Selenium fractionation and speciation in agriculture soils and accumulation in corn (Zea mays L.) under field conditions in Shaanxi Province, China

Aerobic Se(IV) reducing bacteria and their reducing characteristics in estuarine sediment

Microorganisms play a critical role in the biogeochemical cycling of selenium in natural ecosystems, particularly in reducing selenite (Se(IV)) to element selenium (Se(0)) which reduces its mobility and bioavailability. However, Se(IV)-reducing bacteria and their reducing characteristics in estuarine sediments remain inadequately understood. In this study, the reduction of Se(IV) was confirmed to be microbially driven through the cultivation of a mixture of estuarine sediment and Se(IV) under aerobic conditions. Community analysis indicates that Bacillus was primarily involved in the reduction of Se(IV). A strain with high salt tolerance (7.5 % NaCl) and Se(IV) resistance (up to 200 mM), Bacillus cereus SD1, was isolated from an estuarine sediment. The reduction of Se(IV) occurred concomitantly with the onset of microbial growth, and reduction capacity increased approximately 5-fold by adjusting the pH. In addition, Se(IV) reduction in Bacillus cereus SD1 was significantly inhibited by sulfite, and the key enzyme activity tests revealed the possible presence of a sulfite reductase-mediated Se(IV) reduction pathway. These research findings provide new insights into the bioreducing characteristics and the biogeochemical cycling of selenium in estuarine environments.

Fertilizer application alters cadmium and selenium bioavailability in soil-rice system with high geological background levels

The co-occurrence of cadmium (Cd) pollution and selenium (Se) deficiency commonly exists in global soils, especially in China. As a result, there is great interest in developing practical agronomic strategies to simultaneously achieve Cd remediation and Se mobilization in paddy soils, thereby enhancing food quality/safety. To this end, we conducted a field-plot trial on soils having high geological background levels of Cd (0.67 mg kg-1) and Se (0.50 mg kg-1). We explored 12 contrasting fertilizers (urea, potassium sulfate (K2SO4), calcium-magnesium-phosphate (CMP)), amendments (manure and biochar) and their combinations on Cd/Se bioavailability. Soil pH, total organic carbon (TOC), soil available Cd/Se, Cd/Se fractions and Cd/Se accumulation in different rice components were determined. No significant differences existed in mean grain yield among treatments. Results showed that application of urea and K2SO4 decreased soil pH, whereas the CMP fertilizer and biochar treatments increased soil pH. There were no significant changes in TOC concentrations. Three treatments (CMP, manure, biochar) significantly decreased soil available Cd, whereas no treatment affected soil available Se at the maturity stage. Four treatments (CMP, manure, biochar and manure＋urea＋CMP＋K2SO4) achieved our dual goal of Cd reduction and Se enrichment in rice grain. Structural equation modeling (SEM) demonstrated that soil available Cd and root Cd were negatively affected by pH and organic matter (OM), whereas soil available Se was positively affected by pH. Moreover, redundancy analysis (RDA) showed strong positive correlations between soil available Cd, exchangeable Cd and reducible Cd with grain Cd concentration, as well as between pH and soil available Se with grain Se concentration. Further, there was a strong negative correlation between residual Cd/Se (non-available fraction) and grain Cd/Se concentrations. Overall, this study identified the primary factors affecting Cd/Se bioavailability, thereby providing new guidance for achieving safe production of Se-enriched rice through fertilizer/amendment management of Cd-enriched soils.

Rational Combination of Selenium Application Rate and Planting Density to Improve Selenium Uptake, Agronomic Traits, and Yield of Dryland Maize

Exogenous selenium application could effectively improve the selenium absorption of crops affected by different climatic conditions due to changes in the planting environment and planting conditions. We planted maize at planting densities of 67,500 plants ha-1 (D1) and 75,000 plants ha-1 (D2). Five selenium fertilizer gradients of 0 mg m-2 (Se0), 7.5 mg m-2 (Se1), 15.0 mg m-2 (Se2), 22.5 mg m-2 (Se3), and 30.0 mg m-2 (Se4) were applied to investigate the response of the plants to selenium fertilizer application in terms of the gradient selenium absorption and substance accumulation. With the increase in the amount of selenium fertilizer applied, more of the selenium fertilizer will be absorbed and transported from the leaves to the grains, and the selenium content of the grains will gradually increase and exceed the selenium content of leaves. Under the D2 density in 2022, the selenium content of the grains under Se1, Se2, Se3, and Se4 treatments increased by 65.67%, 72.71%, and 250.53%, respectively, compared with that of Se0. A total of 260.55% of the plants showed a gradient of grain > leaf > cob > stalk from the Se2 treatment, and the overall selenium content of the plants increased first and then decreased. Under the D1 density, compared with the Se0, the dry matter mass of the Se1, Se2, Se3, and Se4 treatments significantly improved by 5.84%, 1.49%, and 14.26% in 2021, and significantly improved by 4.84%, 3.50%, and 2.85% in 2022. The 1000-grain weight under Se2, Se3, and Se4 treatments improved by 8.57%, 9.06%, and 15.56% compared to that under the Se0 treatment, and the yield per ha under the Se2, Se3, and Se4 treatments was 18.58%, 9.09%, and 21.42% higher than that under Se0 treatment, respectively. In addition, a reasonable combination of selenium application rate and density could improve the chlorophyll content and stem growth of dryland maize. This lays a foundation for the efficient application of selenium fertilizer and provides an important reference.

Selenosugar, selenopolysaccharide, and putative selenoflavonoid in plants

Selenium (Se) is a naturally occurring essential micronutrient that is required for human health. Selenium supports cellular antioxidant defense and possesses bioeffects such as anti-inflammation, anti-cancer, anti-diabetic, and cardiovascular and liver protective effects arising from Se-enhanced cellular antioxidant activity. Past studies on Se have focused on elucidating Se speciation in foods, biofortification strategies to produce Se-enriched foods to address Se deficiency in the population, and the biochemical activities of Se in health. The bioavailability and toxicity of Se are closely correlated to its chemical forms and may exhibit varying effects on body physiology. Selenium exists in inorganic and organic forms, in which inorganic Se such as sodium selenite and sodium selenate is more widely available. However, it is a challenge for safe and effective supplementation considering inorganic Se low bioavailability and high cytotoxicity. Organic Se, by contrast, exhibits higher bioavailability and lower toxicity and has a more diverse composition and structure. Organic Se exists as selenoamino acids and selenoproteins, but recent research has provided evidence that it also exists as selenosugars, selenopolysaccharides, and possibly as selenoflavonoids. Different food categories contain various Se compounds, and their Se profiles vary significantly. Therefore, it is necessary to delineate Se speciation in foods to understand their impact on health. This comprehensive review documents our knowledge of the recent uncovering of the existence of selenosugars and selenopolysaccharides and the putative evidence for selenoflavonoids. The bioavailability and bioactivities of these food-derived organic Se compounds are highlighted, in addition to their composition, structural features, and structure-activity relationships.

Selenium - An environmentally friendly micronutrient in agroecosystem in the modern era: An overview of 50-year findings

Agricultural productivity is constantly being forced to maintain yield stability to feed the enormously growing world population. However, shrinking arable and nutrient-deprived soil and abiotic and biotic stressor (s) in different magnitudes put additional challenges to achieving global food security. Though well-defined, the concept of macro, micronutrients, and beneficial elements is from a plant nutritional perspective. Among various micronutrients, selenium (Se) is essential in small amounts for the life cycle of organisms, including crops. Selenium has the potential to improve soil health, leading to the improvement of productivity and crop quality. However, Se possesses an immense encouraging phenomenon when supplied within the threshold limit, also having wide variations. The supplementation of Se has exhibited promising outcomes in lessening biotic and abiotic stress in various crops. Besides, bulk form, nano-Se, and biogenic-Se also revealed some merits and limitations. Literature suggests that the possibilities of biogenic-Se in stress alleviation and fortifying foods are encouraging. In this article, apart from adopting a combination of a conventional extensive review of the literature and bibliometric analysis, the authors have assessed the journey of Se in the "soil to spoon" perspective in a diverse agroecosystem to highlight the research gap area. There is no doubt that the time has come to seriously consider the tag of beneficial elements associated with Se, especially in the drastic global climate change era.

