(III) oxides and subjected to intensive nitrogen fertilization. Here, evidence for Feammox in a paddy soil chronosequence with a gradient of microbially reducible iron(III) levels was obtained in Southern China using (15)N-labeled ammonium-based isotopic tracing and acetylene inhibition techniques. Our study demonstrated the occurrence of Feammox in the chronosequence, and direct dinitrogen production was shown to be the dominant Feammox pathway. Within the chronosequence, three paddy soils with higher microbially reducible iron(III) levels had higher Feammox rates (ranged from 0.17 to 0.59 mg N kg(-1) d(-1)) compared to an uncultivated soil (0.04 mg N kg(-1) d(-1)). It is estimated that a loss of 7.8-61 kg N ha(-1) year(-1) is associated with Feammox in the examined paddy soils. Overall, we discover that rice cultivation could enrich microbially reducible iron(III), accelerate Feammox reaction and thus fuel nitrogen loss from soils, and suggest that Feammox could be a potentially important pathway for nitrogen loss in paddy soils.

Consumption of rice contaminated with potentially toxic elements (PTEs) is a major pathway for human exposure to PTEs. This is particularly true in China's so called "Cancer Villages". In this study, sewage sludge biochar (SSBC) was applied to soil (at 5% and 10%) to suppress PTE phytoavailability and as a consequence to reduce PTE levels in rice grown in mining impacted paddy soils. Risk assessment indicated that SSBC addition (10%) markedly (P≤0.05) decreased the daily intake, associated with the consumption of rice, of PTEs (As, Cd, Co, Cu, Mn, Pb and Zn by: 68, 42, 55, 29, 43, 38 and 22%, respectively). In treatments containing SSBC (10%) the health quotient (HQ) indices for PTEs (except for As, Cu and Mn) were <1, indicating that SSBC suppressed the health risk associated with PTEs in rice. The addition of SSBC (10%) markedly (P≤0.01) reduced AsIII (72%), dimethylarsinic acid (DMA) (74%) and AsV (62%) concentrations in rice. Consequentially, following SSBC application (10%), the incremental lifetime cancer (ILTR) value for iAs (AsIII+AsV) associated with the consumption of rice was significantly (P≤0.01) reduced by 66%. These findings suggest that SSBC could be a useful soil amendment to mitigating PTE exposure, through rice consumption, in China's "Cancer Villages"., (Copyright © 2014 Elsevier Ltd. All rights reserved.)

Arsenic (As) uptake by rice is largely determined by As speciation, which is strongly influenced by microbial activities. However, little is known about interactions between root and rhizosphere microbes, particularly on arsenic oxidation and reduction. In this study, two rice cultivars with different radial oxygen loss (ROL) ability were used to investigate the impact of microbially mediated As redox changes in the rhizosphere on As uptake. Results showed that the cultivar with higher ROL (Yangdao) had lower As uptake than that with lower ROL (Nongken). The enhancement of the rhizospheric effect on the abundance of the arsenite (As(III)) oxidase gene (aroA-like) was greater than on the arsenate (As(V)) reductase gene (arsC), and As(V) respiratory reductase gene (arrA), resulting in As oxidation and sequestration in the rhizosphere, particularly for cultivar Yangdao. The community of As(III)-oxidizing bacteria in the rhizosphere was dominated by α-Proteobacteria and β-Proteobacteria and was influenced by rhizospheric effects, rice straw application, growth stage, and cultivar. Application of rice straw into the soil increased As release and accumulation into rice plants. These results highlighted that uptake of As by rice is influenced by microbial processes, especially As oxidation in the rhizosphere, and these processes are influenced by root ROL and organic matter application.

Biochar addition to soil has been proposed to improve plant growth by increasing soil fertility, minimizing bioaccumulation of toxic metal(liod)s and mitigating climate change. Sewage sludge (SS) is an attractive, though potentially problematic, feedstock of biochar. It is attractive because of its large abundance; however, it contains elevated concentrations of metal(loid)s and other contaminants. The pyrolysis of SS to biochar (SSBC) may be a way to reduce the availability of these contaminants to the soil and plants. Using rice plant pot experiments, we investigated the influence of SSBC upon biomass yield, bioaccumulation of nutrients, and metal(loid)s, and green housegas (GHG) emissions. SSBC amendments increased soil pH, total nitrogen, soil organic carbon and available nutrients and decreased bioavailable As, Cr, Co, Ni, and Pb (but not Cd, Cu, and Zn). Regarding rice plant properties, SSBC amendments significantly (P ≤ 0.01) increased shoot biomass (71.3-92.2%), grain yield (148.8-175.1%), and the bioaccumulation of phosphorus and sodium, though decreased the bioaccumulation of nitrogen (except in grain) and potassium. Amendments of SSBC significantly (P ≤ 0.05) reduced the bioaccumulation of As, Cr, Co, Cu, Ni, and Pb, but increased that of Cd and Zn, though not above limits set by Chinese regulations. Finally regarding GHG emissions, SSBC significantly (P < 0.01) reduced N2O emissions and stimulated the uptake/oxidation of CH4 enough to make both the cultivated and uncultivated paddy soil a CH4 sink. SSBC can be beneficial in rice paddy soil but the actual associated benefits will depend on site-specific conditions and source of SS; long-term effects remain a further unknown.