Integrative risk assessment method via combining geostatistical analysis, random forest, and receptor models for potentially toxic elements in selenium-rich soil

Revealing the spatial features and source of associated potentially toxic elements (PTEs) is crucial for the safe use of selenium (Se)-rich soils. An integrative risk assessment (GRRRA) approach based on geostatistical analysis (GA), random forest (RF), and receptor models (RMs) was first established to investigate the spatial distribution, sources, and potential ecological risks (PER) of PTEs in 982 soils from Ziyang City, a typical natural Se-rich area in China. RF combined with multiple RMs supported the source apportionment derived from the RMs and provided accurate results for source identification. Then, quantified source contributions were introduced into the risk assessment. Eighty-three percent of the samples contain Cd at a high PER level in local Se-rich soils. GA based on spatial interpolation and spatial autocorrelation showed that soil PTEs have distinct spatial characteristics, and high values are primarily distributed in this research areas. Absolute principal component score/multiple line regression (APCS/MLR) is more suitable than positive matrix factorization (PMF) for source apportionment in this study. RF combined with RMs more accurately and scientifically extracted four sources of soil PTEs: parent material (48.91%), mining (17.93%), agriculture (8.54%), and atmospheric deposition (24.63%). Monte Carlo simulation (MCS) demonstrates a 47.73% probability of a non-negligible risk (RI > 150) caused by parent material and 3.6% from industrial sources, respectively. Parent material (64.20%, RI = 229.56) and mining (16.49%, RI = 58.96) sources contribute to the highest PER of PTEs. In conclusion, the GRRRA method can comprehensively analyze the distribution and sources of soil PTEs and effectively quantify the source contribution to PER, thus providing the theoretical foundation for the secure utilization of Se-rich soils and environmental management and decision making.

Life-cycle selenium accumulation and its correlations with the rhizobacteria and endophytes in the hyperaccumulating plant Cardamine hupingshanensis

Cardamine hupingshanensis (C. hupingshanensis) is known for its ability to hyperaccumulate selenium (Se). However, the roles of the rhizobacteria or endophytes in Se hyperaccumulation have not been explored in C. hupingshanensis. Here, in-situ-like pot experiments were conducted to investigate the characteristics of Se accumulation throughout C. hupingshanensis growth stages and its correlations with rhizobacteria and endophytes under varying soil Se levels. Results showed that Se levels in roots, stems and leaves increased from the seedling to bolting stage, but remained relatively stable during the flowering and maturity. Leaves exhibited the highest Se levels (736.48 ± 6.51 mg/kg DW), followed by stems (575.39 ± 27.05 mg/kg DW), and lowest in roots (306.62 ± 65.45 mg/kg DW) under high-Se stress. The Se translocation factors from soils to C. hupingshanensis roots was significantly higher (p < 0.05) in low-Se soils compared to medium- and high-Se soils. Rhizobacterial diversity showed significant positive correlations (p < 0.05) with both total and bioavailable soil Se contents. The levels of soil Se and growth stages of C. hupingshanensis were found to have significant effects (p < 0.03) on the compositions of rhizosphere bacteria and C. hupingshanensis endophytes. Low-abundance bacteria (< 5%), including Gemmatimonadetes, Latescibacteria and Nitrospirae, were identified to potentially increase the bioavailable Se levels in the rhizosphere. The Se accumulation significantly decreased (p < 0.05) in C. hupingshanensis grown in sterilized low- (32.4%), medium- (17%) and high-Se (42%) soils. Endophytes in C. hupingshanensis, such as Firmicutes and Proteobacteria, were likely recruited from the rhizobacteria, as evidenced by the isolated bacterial strains, and played an important role in Se hyperaccumulation, particularly during the flowering stage. This study provides new insights into potential mechanism underlying Se hyperaccumulation in C. hupingshanensis.

Irrigation leads to new Se-toxicity paddy fields in and around typical Se-toxicity area

Although the issue has been of much concern and has subsequently been controlled for years, the environmental risk of excess selenium (Se) in farmlands still has not been eliminated in Se-toxicity areas. Different types of farmland utilization can change Se behavior in soil. Thus, located field monitoring and surveys of various farmland soils in and around typical Se-toxicity areas spanning eight years were conducted in the tillage layer and deeper soils. The source of new Se contamination in farmlands was traced along the irrigation and natural waterway. This research indicated that 22 % of paddy fields increased to Se-toxicity in surface soil led by irrigation with high-Se river water. Selenate is the dominant Se species in rivers (90 %) originating from geological background areas with high Se. Both soil organic matter (SOM) and amorphous iron content played important roles in the fixation of input Se. Thus, available Se was increased by more than twofold in paddy fields. The release of residual Se and eventual bounding by organic matter is commonly observed, thus suggesting that stable soil Se availability seems sustainable for a long time. This study is the first report in China that shows how new soil Se-toxicity farmland is caused by high-Se water irrigation. This research warns that external attention should be paid to the selection of irrigation water in high-Se geological background areas to avoid new Se contamination.

A synthetic community of siderophore-producing bacteria increases soil selenium bioavailability and plant uptake through regulation of the soil microbiome

Dietary selenium (Se) is an effective strategy to meet Se requirement of human body, and Se biofortification in crops in seleniferous soils with selenobacteria represents an eco-friendly biotechnique. In this study, we tested the effectiveness of siderophore-producing bacterial (SPB) synthetic communities (SynComs) in promoting plant Se uptake in a subtropical seleniferous soil where the fixation of Se by ferric-oxides is severe. The results indicated that SPB SynComs drastically elevated soil bioavailable Se content by up to 68.7 %, and significantly increased plant Se concentration and uptake by up to 83.1 % and 92.2 %, respectively. Seven out of ten SPB isolates in the SynComs were enriched in soils after 120 days of inoculation. Additionally, variation partitioning analysis (VPA) revealed that the contribution of soil bacterial community (up to 42.8 %) to the increased plant Se uptake was much greater than that of soil bioavailable Se (up to 5.1 %), suggesting a direct pathway other than the pathway of mobilizing Se. The relative abundances of some operational taxonomic units (OTUs) showed significantly positive relationship with plant Se status but not with soil Se status, which supports the results of VPA. Network analysis indicates that some inoculated SPB isolates promoted plant Se uptake by regulating the native bacterial taxa. Taken together, this study demonstrates that SPB can be used in Se biofortification in crops, especially in subtropical soils.

Effects of microorganisms on soil selenium and its uptake by pak choi in selenium-enriched lateritic red soil

Data on selenium (Se) transformation, specifically the mineralization or activation of Se bound by microorganisms in natural Se-enriched soil, is limited. Therefore, this study investigates the effects of microorganisms on Se availability of Se-enriched lateritic red soil and Se uptake by pak choi. Following the incubation of Stenotrophomonas maltophilia S1 and arbuscular mycorrhizal (AM) fungi agent, the available Se content of soils increased from 35 to 66.69-117.04 μg/kg, corresponding to an increase of 90.50-234.40%. The Se bioconcentration and translocation factors in pak choi increased after adding the AM fungi agent and strain S1. The soil acid phosphatase activity, and pak choi root length, surface area, and diameter also increased. Moreover, the soil acid phosphatase activity showed a significant positive correlation with soil available Se and phosphorus content (p < 0.01). Overall, the AM fungi agent and strain S1 increased Se bioavailability by enhancing soil acid phosphatase and promoting root activity, ultimately increasing pak choi's ability to absorb available Se.

Differences in selenium concentration and bioavailability between paddy and dryland soils of China: A study based on literature collection and field sampling

Lack systematic understanding of differences in environmental behavior of selenium between paddy and dryland soils affects Se biofortification and leads to human Se-related health risks. Therefore, this study investigated differences in Se concentration and bioavailability between paddy and dryland soils using data collected from literatures and field sampling. Our analysis showed paddy soil Se concentration in Se-rich area of China was significantly lower than that in dryland soil. Selenium biological concentration factor of rice grain (BCF_grain) in Se-rich area was lower than that in non-Se-rich area attributed to higher percentage of selenite in available Se. Concentration and percentage of available Se were in dryland soil lower than those in paddy soil and this affected BCF_grain of maize, whereas BCF_grain of rice was further influenced by its Se transport capacity. The ranges of Se concentration in Se-rich paddy (0.14-3.63 mg kg^-1) and dryland (0.45-1.17 mg kg^-1) soils were derived using a linear regression model. The current soil Se concentration evaluation standard was only suitable for dryland but overestimated Se-deficiency and Se-toxicity levels in paddy field. The present study provides theoretical foundations for understanding Se concentrations and bioavailability in soils and selecting efficient and safe approach on cultivated land use.

Se(IV)/Se(VI) adsorption mechanisms on natural and on Ca-modified zeolite for Mediterranean soils amended with the modified zeolite: prospects for agronomic applications

In the present study, the ability of a modified CaCl2 zeolite (Ca-Z) to both increase Se(IV) availability and restrict Se(VI) mobility in soils is examined. As it was resulted from batch experiments and verified by X-ray absorption fine structure (XAFS) and X-ray fluorescence (XRF) spectroscopies, higher amounts of both Se species adsorbed on Ca-Z compared to natural zeolite (Z-N) forming outer-sphere complexes while the oxidation state did not alter during agitation of samples. Thereafter, Ca-Z was incorporated in six Greek soils, divided into acid and alkaline, at a 20% (w/w) rate and a series of equilibrium batch experiments were performed with soils alone and soils-Ca-Z mixtures to investigate sorption and desorption processes and mechanisms. The acid soils, either treated with Ca-Z or not, adsorbed higher amounts of Se(IV) than alkaline ones, whereas soils alone did not adsorb Se(VI) but impressively high adsorption of Se(VI) occurred in the Ca-Z-treated soils. Desorption of Se(IV) was higher from the Ca-Z-treated soils and especially from the acid soils. Higher distribution coefficients of desorption than the distribution coefficients of sorption were observed, clearly pointing to a hysteresis mechanism. The experimental data fitted with Langmuir and Freundlich isotherms. In the presence of Ca-Z, the Langmuir qm values increased indicating higher Se(IV) retention while Langmuir bL values decreased suggesting lower bonding strength and higher Se(IV) mobility. Overall, treating the soils with Ca-Z increased Se(IV) adsorption and mobility whereas it provided sites for Se(VI) adsorption that did not exist in the studied soils.

Selenium Species and Fractions in the Rock-Soil-Plant Interface of Maize (Zea mays L.) Grown in a Natural Ultra-Rich Se Environment

Selenium (Se) enrichments or deficiency in maize (Zea mays L.), one of the world's most important staple foods and livestock feeds, can significantly affect many people's diets, as Se is essential though harmful in excess. In particular, Se-rich maize seems to have been one of the factors that led to an outbreak of selenosis in the 1980s in Naore Valley in Ziyang County, China. Thus, this region's geological and pedological enrichment offers some insight into the behavior of Se in naturally Se-rich crops. This study examined total Se and Se species in the grains, leaves, stalks, and roots of 11 maize plant samples, Se fractions of soils around the rhizosphere, and representative parent rock materials from Naore Valley. The results showed that total Se concentrations in the collected samples were observed in descending order of soil > leaf > root > grain > stalk. The predominant Se species detected in maize plants was SeMet. Inorganic Se forms, mainly Se(VI), decreased from root to grain, and were possibly assimilated into organic forms. Se(IV) was barely present. The natural increases of Se concentration in soils mainly affected leaf and root dry-weight biomasses of maize. In addition, Se distribution in soils markedly correlated with the weathered Se-rich bedrocks. The analyzed soils had lower Se bioavailability than rocks, with Se accumulated predominantly as recalcitrant residual Se. Thus, the maize plants grown in these natural Se-rich soils may uptake Se mainly from the oxidation and leaching of the remaining organic-sulfide-bound Se fractions. A viewpoint shift from natural Se-rich soils as menaces to possibilities for growing Se-rich agricultural products is also discussed in this study.

Topographic constraints on the distribution of selenium in the supergene environment: A case study at Yutangba, China

The local topography and leaching conditions significantly affect the spatial distribution of selenium (Se) in the local environment. However, the driving factors controlling Se distribution have not been well addressed. In this paper, taking Yutangba, a village known for human selenosis in China, as an example, we demonstrate how topographic factors influence the spatial distribution of Se in soils and plants. In the scenarios of slope ≤25°, the correlations among slope and soil/extractable/plant Se are significantly negative (P < 0.05), whereas they become weak or unclear when the slope is > 25°, suggesting that 25° of slope is a critical transition boundary. Similar observations were further verified by the soil erosion modulus (SEM) and the surface runoff intensity index (SRI), indicating that Se transport via soil erosion is limited and accounts for 11.2-17% of the soil Se, while surface runoff plays a dominant role in the Se distribution, accounting for 83-88.1%. Soil extractable Se is negatively correlated with SRI (Pearson r = -0.87 at slope < 25°), showing that the migration capacity of Se is higher at steep terrain and controlled by topography through soil erosion and surface runoff. The positive relationship between plant Se and soil/extractable Se demonstrates that topography indirectly influences plant Se through soil Se bioavailability. Abnormally local Se enrichment observed at the elevated steep hillside (>25°) in northwestern Yutangba primarily was resulted from the weathering of Se-rich rocks. These observations confirm that the topographic slope gradient influences the transport and spatial distribution of soil Se, implying that topography should be considered when studying the spatial distribution of soil Se at a regional scale, especially for the Se-poor belt in China.

Influence of exogenous selenomethionine and selenocystine on uptake and accumulation of Se in winter wheat (Triticum aestivum L. cv. Xinong 979)

Soil selenium (Se) is mainly inorganic including selenate and selenite but organic forms such as selenomethionine (SeMet) and selenocystine (SeCys₂) are commonly present. Although organic Se is bioavailable or potentially bioavailable to plants, whether the effects of the organic Se on uptake and accumulation of Se in winter wheat differ in forms is still not clear. Both hydroponic experiments and a pot trial of whole plant growth stage were conducted to investigate the effects of SeMet and L-selenocystine (SeCys₂) on uptake and accumulation of Se in winter wheat (Triticum aestivum L. cv. Xinong 979). Not only metabolic inhibitor (carbonyl cyanide m-chlorophenylhydrazone (CCCP)) inhibited SeMet (44%) influx into wheat roots but also aquaporin inhibitor (AgNO₃) or putative inhibitor (H₂SiO₄ and H₃BO₃) suppressed 83%, 62%, or 64% SeMet influx into the roots. However, these inhibitors had insignificant effects on SeCys₂ influx into the roots. Wheat grain possessed more effective Se accumulation under SeCys₂ treatments than under SeMet treatments, which was contributed to more efficiently translocation of Se from husk to grain, more remobilization of tissue Se to grain, and significantly higher concentration of soluble Se (SOL-Se) and exchangeable and carbonate-bound Se (EXC-Se) in the rhizosphere of winter wheat. The present study indicated that the effects of organic Se on uptake and accumulation of Se in winter wheat differed in forms and that SeCys₂ exhibited the potential to increase grain Se concentration in winter wheat. The results from the present study will replenish information about the effects and related mechanisms of SeMet or SeCys₂ on uptake and accumulation of Se in winter wheat and provide insights of effects of organic Se on wheat grain Se accumulation.

Highland barley grain and soil surveys reveal the widespread deficiency of dietary selenium intake of Tibetan adults living along Yalung Zangpo River

Objective

In order to assess selenium (Se) flux through the soil-plant-human chain in Tibet plateau and explore the reason why local Tibetan adult residents from large scale agricultural production areas in Tibet lacked daily Se intake.

Methods

A total of 210 intact highland barley plants and their corresponding cultivated topsoil samples were collected in fields of 14 agricultural counties along Yalung Zangpo River and quantitative dietary data were collected from a cross-sectional survey using a cultural-specific food frequency questionnaire that contained all local Tibetan foods in 2020.