Methylation of arsenic in soil influences its environmental behavior and accumulation by plants, but little is known about the factors affecting As methylation. As speciation was determined in the pore waters of six soils from diverse geographical locations over 54 days of incubation under flooded conditions. The concentration of methylated As (monomethylarsonic acid, MMA, and dimethylarsinic acid, DMA) varied from 0 to 85 μg L(-1) (0 - 69% of the total As in pore water). Two Bangladeshi paddy soils contaminated by irrigation of As-laden groundwater produced large concentrations of inorganic As but relatively little methylated As. Two contaminated paddy soils from China produced a transient peak of DMA during the early phase of incubation. Methylated As represented considerable proportions of the total soluble As in the two uncontaminated soils from the UK and U.S. The copy number of the microbial arsenite methyltransferase gene (arsM) correlated positively with soil pH. However, pore-water methylated As correlated negatively with pH or arsM copy number, and positively with dissolved organic C. GeoChip assay revealed considerable arsM diversity among the six soils, with 27-35 out of 66 sequences in the microarray being detected. As speciation in rice plants grown in the soils generally mirrored that in the pore water. The results suggest that methylated As species in plants originated from the soil and As methylation in soil was influenced strongly by the soil conditions.

Rice is a major source of inorganic arsenic (iAs) in the human diet because paddy rice is efficient at accumulating As. Rice As speciation is dominated by iAs and dimethylarsinic acid (DMA). Here we review the global pattern in rice As speciation and the factors causing the variation. Rice produced in Asia shows a strong linear relationship between iAs and total As concentration with a slope of 0.78. Rice produced in Europe and the United States shows a more variable, but generally hyperbolic relationship with DMA being predominant in U.S. rice. Although there is significant genotypic variation in grain As speciation, the regional variations are primarily attributed to environmental factors. Emerging evidence also indicates that methylated As species in rice are derived from the soil, while rice plants lack the As methylation ability. Soil flooding and additions of organic matter increase microbial methylation of As, although the microbial community responsible for methylation is poorly understood. Compared with iAs, methylated As species are taken up by rice roots less efficiently but are transported to the grain much more efficiently, which may be an important factor responsible for the spikelet sterility disorder (straight-head disease) in rice. DMA is a weak carcinogen, but the level of ingestion from rice consumption is much lower than that of concern. Questions that require further investigations are identified.

Methylated arsenic (As) species are a common constituent of rice grains accounting for 10-90% of the total As. Recent studies have shown that higher plants are unlikely to methylate As in vivo suggesting that As methylation is a microbial mediated process that occurs in soils prior to plant uptake. In this study, we designed primers according to the conserved essential amino acids and structural motifs of arsenite S-adenosylmethionine methyltransferase (ArsM). We report for the first time the successful amplification of the prokaryotic arsM gene in 14 tested soils with wide ranging As concentrations. The abundance and diversity of the arsM gene in the rice rhizosphere soil and roots were analyzed using the designed primers. Results showed that microbes containing arsM genes were phylogenetically diverse, as revealed by the clone library and terminal restriction fragment length polymorphism (T-RFLP) analysis, and were branched into various phyla. Concentration of methylated As species in the soil solution was elevated in the rhizosphere soil and also by the addition of rice straw into the paddy soil, corresponding to the elevated abundance of the arsM gene in the soil. These results, together with evidence of horizontal gene transfer (HGT) of the arsM gene, suggest the genes encoding ArsM in soils are widespread. These findings demonstrate why most rice, when compared with other cereals, contains unusually high concentrations of methylated As species.