Results

The mean value of The estimated daily Se dietary intake by each participant was 17.1 ± 1.9 μg/day/adult, the Se concentration in topsoil and highland barley grain were 0.128 ± 0.015 mg/kg and 0.017 ± 0.003 mg/kg, respectively. Although highland barley was the first contributor of dietary Se in local adult residents (34.2%), the dietary Se intake provided by highland barley only about 10% of the EAR value (50 μg/day/adult) currently. A significantly positive relationship was determined between soil total Se content (STSe), available Se content (SASe) and highland barley grain Se content (GSe). The amount of Se in food system depends on a number of soil properties (TOC, pH, clay content, Fe/Mn/Al oxides), climate variables (MAP, MAT) and terrain factor (altitude).

Conclusion

To sum up, it can be inferred that the insufficient dietary Se intake of Tibetan adult population living along Yalung Zangbo River is mainly caused by the low Se content in highland barley grain, which was result from the low Se content in cultivated soil. In order to enable adult participants in the present study to achieve recommended dietary Se-intake levels, agronomic fortification with selenised fertilizers applied to highland barley could be a great solution. It is necessary to combine the influencing factors, and comprehensively consider the spatial variation of local soil properties, climatic and topographic conditions, and planting systems.

Separating selenium species by diffusion in Brazilian bentonite: a mathematical modeling approach

Bentonite was applied in diffusive studies for selenium, an emerging contaminant. The planar source method was used to determine the apparent and effective diffusion coefficients and assess the mobility of the selenium species. A double Gaussian function described the results. Different diffusion coefficients were associated with different mobilities, and consequently, to the coexistence of two selenium species: selenite and selenate. Apparent diffusion coefficients were higher for selenate, around 10^- 10 m² s^- 1, than for selenite, around 10^- 12 m² s^- 1. Results from sequential extraction and distribution coefficient justified selenate's greater mobility than selenite. Since the increase in redox potential from 448 to 511 mV may be associated with selenite oxidation in an interconversion process, the diffusion in bentonite demonstrates that applications in geological barriers deserve attention regarding the mobilization of selenium species. Interconversions can mobilize selenium, as reduced species can shift to more oxidized and mobile species, enhancing environmental contamination.

Probability of cultivating Se-rich maize in Se-poor farmland based on intensive field sampling and artificial neural network modelling

Selenium (Se) is a necessary micronutrient for humans, and its supplementation from crop grains is important to address the ubiquitous Se deficiency in people worldwide. Se uptake by crops largely depend on soil bioavailable Se rather than soil total Se content, which provides possibilities to explore the Se-rich crops in Se-poor area. Here, the possibility of cultivating Se-rich maize grains in Se-poor farmland was tested based on intensive field sampling and mathematical modelling. Sampling was conducted at county scale, and a total of 7779 topsoil samples and 109 maize samples with paired rhizosphere soils samples were collected. Results showed that although the soil Se content in the study county from southwestern China was at a low level (0.01-2.75 mg kg^-1), 54.1% of the maize grain samples satisfied the standard for Se-rich products (0.02-0.30 mg kg^-1). Soil organic matter, iron oxide, and phosphorus levels were correlated negatively with Se bioconcentration factor (BCF) of maize grain. Compared with the multivariate linear regression model, the artificial neural network (ANN) model was more accurate and reliable in predicting maize Se BCF. Prediction using the ANN model showed that 22.7% of the county's farmland was suitable for cultivating naturally Se-rich maize, which increased 21.3% growing areas than that from cultivation based on simply soil total Se. This study provided a new methodological framework for natural Se-rich maize production and verified the probability of cultivating naturally Se-rich maize in Se-poor farmland.

Effects of Basal Selenium Fertilizer Application on Agronomic Traits, Yield, Quality, and Se Content of Dryland Maize

To explore the efficiency of selenium (Se) fertilizer application in dryland maize, we tested five Se fertilizer application treatments: 0 g ha^-1 (Se0), 75 g ha^-1 (Se1), 150 g ha^-1 (Se2), 225 g ha^-1 (Se3), and 300 g ha^-1 (Se4). Compared with Se0, Se2 increased the leaf area, chlorophyll content, internode length, plant height, and ear height by 7.95%, 3.20%, 13.19%, 1.89%, and 7.98%, respectively. Se2 and Se3 significantly affected the stem internode diameter, cortex thickness, and cellulose content, which were positively correlated with lodging resistance. Compared with Se0, Se3 promoted the contents of soluble sugar, crude protein, crude fat, and starch in grains, which increased by 9.48%, 6.59%, 1.56%, and 4.82%, respectively. It implies that maize grain application of Se significantly improves their Se content. Se1 did not observably influence the growth of maize, and the promoting effect of Se4 on maize decreased. The lodging resistance of maize as analyzed by Pearson correlation analysis correlated with the application of Se fertilizer. It proved that higher yield, grain quality, grain Se content, and lodging resistance of stems were concerned with Se fertilizer application in the range of 150-225 g ha^-1. The results provide useful information for Se fertilizer treatment in dryland maize.

Understanding soil selenium accumulation and bioavailability through size resolved and elemental characterization of soil extracts

Dietary deficiency of selenium is a global health threat related to low selenium concentrations in crops. Despite the chemical similarity of selenium to the two more abundantly studied elements sulfur and arsenic, the understanding of its accumulation in soils and availability for plants is limited. The lack of understanding of soil selenium cycling is largely due to the unavailability of methods to characterize selenium species in soils, especially the organic ones. Here we develop a size-resolved multi-elemental method using liquid chromatography and elemental mass spectrometry, which enables an advanced characterization of selenium, sulfur, and arsenic species in soil extracts. We apply the analytical approach to soils sampled along the Kohala rainfall gradient on Big Island (Hawaii), which cover a large range of organic carbon and (oxy)hydroxides contents. Similarly to sulfur but contrarily to arsenic, a large fraction of selenium is found associated with organic matter in these soils. However, while sulfur and arsenic are predominantly found as oxyanions in water extracts, selenium mainly exists as small hydrophilic organic compounds. Combining Kohala soil speciation data with concentrations in parent rock and plants further suggests that selenium association with organic matter limits its mobility in soils and availability for plants.

Insight into the fraction variations of selenium and their effects on humification during composting

This study investigated the variation of selenium fractions and their effects on humification during composting. Selenite and selenate were added to a mixture of goat manure and wheat straw for composting. The results demonstrated that the bioavailable Se in the selenite added treatment (9.3-13.8%) was lower than in the selenate added treatment (18.1-47.3%). Meanwhile, the HA/FA of selenite and selenate added treatments were higher than in control, indicating that the selenium addition (especially selenite) promoted the humification of composting. Importantly, selenite enriched the abundance of Tepidimicrobium and Virgibacillus which were responsible to improve humification performance. Selenate increased the abundance of Thermobifida and Cellvibrio which facilitated the composting humification. The genes encoding CAZymes involved in the degradation of organic materials were also analyzed, and selenium could contribute to the synthesis of humus. KEGG pathway analysis revealed that the selenite addition promoted amino acids and carbohydrate metabolism compared to the control.

Chemical Structure, Hypoglycemic Activity, and Mechanism of Action of Selenium Polysaccharides

Selenium polysaccharides (Se-polysaccharides) are one of important forms of organic Se, in which selenium (Se) and polysaccharides are joined by covalent bonds. In the present review, recent progress in chemical structure and hypoglycemic activity of Se-polysaccharides is summarized. In particular, the mechanism underlying hypoglycemic capacity of Se-polysaccharides is discussed, and the relationship between hypoglycemic activity and chemical structure is analyzed. Besides, strategies for further research into chemical structure and hypoglycemic activity of Se-polysaccharides are proposed. Hypoglycemic activity of Se-polysaccharides is closely related to their inhibitory effect on α-amylase and α-glucosidase, influence on insulin signal pathway especially IRS-PI3K-Akt signaling pathway, and protection capacity against oxidative stress.