Fish consumption is considered as the primary pathway of human methylmercury (MeHg) exposure. However, recent studies highlighted that, rice, rather than fish, is the main route of human MeHg exposure in Guizhou, inland China. China is considered as the largest anthropogenic source of mercury (Hg) emission in the world, which has led to serious environmental Hg pollution. But there are no comprehensive studies regarding this environmental health problem to evaluate human Hg exposure and associated health effects. This study aimed to estimate daily MeHg intake and health risk in 7 provinces in southern China, and to assess the relative contribution from rice and fish consumption. The average levels of total mercury (THg) and MeHg in rice samples were generally low at 10.1 ng·g⁻¹ and 2.47 ng·g⁻¹, respectively. But a total of 36 rice samples (12.7%) had THg concentration exceeding the national limit (20 ng·g⁻¹). Generally, rural population had significantly higher Probable Daily Intakes (PDIs) of MeHg than urban population from rice consumption and its relative contribution to MeHg exposure increased significantly from coastal to inland area. The averages of PDIs of MeHg were 0.020 μg·kg⁻¹·d⁻¹ and 0.028 μg·kg⁻¹·d⁻¹ for urban and rural population in southern China, respectively. Despite the serious environmental Hg pollutions in China, the general population in southern China had low risk of MeHg exposure. But rice is an important route of human MeHg exposure in southern China, especially for the rural population in inland area. The findings indicate that rice consumption should be considered when evaluating MeHg exposure in rice eating population in southern China., (Copyright © 2012 Elsevier Ltd. All rights reserved.)

Recent studies have shown that higher plants are unable to methylate arsenic (As), but it is not known whether methylated As species taken up by plants can be volatilized. Rice (Oryza sativa L.) plants were grown axenically or in a nonsterile soil using a two-chamber system. Arsenic transformation and volatilization were investigated. In the axenic system, uptake of As species into rice roots was in the order of arsenate (As(V)) > monomethylarsonic acid (MMAs(V)) > dimethylarsinic acid (DMAs(V)) > trimethylarsine oxide (TMAs(V)O), but the order of the root-to-shoot transport index (Ti) was reverse. Also, volatilization of trimethylarsine (TMAs) from rice plants was detected when plants were treated with TMAs(V)O but not with As(V), DMAs(V), or MMAs(V). In the soil culture, As was volatilized mainly from the soil. Small amounts of TMAs were also volatilized from the rice plants, which took up DMAs(V), MMAs(V), and TMAs(V)O from the soil solution. The addition of dried distillers grain (DDG) to the soil enhanced As mobilization into the soil solution, As methylation and volatilization from the soil, as well as uptake of different As species and As volatilization from the rice plants. Results show that rice is able to volatilize TMAs after the uptake of TMAs(V)O but not able to convert inorganic As, MMAs(V) or DMAs(V) into TMAs and that the extent of As volatilization from rice plants was much smaller than that from the flooded soil.

Foods produced on soils impacted by antimony (Sb) mining activities are a potential health risk due to plant uptake of the contaminant metalloids (Sb) and arsenic (As). Here we report for the first time the chemical speciation of Sb in soil and porewater of flooded paddy soil, impacted by active Sb mining, and its effect on uptake and speciation in rice plants (Oryza sativa L. cv Jiahua). Results are compared with behavior and uptake of As. Pot experiments were conducted under controlled conditions in a climate chamber over a period of 50 days. In pots without rice plants, flooding increased both the concentration of dissolved Sb (up to ca. 2000 μg L(-1)) and As (up to ca. 1500 μg L(-1)). When rice was present, Fe plaque developing on rice roots acted as a scavenger for both As and Sb, whereby the concentration of As, but not Sb, in porewater decreased substantially. Dissolved Sb in porewater, which occurred mainly as Sb(V), correlated with Ca, indicating a solubility governed by Ca antimonate. No significant differences in bioaccumulation factor and translocation factor between Sb and As were observed. Greater relative concentration of Sb(V) was found in rice shoots compared to rice root and porewater, indicating either a preferred uptake of Sb(V) or possibly an oxidation of Sb(III) to Sb(V) in shoots. Adding soil amendments (olivine, hematite) to the paddy soil had no effect on Sb and As concentrations in porewater.

Wetland plants possess the unique ability to release oxygen as well as organic matter into the rhizosphere. It is understood that microbial fuel cells (MFCs) can use organic matter from plants as key electron donors, but the effect of root excreted oxygen on MFCs is presently unknown. In this study, a novel biocathode was buried in the rice rhizosphere and found to be capable of delivering electrons to root excreted oxygen for oxygen reduction reactions. The voltages between electrodes in the rhizosphere and bulk soil were found to increase initially, but dissipate after approximately 1 month. Results from the MFC and oxygen microelectrode experiments indicated that the oxygen efflux rate from rice roots was dependent on the root maturity. Furthermore, the excreted oxygen from wetland plant roots could be used for the construction of highly efficient biocathodes., (Copyright © 2011 Elsevier Ltd. All rights reserved.)

• Inorganic arsenic (As(i) ) in rice (Oryza sativa) grains is a possible threat to human health, with risk being strongly linked to total dietary rice consumption and consumed rice As(i) content. This study aimed to identify the range and stability of genetic variation in grain arsenic (As) in rice. • Six field trials were conducted (one each in Bangladesh and China, two in Arkansas, USA over 2 yr, and two in Texas, USA comparing flooded and nonflood treatments) on a large number of common rice cultivars (c. 300) representing genetic diversity among international rice cultivars. • Within each field there was a 3-34 fold range in grain As concentration which varied between rice subpopulations. Importantly, As(i) correlated strongly with total As among a subset of 40 cultivars harvested in Bangladesh and China. • Genetic variation at all field sites was a large determining factor for grain As concentration, indicating that cultivars low in grain As could be developed through breeding. The temperate japonicas exhibited lower grain As compared with other subpopulations. Effects for year, location and flooding management were also statistically significant, suggesting that breeding strategies must take into account environmental factors., (© 2011 The Authors. New Phytologist © 2011 New Phytologist Trust.)

• Biotransformation of arsenic includes oxidation, reduction, methylation, and conversion to more complex organic arsenicals. Members of the class of arsenite (As(III)) S-adenosylmethyltransferase enzymes catalyze As(III) methylation to a variety of mono-, di-, and trimethylated species, some of which are less toxic than As(III) itself. However, no methyltransferase gene has been identified in plants. • Here, an arsM gene from the soil bacterium Rhodopseudomonas palustris was expressed in Japonica rice (Oryza sativa) cv Nipponbare, and the transgenic rice produced methylated arsenic species, which were measured by inductively coupled plasma mass spectrometry (ICP-MS) and high-performance liquid chromatography-inductively coupled plasma mass spectrometry (HPLC-ICP-MS). • Both monomethylarsenate (MAs(V)) and dimethylarsenate (DMAs(V)) were detected in the roots and shoots of transgenic rice. After 12 d exposure to As(III), the transgenic rice gave off 10-fold greater volatile arsenicals. • The present study demonstrates that expression of an arsM gene in rice induces arsenic methylation and volatilization, theoretically providing a potential stratagem for phytoremediation., (© 2011 The Authors. New Phytologist © 2011 New Phytologist Trust.)

• In order to gain insights into the transport and distribution of arsenic (As) in intact rice (Oryza sativa) plants and its unloading into the rice grain, we investigated the spatial distribution of As and the temporal variation of As concentration in whole rice plants at different growth stages. To the best of our knowledge, this is the first time that such a study has been performed. • Inductively coupled plasma mass spectroscopy (ICP-MS) and high-performance liquid chromatography (HPLC)-ICP-MS were used to analyze total As concentration and speciation. Moreover, synchrotron-based X-ray fluorescence (SXRF) was used to investigate in situ As distribution in the leaf, internode, node and grain. • Total As concentrations of vegetative tissues increased during the 2 wk after flowering. The concentration of dimethylarsinic acid (DMA) in the caryopsis decreased progressively with its development, whereas inorganic As concentration remained stable. The ratios of As content between neighboring leaves or between neighboring internodes were c. 0.6. SXRF revealed As accumulation in the center of the caryopsis during its early development and then in the ovular vascular trace. • These results indicate that there are different controls on the unloading of inorganic As and DMA; the latter accumulated mainly in the caryopsis before flowering, whereas inorganic As was mainly transported into the caryopsis during grain filling. Moreover, nodes appeared to serve as a check-point in As distribution in rice shoots., (© The Authors (2010). Journal compilation © New Phytologist Trust (2010).)

Some paddy soils in the Bengal delta are contaminated with arsenic (As) due to irrigation of As-laden groundwater, which may lead to yield losses and elevated As transfer to the food chain. Whether these soils have a higher As bioavailability than other soils containing either geogenic As or contaminated by mining activities was investigated in a pot experiment. Fourteen soils varying in the source and the degree (4-138 mg As kg 1⁻¹) of As contamination were collected, 10 from Bangladeshi paddy fields (contaminated by irrigation water) and two each from China and the UK (geogenic or mining impacted), for comparison. Bangladeshi soils had higher percentages of the total As extractable by ammonium phosphate (specifically sorbed As) than other soils and also released more As into the porewater upon flooding. Porewater As concentrations increased with increasing soil As concentrations more steeply in Bangladeshi soils, with arsenite being the dominant As species. Rice growth and grain yield decreased markedly in Bangladeshi soils containing > 13 mg As kg 1⁻¹, but not in the other soils. Phosphate-extractable or porewater As was a better indicator of As bioavailability than total soil As. Rice straw As concentrations increased with increasing soil As concentrations; however, As phytotoxicity appeared to result in lower grain As concentrations. The relative proportions of inorganic As and dimethylarsinic acid (DMA) in grain varied among soils, and the percentage DMA was larger in greenhouse-grown plants than grain samples collected from the paddy fields of the same soil and the same rice cultivar, indicating a strong environmental influence on As species found in rice grain. This study shows that Bangladeshi paddy soils contaminated by irrigation had a higher As bioavailability than other soils, resulting in As phytotoxicity in rice and substantial yield losses.