Selenium distribution in cultivated Argosols and Gleyosols of dry and paddy lands: A case study in Sanjiang Plain, Northeast China

Distribution pattern of selenium (Se) fractions in soil could influence Se content in crops and thereby intake of Se in human body. In order to investigate the effects of soil types and farming conditions on Se distribution in small-scaled cultivated land developed under the same conditions of climate, topography and parent materials, two types of soils (i.e., Argosols and Gleyosols) from paddy and dry lands in the Sanjiang Plain of Northeast China were selected. Total Se (T-Se) content in Argosols was influenced by organic carbon (Org C) content and pH of bulk topsoil. In Gleyosols, it was mainly affected by Org C content in dry land and pH in paddy land, respectively. In rice root associated topsoil, organic matter associated Se (OM-Se) accounted for 70% of T-Se. Compared with pH (median 6.10) and OM weakly bound Se (OW-Se) (0.14 ± 0.04 mg kg^-1) of Argosols, the higher pH (median 6.77) resulted in less OW-Se (0.10 ± 0.04 mg kg^-1) of Gleyosols. Vertical distribution of Se in borehole cores within the depth of 0-900 cm was mainly affected by the soil type. Se accumulated mainly within 0-150 cm depth (horizon A, E and B) in Argosols and above 40 cm depth (horizon H), existing prominently as OM strongly bound Se (OS-Se), in Gleyosols. Within the depth of 0-150 cm, various Se fractions for both soils were probably controlled by reductive fixation and complexation of Org C; In the alkaline paddy land, DOM-complexed Se was the main composition of A-Se. The findings of this study could help in understanding the mechanisms of Se distribution and enrichment in soils developed under different formation processes and farming conditions.

Microbial oxidation of organic and elemental selenium to selenite

Selenium (Se) is an essential trace element for life. Se reduction has attracted much attention in the microbial Se cycle, but there is less evidence for Se oxidation. In particular, it is unknown whether microorganisms oxidise organic Se(-II). In this study, four strains of bacteria, namely Dyella spp. LX-1 and LX-66, and Rhodanobacter spp. LX-99 and LX-100, isolated from seleniferous soil, were involved in the oxidation of selenomethionine (SeMet), selenocystine (SeCys2), selenourea and Se(0) to selenite (Se(IV)) in pure cultures. The oxidation rates of organic Se were more rapidly than those of Se(0) in liquid media. Then Se(0) and SeMet were used as examples, microbial oxidation was the predominant process for both additional Se(0) and SeMet in sterilised alkaline or acidic soils. The Se(IV) concentrations were significantly higher at pH 8.56 than at pH 5.25. In addition, water-soluble Se (SOLSe) and exchangeable and carbonate-bound Se (EXC-Se) fractions increased dramatically with these four Se-oxidising bacteria in unsterilised seleniferous soil. To our knowledge, this is the first study to find that various bacteria are involved in the oxidation of organic Se to Se oxyanions, bridging the gap of Se redox in the Se biogeochemical cycle.

Novel mechanistic insights of selenium induced microscopic, histochemical and physio-biochemical changes in tomato (Solanum lycopersicum L.) plant. An account of beneficiality or toxicity

Selenium (Se) is a well-known beneficial element in plants. The window of Se between toxic and optimal concentration is narrow and uneven which fluctuates with plants species. This experiment was aimed to investigate the morpho-physiological, microscopic and histochemical responses of two different varieties of tomato (S-22 and PKM-1), exposed to different concentrations of Se (0, 10, 40 or 80 µM), applied to soil at 30 days after transplantation (DAT). At 40 DAT, it was observed that high concentrations (40 or 80 µM) of Se radically increased oxidative stress examined by elevated reactive oxygen species (ROS), malondialdehyde (MDA) content, cell death, electrolyte leakage and decreased chlorophyll content leading phenotypic symptoms of Se-induced toxicity like stunted growth and chlorosis. Furthermore, high doses of Se altered the chloroplast and stomatal organisation, and adversely affected the photosynthetic performance of plants. But low concentration of Se improved the plant dry mass, photosynthesis, Rubisco activity, protein content and maintained the steady-state equilibrium among ROS generation and antioxidant enzymes like catalase, peroxidase and superoxide dismutase. Our outcomes proposed that high concentration of Se generated toxicity (phyto-selenosis), whereas lower concentration of Se-triggered positive impact by improving growth, photosynthetic traits and maintaining steady-state equilibrium between scavenging-system and ROS generation.

Selenium uptake and accumulation in winter wheat as affected by level of phosphate application and arbuscular mycorrhizal fungi

Selenium (Se) is an advantageous element to crops. However, the influence of arbuscular mycorrhizal fungi (AMF), phosphate (P) and selenite in soil on Se uptake by winter wheat remain elusive. Pot trials were carried out including seven levels of P (0, 12.5, 25, 50, 100, 200 or 400 mg kg^-1) and non-mycorrhizal inoculation (NM), inoculation of Funneliformis mosseae (F.m) or Glomus versiforme (G.v). The present results found that grain phosphorus concentration increased with increase of P level from 0 to 100 mg kg^-1 and then tended to plateau, while grain Se concentration decreased with the level of P from 0 to 400 mg kg^-1. Based on mathematical modeling, inoculation of F.m or G.v dramatically improved grain Se concentration by 16.90% or 12.53% under the lower level of P (48.76 mg kg^-1). Furthermore, partial least squares path modeling (PLS-PM) identified that both up-regulated of the expression of AMF-inducible phosphate transporter and improved Se bioavailability in rhizosphere soil contributed to enhancing plant Se concentration under P levels ≤ 100 mg kg^-1. The present study demonstrated that AMF combined with 48.76 mg kg^-1 P applied in soil can not only achieve high grain yield, but also fully exploit the biological potential of Se uptake in wheat.

Selenium Uptake by Lettuce Plants and Se Distribution in Soil Chemical Phases Affected by the Application Rate and the Presence of a Seaweed Extract-Based Biostimulant

To tackle selenium (Se) malnutrition, biofortification is among the proposed strategies. A biostimulant application in soils is thought to support a plant’s growth and productivity. Biofortification with Se(VI) may lead to a leaching hazard due to the high mobility of Se(VI) in the soil environment. In this study, the effect of the application of two Se(VI) rates—5 and 10 mg kg−1 soil—and a biostimulant on the Se uptake by lettuce plants and on the Se(VI) distribution in soil fractions following the plants harvest, was investigated. Phosphorus (P) and sulfur (S) concentrations in plants were also determined. A high Se(VI) rate suppressed plant growth, leading to a significant fresh weight decrease from 12.28 to 7.55 g and from 14.6 to 2.43 g for the control and high Se(VI) without and with biostimulants, respectively. Impaired plant growth was verified by the SPAD, NDVI and NDRE measurements. The significantly highest Se concentration in plants, 325 mg kg−1, was recorded for the high Se(VI) rate in the presence of the biostimulant. Compared to controls, the low Se(VI) rate significantly decreased P and increased the S concentrations in plants. The post-harvest soil fractionation revealed that, in the presence of the biostimulant, the Se(VI) soluble fraction increased from 0.992 to 1.3 mg kg−1 at a low Se(VI) rate, and decreased from 3.T85 to 3.13 mg kg−1 at a high Se(VI) rate. Nevertheless, at a low Se(VI) rate, 3.6 and 3.1 mg kg−1 of the added Se(VI) remained in the soil in less mobile forms, in the presence or absence of the biostimulant, respectively. This study indicated that the exogenous application of Se in soil exerted dual effects on lettuce growth and Se availability, depending on the level of selenate applied.

Size-dependent transformation, uptake, and transportation of SeNPs in a wheat-soil system

Foliar application of selenium nanoparticles (SeNPs) has been used to enhance Se concentration in winter wheat, but soil application of SeNPs on Se uptake in the crop and their transformation in soil are still limited. This study investigated the effects of varying sizes (50, 100, 200 nm) and concentrations (0, 2, 5, 25, 100 mg kg^-1) of chemical synthesized SeNPs in soil on uptake and accumulation of Se in the crop at maturity and related mechanisms. SeNPs not only posed very low toxic to plant growth, except for leaf, but also significantly enhanced grain Se concentration. Regardless of concentration of SeNPs added to soil, the transformation rate of the larger sized SeNPs (200 nm) in soil was significantly (p < 0.05) higher than that of the smaller one, which is mainly due to the latter was more easily adsorbed onto soil organic matter and reluctant to be oxidized. Significantly higher grain Se concentration under the larger sized SeNPs contributed to significantly higher transformation rate of SeNPs and concentration of available Se in soil. The present study showed that the larger sized SeNPs in soil had significant advantages including higher grain Se concentration and Se utilization efficiency for wheat Se biofortification.