Selenium, an essential micronutrient for humans, is insufficient in dietary intake for millions of people worldwide. Rice as the most popular staple food in the world is one of the dominant selenium (Se) sources for people. The distribution and translocation of Se from soil to grain were investigated in a Se-rich environment in this study. The Se levels in soils ranged widely from 0.5 to 47.7 mg kg(-1). Selenium concentration in rice bran was 1.94 times higher than that in corresponding polished rice. The total Se concentrations in the rice fractions were in the following order: straw>bran>whole grain>polished rice>husk. Significant linear relationships between different rice fractions were observed with each other, and Se in the soil has a linear relationship with different rice fractions as well. Se concentration in rice can easily be predicted by soil Se concentrations or any rice fractions and vice versa according to their linear relationships. In all rice samples for Se speciation, SeMet was the major Se species, followed by MeSeCys and SeCys. The average percentage for SeMet (82.9%) and MeSeCys (6.2%) was similar in the range of total Se from 2.2 to 8.4 mg kg(-1) tested. The percentage of SeCys decreased from 6.3 to 2.8%, although its concentration elevated with the increase in total Se in rice. This could be due to the fact that SeCys is the precursor for the formation of other organic Se compounds. The information obtained may have considerable significance for assessing translocation and accumulation of Se in plant.

A reconnaissance of 23 paddy fields, from three Bangladesh districts, encompassing a total of 230 soil and rice plant samples was conducted to identify the extent to which trace element characteristics in soils and irrigation waters are reflected by the harvested rice crop. Field sites were located on two soil physiographic units with distinctly different As soil baseline and groundwater concentrations. For arsenic (As), both straw and grain trends closely fitted patterns observed for the soils and water. Grain concentration characteristics for selenium (Se), zinc (Zn), and nickel (Ni), however, were markedly different. Regressions of shoot and grain As against grain Se, Zn, and Ni were highly significant (P < 0.001), exhibiting a pronounced decline in grain trace-nutrient quality with increasing As content. To validate this further, a pot experiment cultivar screening trial, involving commonly cultivated high yielding variety (HYV) rice grown alongside two U.S. rice varieties characterized as being As tolerant and susceptible, was conducted on an As-amended uniform soil. Findings from the trial confirmed that As perturbed grain metal(loid) balances, resulting in severe yield reductions in addition to constraining the levels of Se, Zn, and Ni in the grain.

For up to 1 billion people worldwide, insufficient dietary intake of selenium (Se) is a serious health constraint. Cereals are the dominant Se source for those on low protein diets, as typified by the global malnourished population. With crop Se content constrained largely by underlying geology, regional soil Se variations are often mirrored by their locally grown staples. Despite this, the Se concentrations of much of the world's rice, the mainstay of so many, is poorly characterized, for both total Se content and Se speciation. In this study, 1092 samples of market sourced polished rice were obtained. The sampled rice encompassed dominant rice producing and exporting countries. Rice from the U.S. and India were found to be the most enriched, while mean average levels were lowest in Egyptian rice: approximately 32-fold less than their North American equivalents. By weighting country averages by contribution to either global production or export, modeled baseline values for both were produced. Based on a daily rice consumption of 300 g day(-1), around 75% of the grains from the production and export pools would fail to provide 70% of daily recommended Se intakes. Furthermore, Se localization and speciation characterization using X-ray fluorescence (micro-XRF) and X-ray absorption near edge structure (micro-XANES) techniques were investigated in a Se-rich sample. The results revealed that the large majority of Se in the endosperm was present in organic forms.

Rice has been demonstrated to be one of the major contributors to arsenic (As) in human diets in addition to drinking water, but little is known about rice products as an additional source of As exposure. Rice products were analyzed for total As and a subset of samples were measured for arsenic speciation using high performance liquid chromatography interfaced with inductively coupled plasma-mass spectrometry (HPLC-ICP-MS). A wide range of rice products had total and inorganic arsenic levels that typified those found in rice grain including, crisped rice, puffed rice, rice crackers, rice noodles and a range of Japanese rice condiments as well as rice products targeted at the macrobiotic, vegan, lactose intolerant and gluten intolerance food market. Most As in rice products are inorganic As (75.2-90.1%). This study provides a wider appreciation of how inorganic arsenic derived from rice products enters the human diet.