Linking microbial community composition to farming pattern in selenium-enriched region: Potential role of microorganisms on Se geochemistry

Selenium (Se) is an essential micronutrient for lives. Indigenous microbial communities play an important role on Se geochemistry in soils. In this study, the microbial community composition and functions of 53 soil samples were investigated using high-throughput sequencing. Samples were divided into 3 groups with different farming types based on the measured geochemical parameters and microbial functional structures. Results indicated that putative Se related bacteria Bacillus, Dyella, Paenibacillus, Burkholderia and Brevibacillus were dominant in dryland plantation soils which were characterized with higher available Se and low contents of H₂O, total organic carbon (TOC), NH₄⁺ and NO₂^-. In contrast, the putative denitrifier Pseudomonas dominated in flooded paddy soils with higher TOC, NO₃^- and organic Se, whereas genera Rhizobium, Nitrosospira, and Geobacter preferred woodland soils with higher oxidation-reduction potential (ORP), pH, NH₄⁺ and Fe. Farming patterns resulted in distinct geochemical parameters including moisture, pH, ORP, TOC, and contents of soluble Fe, NO₂^- and NH₄⁺, shaping the microbial communities, which in turn affected Se forms in soils. This study provides a valuable insight into understanding of Se biogeochemistry in soils and prospective strategy for Se-rich agriculture production.

Selenium in Soil-Plant-Microbe: A Review

Selenium (Se) plays an important role in geochemistry and is an essential trace element for humans and animals. This review summarizes the transformation and accumulation of Se in the plant-soil-microbe system. As one of the important reservoirs of Se, soil is an important material basis of its entry into the food chain through plants. Soil with an appropriate amount of Se is beneficial for plant growth and plays a valuable role in a stress-resistant environment. Among the many migration and transformation pathways, the transformation of Se by microorganisms is particularly important and is the main form of Se transformation in the soil environment. In this review, the role and form transformation of Se in plants, soil, and microorganisms; the role of Se in plants; the form, input, and output of Se in soil; the absorption and transformation of Se by plants; and the role of microorganisms in Se transformation are presented. In addition to describing the migration and transformation laws of Se in the environment, this review expounds on the main directions and trends of Se research in the agricultural field as well as current gaps and difficulties in Se-related research. Overall, this reviews aims to provide necessary information and theoretical references for the development of Se-rich agriculture.

Association of selenium levels with the prevention and control of Keshan disease: A cross-sectional study

PURPOSE

The prevalence of Keshan disease (KD) is low and has reached controlled or eliminated levels even in counties that had a high KD prevalence in the past. Few nationwide surveys on selenium levels in KD areas have been conducted in the past 2 decades. We conducted a cross-sectional study to investigate the selenium levels and their association with KD control and prevention in areas where KD is prevalent.

METHODS

We collected 2143 human-hair, 698 soil, 701 rice, 607 flour, 521 corn, and 330 other-food samples from 49 counties with KD and 19 non-KD counties of nine KD provinces of China. The selenium content of samples was examined with hydride generation atomic fluorescence spectrometry. The difference in selenium levels between the KD and non-KD areas was analyzed. Cochran-Armitage trend tests were used to evaluate the association between selenium levels and KD control.

RESULTS

The selenium levels in human hair, soil, staple foods, and other foods in the KD areas (0.2996 mg/kg, 0.1380 mg/kg, 0.0190 mg/kg and 0.0076 mg/kg, respectively) were lower than those in the non-KD areas (0.3700 mg/kg, 0.1930 mg/kg, 0.0240 mg/kg and 0.0165 mg/kg, respectively). The Cochran-Armitage tests showed that there was a trend for the selenium standard ratio in the counties to increase in the order of KD uncontrolled, to controlled, to eliminated (Z = 2.229, P < 0.05).

CONCLUSION

The residents in the KD areas were found to be selenium-deficient. Improving the supply of staple foods containing selenium levels exceeding 0.025 mg/kg and abundant foodstuffs might contribute to KD control and prevention.

Arbuscular mycorrhizal fungi reverse selenium stress in Zea mays seedlings by improving plant and soil characteristics

Selenium (Se) is a beneficial trace element for certain animals including humans, while remaining controversial for plants. High Se concentration in soil is toxic to plants especially at seedling stage of the plants. Although, arbuscular mycorrhizal fungi (AMF) are important for plant stress resistance; but the mechanisms by which AMF alleviate Se stress in crop seedlings are unclear. Therefore, we investigated the potential strategies of AMF symbiosis to alleviate Se stress in maize (Zea mays) from plants and soil perspectives. Results showed that Se stress (Se application level > 5 mg kg-1) significantly inhibited leaf area, shoot dry weight, and root dry weight of maize (P < 0.05). In contrast, AM symbiosis significantly improved root morphology, increased nitrogen and phosphorus nutrition, promoted shoot growth, inhibited the transport of Se from soil/roots to shoots, and then diluted the concentration of Se in shoots (32.65-52.80%). In general, the response of maize growth to AMF was mainly observed in shoots rather than roots. In addition, AMF inoculation significantly increased the easily extractable glomalin-related soil protein and organic matter contents and decreased the availability of soil Se to the plant. Principal component analysis showed that AMF promoted growth and nutrition uptake of maize was the most dominant effect of Se stress alleviation, followed by the decrease of soil Se availability, limiting Se transport from soil/roots to shoots. Moreover, the expression of Se uptake-related ion transporter genes (ZmPht2, ZmNIP2;1, and ZmSultr1;3) in maize roots were down-regulated upon AM symbiosis which resultantly inhibited the uptake and transport of Se from soil to maize roots. Thus, AMF could impede Se stress in maize seedlings by improving plant and soil characteristics.

Multiple geochemical factors may cause iodine and selenium deficiency in Gilgit-Baltistan, Pakistan

Deficiencies of the micronutrients iodine and selenium are particularly prevalent where populations consume local agricultural produce grown on soils with low iodine and selenium availability. This study focussed on such an area, Gilgit-Baltistan in Pakistan, through a geochemical survey of iodine and selenium fractionation and speciation in irrigation water and arable soil. Iodine and selenium concentrations in water ranged from 0.01-1.79 µg L^-1 to 0.016-2.09 µg L^-1, respectively, which are smaller than levels reported in similar mountainous areas in other parts of the world. Iodate and selenate were the dominant inorganic species in all water samples. Average concentrations of iodine and selenium in soil were 685 µg kg^-1 and 209 µg kg^-1, respectively, much lower than global averages of 2600 and 400 µg kg^-1, respectively. The 'reactive' fractions ('soluble' and 'adsorbed') of iodine and selenium accounted for < 7% and < 5% of their total concentrations in soil. More than 90% of reactive iodine was organic; iodide was the main inorganic species. By contrast, 66.9 and 39.7% of 'soluble' and 'adsorbed' selenium, respectively, were present as organic species; inorganic selenium was mainly selenite. Very low distribution coefficients (kd = adsorbed/soluble; L kg^-1) for iodine (1.07) and selenium (1.27) suggested minimal buffering of available iodine and selenium against leaching losses and plant uptake. These geochemical characteristics suggest low availability of iodine and selenium in Gilgit-Baltistan, which may be reflected in locally grown crops. However, further investigation is required to ascertain the status of iodine and selenium in the Gilgit-Baltistan food supply and population.