An extensive data set of total arsenic analysis for 901 polished (white) grain samples, originating from 10 countries from 4 continents, was compiled. The samples represented the baseline (i.e., notspecifically collected from arsenic contaminated areas), and all were for market sale in major conurbations. Median total arsenic contents of rice varied 7-fold, with Egypt (0.04 mg/kg) and India (0.07 mg/kg) having the lowest arsenic content while the U.S. (0.25 mg/kg) and France (0.28 mg/kg) had the highest content. Global distribution of total arsenic in rice was modeled by weighting each country's arsenic distribution by that country's contribution to global production. A subset of 63 samples from Bangladesh, China, India, Italy, and the U.S. was analyzed for arsenic species. The relationship between inorganic arsenic contentversus total arsenic contentsignificantly differed among countries, with Bangladesh and India having the steepest slope in linear regression, and the U.S. having the shallowest slope. Using country-specific rice consumption data, daily intake of inorganic arsenic was estimated and the associated internal cancer risk was calculated using the U.S. Environmental Protection Agency (EPA) cancer slope. Median excess internal cancer risks posed by inorganic arsenic ranged 30-fold for the 5 countries examined, being 0.7 per 10,000 for Italians to 22 per 10,000 for Bangladeshis, when a 60 kg person was considered.

Paddy rice has been likened to nictiana sp in its ability to scavenge cadmium (Cd) from soil, whereas arsenic (As) accumulation is commonly an order of magnitude higher than in other cereal crops. In areas such as those found in parts of Hunan province in south central China, base-metal mining activities and rice farming coexist. Therefore there is a considerable likelihood that lead (Pb), in addition to Cd and As, will accumulate in rice grown in parts of this region above levels suitable for human consumption. To test this hypothesis, a widespread provincial survey of rice from mine spoilt paddies (n = 100), in addition to a follow-up market grain survey (n = 122) conducted in mine impacted areas was undertaken to determine the safety of local rice supply networks. Furthermore, a specific Cd, As, and Pb biogeochemical survey of paddy soil and rice was conducted within southern China, targeting sites impacted by mining of varying intensities to calibrate rice metal(loid) transfer models and transfer factors that can be used to predict tissue loading. Results revealed a number of highly significant correlations between shoot, husk, bran, and endosperm rice tissue fractions and that rice from mining areas was enriched in Cd, As, and Pb. Sixty-five, 50, and 34% of all the mine-impacted field rice was predicted to fail national food standards for Cd, As, and Pb, respectively. Although, not as elevated as the grains from the mine-impacted field survey, it was demonstrated that metal(loid) tainted rice was entering food supply chains intended for direct human consumption.

Rice is more elevated in arsenic than all other grain crops tested to date, with whole grain (brown) rice having higher arsenic levels than polished (white). It is reported here that rice bran, both commercially purchased and specifically milled for this study, have levels of inorganic arsenic, a nonthreshold, class 1 carcinogen, reaching concentrations of approximately 1 mg/kg dry weight, around 10-20 fold higher than concentrations found in bulk grain. Although pure rice bran is used as a health food supplement, perhaps of more concern is rice bran solubles, which are marketed as a superfood and as a supplement to malnourished children in international aid programs. Five rice bran solubles products were tested, sourced from the United States and Japan, and were found to have 0.61-1.9 mg/kg inorganic arsenic. Manufactures recommend approximately 20 g servings of the rice bran solubles per day, which equates to a 0.012-0.038 mg intake of inorganic arsenic. There are no maximum concentration levels (MCLs) set for arsenic or its species in food stuffs. EU and U.S. water regulations, set at 0.01 mg/L total or inorganic arsenic, respectively, are based on the assumption that 1 L of water per day is consumed, i.e., 0.01 mg of arsenic/ day. At the manufacturers recommended rice bran solubles consumption rate, inorganic arsenic intake exceeds 0.01 mg/ day, remembering that rice bran solubles are targeted at malnourished children and that actual risk is based on mg kg(-1) day(-1) intake.

The uptake of selected polycyclic aromatic hydrocarbons (PAHs) by rice (Oryza sativa) seedlings from spiked aged soils was investigated. When applied to soils aged for 4 months, naphthalene, phenanthrene, and pyrene exhibited volatilization loss of 98, 95, and 30%, respectively, with the remaining fraction being fixed by soil organic matter and/or degraded by soil microbes. In general, concentrations of the three PAHs in rice roots were greater than those in the shoots. The concentrations of root associated PHN and PYR increased proportionally with both soil solution and rhizosphere concentrations. PAH concentrations in shoots were largely independent of those in soil solution, rice roots, or rhizosphere soil. The relative contributions of plant uptake and plant-promoted rhizosphere microbial biodegradation to the total mass balance were 0.24 and 14%, respectively, based on PYR concentrations in rhizosphere and non-rhizosphere soils, the biomass of rice roots, and the dry soil weight.

Rice has a preference for uptake of ammonium over nitrate and can use ammonium-N efficiently. Consequently, transporters mediating ammonium uptake have been extensively studied, but nitrate transporters have been largely ignored. Recently, some reports have shown that rice also has high capacity to acquire nitrate from growth medium, so understanding the nitrate transport system in rice roots is very important for improving N use efficiency in rice. The present study identified four putative NRT2 and two putative NAR2 genes that encode components of the high-affinity nitrate transport system (HATS) in the rice (Oryza sativa L. subsp. japonica cv. Nipponbare) genome. OsNRT2.1 and OsNRT2.2 share an identical coding region sequence, and their deduced proteins are closely related to those from mono-cotyledonous plants. The two NAR2 proteins are closely related to those from mono-cotyledonous plants as well. However, OsNRT2.3 and OsNRT2.4 are more closely related to Arabidopsis NRT2 proteins. Relative quantitative reverse transcription-polymerase chain reaction analysis showed that all of the six genes were rapidly upregulated and then downregulated in the roots of N-starved rice plants after they were re-supplied with 0.2 mM nitrate, but the response to nitrate differed among gene members. The results from phylogenetic tree, gene structure and expression analysis implied the divergent roles for the individual members of the rice NRT2 and NAR2 families. High-affinity nitrate influx rates associated with nitrate induction in rice roots were investigated and were found to be regulated by external pH. Compared with the nitrate influx rates at pH 6.5, alkaline pH (pH 8.0) inhibited nitrate influx, and acidic pH (pH 5.0) enhanced the nitrate influx in 1 h nitrate induced roots, but did not significantly affect that in 4 to 8 h nitrate induced roots.

A pot microcosm experiment was conducted to investigate the effect of different forms of N fertilizers on As uptake by rice. Compared to a nontreated control, addition of nitrate reduced Fe(II) concentration in soil solution, while treatment with ammonium enhanced Fe(III) reduction, probably coupled to NH(4)(+) oxidation in the nonrhizosphere. Most-probable-number (MPN) enumerations revealed high densities of nitrate-dependent Fe(II)-oxidizing microorganisms. The addition of nitrate decreased Fe plaque formation on the root surface, accompanied by much lower dissolved Fe(II) concentrations in the rhizosphere soil solution compared to the nonamended control. Nitrate addition also reduced As uptake by the rice plant. These results suggest that nitrate may inhibit Fe(III) reduction and/or stimulate nitrate-dependent Fe(II) oxidation, leading to As coprecipitation with, or adsorption to, Fe(III) minerals in the soil. Although Fe plaque formation on the root surface is reduced, nitrate-dependent stimulation of Fe(II) oxidation and/or inhibition of Fe(III) reduction in the bulk soil sequesters mobile As in the soil, resulting in reduced As uptake by rice.

Inorganic arsenic is a chronic exposure carcinogen. Analysis of UK baby rice revealed a median inorganic arsenic content (n=17) of 0.11 mg/kg. By plotting inorganic arsenic against total arsenic, it was found that inorganic concentrations increased linearly up to 0.25 mg/kg total arsenic, then plateaued at 0.16 mg/kg at higher total arsenic concentrations. Inorganic arsenic intake by babies (4-12 months) was considered with respect to current dietary ingestion regulations. It was found that 35% of the baby rice samples analysed would be illegal for sale in China which has regulatory limit of 0.15 mg/kg inorganic arsenic. EU and US food regulations on arsenic are non-existent. When baby inorganic arsenic intake from rice was considered, median consumption (expressed as microg/kg/d) was higher than drinking water maximum exposures predicted for adults in these regions when water intake was expressed on a bodyweight basis.