Selenium Biofortification Modulates Plant Growth, Microelement and Heavy Metal Concentrations, Selenium Uptake, and Accumulation in Black-Grained Wheat

In Se-deficient populations, Selenium- (Se-) enriched wheat is a source of Se supplementation, and Se content can be improved by agronomic biofortification. Thus, black-grained wheat (BGW) and white-grained wheat (WGW) (as the control) were grown in Se naturally contained soils at different concentrations (11.02, 2.21, 2.02, and 0.20 mg·kg-1). Then, a field experiment was conducted to assess agronomic performance, the concentration of microelements and heavy metals, and the uptake and distribution of Se in the BGW under the application of Se ore powder. The results showed that the grain yield and grain Se concentration of wheat respectively show a significant increase and decrease from high Se to low Se areas. Higher grain yield and crude protein content were observed in Se-rich areas. The soil application of Se ore powder increased wheat grain yield and its components (biomass, harvest index, grain number, and 1,000 kernels weight). The concentrations of Zn, Fe, Mn, total Se, and organic Se in the grains of wheat were also increased, but Cu concentration was decreased. The concentrations of Pb, As, Hg, and Cr in wheat grains were below the China food regulation limits following the soil application of Se ore powder. Compared with the control, Se ore powder treatment increased the uptake of Se in various parts of wheat plants. More Se accumulation was observed in roots following Se ore powder application, with a smaller amount in grains. In addition, compared with the control, BGW had significantly higher concentrations of Zn, Fe, and Mn and accumulated more Se in grains and shoots and less Se in roots. The results indicate that wheat grown in Se-rich areas increases its grain yield and crude protein content. The soil application of Se ore powder promotes wheat growth and grain yield. Compared with WGW, BGW accumulated more Se in grains and had a higher concentration of organic Se in grains. In conclusion, the application of Se ore powder from Ziyang as Se-enriched fertilizer could be a promising strategy for Se biofortification in the case of wheat, and BGW is the most Se-rich potential genotype.

Phosphorus and selenium uptake, root morphology, and carboxylates in the rhizosheath of alfalfa (Medicago sativa) as affected by localised phosphate and selenite supply in a split-root system

Low availability of phosphorus (P) is a key limiting factor for the growth of many crops. Selenium (Se) is a nutrient for humans that is acquired predominantly from plants. Localised P and Se supply may affect P- and Se-uptake efficiency. Our aim was to examine the mechanisms of alfalfa (Medicago sativa L.) to acquire P and Se when the elements are heterogeneously or homogeneously distributed in soil, and how P and Se supply affect plant growth and uptake of P and Se. We conducted a split-root experiment growing alfalfa in a loess soil with two distribution patterns (i.e. heterogeneous and homogeneous) of P and Se. The application rates of P (KH2PO4) and Se (Na2SeO3) were 0 and 20mgPkg-1, and 0 and 1mgSekg-1, respectively. Our results showed that plants absorbed more Se when both P and Se were supplied homogeneously than when supplied heterogeneously. Supplying Se had a positive effect on plant P content. Localised P supply resulted in the exudation of more carboxylates by roots than homogeneous P supply did. Soil microbial biomass P was significantly greater when P was supplied homogeneously. Shoot-to-root translocation of Se had a positive effect on P-uptake efficiency. These results indicated that, compared with homogeneous P supply, localised P supply promoted P and Se uptake by increasing the amount of rhizosheath carboxylates and weakening the competition between roots and microbes. Translocation of Se within plant organs was promoted by the application of P, thus enhancing the P-uptake efficiency of alfalfa.

Risk-Benefit Assessment of Consumption of Rice for Adult Men in China

Objective: To evaluate the health impact of current and alternative patterns of rice consumption in Chinese adult men (40-79 years of age). Methods: We applied a risk-benefit assessment (RBA) model that took into account the health effects of selenium (Se), cadmium (Cd), and inorganic arsenic (i-As). The health effects included the prevention of prostate cancer associated with exposure to Se, and an increased risk of lung, bladder, and skin cancer for i-As and chronic kidney disease (CKD) for Cd. We defined the baseline scenario (BS) as the current individual mean daily consumption of rice in the population of interest and two alternative scenarios (AS): AS1 = 50 g/day and AS2 = 200 g/day. We estimated the health impact for different age groups in terms of change in Disability-Adjusted Life Years (ΔDALY). Results: The BS of rice consumption was 71.5-105.4 g/day in different age groups of adult men in China. We estimated that for AS1, the mean ΔDALY was -2.76 to 46.2/100,000 adult men of 40-79 years old. For AS2, the mean ΔDALY was 41.3 to 130.8/100,000 individuals in this population group. Conclusion: Our results showed that, based on associated exposure to selenium, cadmium, and i-As in rice, the current consumption of rice does not pose a risk to adult men in China. Also, a lower (50 g/day) or higher (200 g/day) rice consumption will not bring larger beneficial effects.

Selenium distribution in French forests: Influence of environmental conditions

Selenium is a trace element and an essential nutrient. Its long-lived radioisotope, selenium 79 is of potential radio-ecological concern in surface environment of deep geological repository for high-level radioactive waste. In this study, the influence of environmental, climatic and geochemical conditions on stable Se (as a surrogate of ⁷⁹Se) accumulation was statistically assessed (PCA analysis, Kruskall-Wallis and Spearman tests) based on the analysis of its concentration in litterfall, humus, and soil samples collected at 51 forest sites located in France. Selenium concentrations were in the ranges: 22-369, 57-1608 and 25-1222 μg kg^-1 respectively in litterfall, humus, and soil. The proximity of the ocean and oceanic climate promoted Se enrichment of litterfall, likely due to a significant reaction of wet deposits with forest canopy. Se content was enhanced by humification (up to 6 times) suggesting that Se concentrations in humus were affected by atmospheric inputs. Selenium stock in humus decreased in the order of decreasing humus biomass and increasing turnover of organic matter: mor > moder > mull. Positive correlations between Se content and geochemical parameters such as organic carbon content, total Al and total Fe confirmed the important role of organic matter (OM) and mineral Fe/Al oxides in Se retention in soils.

Selenium bioavailability in soil-wheat system and its dominant influential factors: A field study in Shaanxi province, China

Selenium (Se) content of crops depends on the local soil Se content and/or its bioavailability, and identifying the influence factors of soil Se bioavailability is a significant basis for adopting targeted agronomic measures to improve the Se nutritional status of humans. In this study, the main wheat-producing region in Shaanxi province with similar parent material and climate conditions was selected as the study area. The total Se contents of 602 soil samples and their corresponding wheat grains were determined, and the distribution characteristics of soil Se bioavailability and its dominant influential factors were investigated. Results showed that the total Se content ranged from 0.02 mg/kg to 1.67 mg/kg (average of 0.25 ± 0.25 mg/kg) in soil, which was lower than that content in China (0.29 mg/kg). The Se content of wheat grain was 0.001-1.50 mg/kg (average of 0.11 ± 0.19 mg/kg). The distribution trend of the Se content in wheat grains was different from that of the total soil Se, but it was consistent with the distribution of soil bioavailable Se content. The bioavailable Se accounted for 11.1% of the total soil Se. This could be attributed to relatively high soil Se bioavailability of the study area belonging to alkaline soil (with a pH of approximately 8). Both redundancy analysis and path analysis revealed that soil pH and organic matter were the dominant influential factors of soil Se bioavailability in Shaanxi wheat-producing area, and the soil Se bioavailability increased with these two parameters raising. On this basis, a prediction model was established to predict the Se content in wheat grain. The results show that the various agronomic measures could be used to produce Se-enriched wheat by regulating the soil pH and the organic matter content in Se biofortification practice.

Ratio of selenium concentrations between soil, forage plants and blood serum of beef cattle studied in organic and conventional farms

The aim of the study was to compare the selenium (Se) status of beef cattle from conventional farms with the status of cattle from organic farms located in western Pomerania and to determine the ratios of Se concentration between soil, forage plants and animals at these locations. The mean total Se (Se_T) content in soil was 0.208 mg/kg dry matter (DM) on organic farms and 0.254 mg/kg DM on conventional farms. Animals from conventional farms had significantly (p < 0.05) higher serum Se concentration than those on organic farms. As Se deficiency were classified in 75% of animals from organic farms and in 42% animals from conventional farms. A lack of Se supplementation is associated with a serious risk of deficiency in ruminants, particularly in areas with low Se levels in the soil-plant system. Therefore, more attention should be devoted to increasing the bioavailability of Se for plants by enhancing the physicochemical properties of soil. In addition, the composition of swards for grazing should be adjusted to increase the share of forage plants capable of collecting larger amounts of Se from soil.