A compartmented soil-glass bead culture system was used to investigate characteristics of arsenic accumulation in iron plaque and in mature rice plants irrigated using water with arsenic in greenhouse. Arsenic was supplied as a solution of Na3AsO4 * 12H2O at the following stages: tillering, stem elongation, booting, flowering and grain filling. The whole plant was separated into four parts and As concentrations were analyzed in DCB (dithionite-citrate-bicarbonate)-extraction, root, straw, rice husk and grain respectively. The results show that irrigation-water with arsenic has no significant effect on biomass of straw and grain. Arsenic concentrations are distributed in different components of mature rice with the ranking of iron plaque > root > straw > husk > grain. Arsenic in straw and grain just derive from soil in control, and derive from soil and irrigation-water in arsenic treatment. About 76.5% and 71.0% of total arsenic in rice straw are from soil for lines of YY-1 and 94D-64 respectively. There is no significant difference between two lines. However, about 33.6% of total arsenic in grain of YY-1 comes from irrigation-water with arsenic, and only 15.2% of total arsenic in grain of 94D1-64 is from irrigation-water with arsenic. There is a significant difference between YY-1 and 94D-64. Arsenic concentrations in rice grain are lower than the food safety limitation in China (0.7 mg x kg(-1)).

The quantitative trait loci (QTLs) associated with arsenic (As) accumulation in rice were mapped using a doubled haploid population established by anther culture of F1 plants from a cross between a Japonica cultivar CJ06 and an Indica cultivar TN1 (Oryza sativa). Four QTLs for arsenic (As) concentrations were detected in the map. At the seedling stage, one QTL was mapped on chromosome 2 for As concentrations in shoots with 24.4% phenotypic variance and one QTL for As concentrations in roots was detected on chromosome 3. At maturity, two QTLs for As concentrations in grains were found on chromosomes 6 and 8, with 26.3 and 35.2% phenotypic variance, respectively. No common loci were detected among these three traits. Interestingly, the QTL on chromosome 8 was found to be colocated for As concentrations in grain at maturity and shoot phosphorus (P) concentrations at seedling stage. These results provide an insight into the genetic basis of As uptake and accumulation in rice, and will be useful in identifying genes associated with As accumulation.

Uptake of six organic compounds, dinitrobenzene (DNB), dinitrotoluene (DNT), lindan (LIN), 1,2,3-triclorobenzene (TCB), phenanthrene (PHN) and pyrene (PYR) by freshly excised rice roots and dead rice roots (heated for 40min at 105 degrees C) were investigated. Results indicated that the uptake by the two types of roots did not increase proportionally with those in external solution. There appears to be some special chemical function of root cells other than simple absorption by the cells. The contribution of this special function can be roughly estimated by deducting the partition uptake into cells from the total uptake. Both time-dependent uptake data and concentration-dependent uptake curves demonstrate that, DNT and DNB transport is achieved presumably mainly via the symplastic pathway, while PHN and PYR transport mainly via the apoplastic pathway. For LIN and TCB, apoplastic transport pathway plays major roles in the first 2h of uptake, then symplastic transport pathway dominates uptake.

The effects of two sulfur (S) sources (SO(4)(2-), S(0)), and three rates of S application (0, 30, 120 mgS/kg) on the formation of iron plaque in the rhizosphere, and on the root surface of rice, and As (arsenic) uptake into rice (Oryza sativa L.) were studied in a combined soil-sand culture experiment. Significant differences in As uptake into rice between +S and -S treatments were observed in relation to S sources, and rates of S application. Concentrations of As in rice shoots decreased with increasing rates of S application. The mechanism could be ascribed to sulfur, induced the formation of iron plaque, since concentrations of Fe in iron plaque on quartz sands in the rhizosphere, and on the root surface of rice increased with increasing rates of S application. The results suggest that sulfur fertilization may be important for the development approaches to reducing As accumulation in rice.

Root uptake of atrazine (ATR) by rice seedlings (Oryza sativa L.) from nutrient solution was investigated with exposure periods of 48, 96, and 240h. A similar ATR uptake was carried out with two co-existing organic compounds (o-chlorophenol (CP) and 2,4-dichlorophenol (DCP)) with 48h exposure. In contact with the seedlings, the ATR level in nutrient solution decreased sharply during the early exposure and then reached relatively steady levels after 48h. It was observed that the ATR levels within whole seedlings approached the estimated equilibrium partition limits in about 48h, according to the partition-limited model utilizing the measured plant water and organic matter contents and the ATR partition coefficients with whole seedlings. However, when roots and shoots were consisted separately, the detected ATR levels in roots were lower than estimated equilibrium limit while the levels in shoots exceeded the equilibrium limit. The data with roots suggested the occurrence of rapid ATR degradation in roots. The results with shoots are intrinsically consistent with the suggested complex formation of ATR with free metal ions in shoots. The ATR levels in roots and shoots varied to a moderate extent when the seedlings were exposed to different levels of ATR-CP-DCP mixtures. The variation results presumably from the interference of coexisting CP and DCP and the phytotoxicity of the mixed chemicals.