Assessing the potential availability of selenium in the soil-plant system with manure application using diffusive gradients in thin-films technique (DGT) and DOM-Se fractions extracted by selective extractions

Knowledge of the Se fractionation and the role of dissolved organic matter (DOM) in soil is the key to understanding Se mobility and its bioavailability in the soil-plant system. In this study, single extractions using phosphate-buffer (PBS), sequential extraction procedures (SEP), and diffusive gradients in thin-films (DGT) were used to measure Se bioavailability in soil supplemented with selenite and organic amendment (cow and chicken manures). Selenium fraction was isolated into DOM-Se fractions, such as hydrophilic acid-bound Se (HY-Se), fulvic acid-bound Se (FA-Se), humic acid-bound Se (HA-Se), and hydrophobic organic neutral-bound Se (HON-Se), by a rapid batch technique using XAD-8 resin (AMBERLITE XAD™, USA). Simultaneous application of either cow or chicken manure with selenite could result in the decrease of Se availability in the soil. Isolating Se available fraction into DOM-Se fractions showed that low-molecular-weight DOM-Se as an available fraction and even HY-Se as a less available fraction (OM-Se) were likely the major sources for Brassica juncea (L.) Czern. et Coss uptake in soil. Moreover, knowledge of the DOM-Se composition, especially the low-molecular-weight DOM-Se fractions, is important for assessing the bioavailability of Se in soil, the results of which are more accurate than the chemical extraction method. The high value of Pearson correlation coefficients between CDGT-Se and Se concentrations in shoots, tubers and roots of Brassica juncea (L.) Czern. et Coss in cow and chicken manures treatment were 0.95 and 0.99, 0.96 and 0,96, and 0.89 and 0.97 (p < 0,05), respectively, indicating that DGT-Se can reflect the Se uptake ability by plants and can be used to predict the bioavailability of Se when manure and selenite are simultaneously applied.

Selenium bioaccessibility in native seleniferous soil and associated plants: Comparison between in vitro assays and chemical extraction methods

Selenium (Se) bioaccessibility in soil and crops from seleniferous areas is closely relevant to Se intake risks of local residents. The current in vitro digestion methods used for Se bioaccessibility evaluation are single and inconsistent, and most of them are only for food and neglect soil. In this study, 14 Se-contaminated soils and their corresponding crops in Naore Village (seleniferous area) were used as the research objects. Four in vitro digestion assays, including Solubility Bioaccessibility Research Consortium method (SBRC), physiologically-based extraction test (PBET), in vitro gastrointestinal method (IVG), and Unified Bioaccessibility Method (UBM) were used to determine the bioaccessible Se concentration in soil and edible parts of crops. Results showed that the Se in natural seleniferous soil mainly existed in relatively stable forms, i.e., residual and Fe-Mn oxide-bound Se (average of 80%). Only 10.6% of the total Se was distributed in water-soluble and exchangeable Se fractions. The Se content in crops was significantly positively correlated with the organic-bound and phosphate-extractable Se contents in the corresponding soil (p < 0.05). The organic-bound Se was clearly a potentially bioavailable Se source in soil. The Se bioaccessibility in soil and crops measured using the four in vitro methods in gastric/intestinal digestions were in the same order, which was PBET > UBM > SBRC > IVG. Similar to the absorption and utilization of soil Se fractions by crops, the water-soluble, organic-bound and exchangeable Se in soil were the main contributors of bioaccessible Se in the digestive juices in various in vitro methods. Furthermore, the bioaccessible Se in crops and soil measured via PBET method demonstrated the most significant correlation between the total Se in crops and the phosphate-extractable Se in soil. Therefore, the PBET method was the optimum in vitro method for the evaluation of Se bioaccessibility in crops and soil.

Distribution, origin and speciation of soil selenium in the black soil region of Northeast China

Selenium (Se) is an essential trace element within human beings that hold with crucial biological functions. Investigating the complex origin of soil Se is of great importance to scientifically approach the land use of Se-rich land use, and the respective promotion of regional economic development. In this study, 160 soil samples from 10 profiles in farmland and woodland were collected in Hailun city, which is a typical black soil region in Northeast China, in order to characterize the distribution and speciation of Se in the black soil, and to identify the origin of soil Se. The total selenium content in the soil ranges from 0.045 to 0.444 μg g^-1, with an average selenium content in black soil (0.318 μg g^-1) of three times greater than that found in the yellow-brown soil (0.114 μg g^-1). The land-use type has a significant influence on the distribution of selenium in the black soil. Moreover, Se and heavy metals have a significant (positive or negative) correlation, in which TOC plays an important role. The black soil presents a consistent REE distribution pattern with underlying yellow-brown soil indicating black soil originates from yellow-brown soil. REE geostatistical analysis suggests that the soil Se partly originates from shale weathering and enriches in black soil. Moreover, elemental geochemical analysis and XRD results show that the paleoclimate change from humid and warm to dry and cold is favorable for organic matter accumulation, resulting in less leaching and enhanced adsorption of selenium into the black soil.

Characteristics of selenium enrichment and assessment of selenium bioavailability using the diffusive gradients in thin-films technique in seleniferous soils in Enshi, Central China

Agricultural products from seleniferous areas commonly face problems associated with substantial variation in selenium (Se) concentration, which is mainly caused by the heterogeneity of Se bioavailability in soil. Many studies have assessed the bioavailability of Se and its influencing factors using soil samples treated with exogenous Se. Given the distinctly different characteristics of Se-spiked soils and naturally seleniferous soils, exploring Se bioavailability in naturally seleniferous soils is crucial to the stable production of Se-enriched agricultural products. In this study, we used the classical sequential extraction method to determine the Se fractionation and then applied the diffusive gradients in thin-films (DGT) technique to assess the Se bioavailability in naturally seleniferous soils. The results indicated that soluble and exchangeable Se fractions with high bioavailability accounted for only 0.7% and 5.1% of total Se, respectively. Both soluble and exchangeable Se concentrations were significantly positively correlated with soil pH (r = 0.329 and 0.262, respectively; P < 0.01). Se mainly exists in Fe-Mn oxide-bound, organic matter-bound, and residual Se fractions with low mobility (94.2% of total Se), among which organic matter-bound Se was the predominant fraction (49.5% of total Se). A significant positive correlation was found between total Se and soil organic matter (r = 0.539; P < 0.01). Multiple regression analysis revealed that the DGT-determined Se was mostly derived from soluble and exchangeable Se. The high correlation between the DGT-determined Se fraction and Se uptake by rice (r = 0.91; P < 0.01) confirmed that DGT can accurately assess Se bioavailability in naturally seleniferous soils in Enshi and other similar environmental settings.

Selenium volatilization from tundra soils in maritime Antarctica

Maritime Antarctica harbors a large number of penguins and seals that provide considerable input of selenium (Se) originating as guano into terrestrial ecosystems. Subsequent Se emissions via biomethylation and volatilization from these sources of Se have not been studied. Here, penguin colony soils (PCS) and adjacent tundra marsh soils (TMS), seal colony soils (SCS) and adjacent tundra soils (STS), and normal upland tundra soils (NTS) were collected in maritime Antarctica. For the first time, Se volatilization and speciation were investigated in these soils through incubation experiments using chemo-trapping method. The Se contents in PCS, SCS, STS and TMS were highly enriched compared with NTS, with organic matter-bound Se accounting for 70%-80%. Laboratory incubations yielded the greatest Se volatilization rates (VR_Se) in PCS (0.20 ± 0.01 μg kg^-1 d^-1), followed by SCS (0.14 ± 0.01 μg kg^-1 d^-1) at low temperature (4 °C). Soil frozen-thawing induced 1-4 fold increase in VR_Se, and the VR_Se continuously increased until the soils fully thawed. The VR_Se showed a significant positive correlation (R² = 0.96, p < 0.01) with soil temperature. Methylated Se species were dominated by dimethylselenide (DMSe) in PCS and dimethyldiselenide (DMDSe) in SCS. Our results imply that the combination of climate warming, frozen-thawing processes, and high-Se inputs from sea animals will significantly increase tundra soil Se volatilization in maritime Antarctica. High VR_Se from penguin colony soils, and significantly elevated Se levels in the mosses close to penguin colony, suggest that volatilization of Se from penguin colony soils play an important role in the mobilization and regional biogeochemical cycling of Se in maritime Antarctica.

The Interaction of Arbuscular Mycorrhizal Fungi and Phosphorus Inputs on Selenium Uptake by Alfalfa (Medicago sativa L.) and Selenium Fraction Transformation in Soil

Selenium (Se) is a beneficial element to plants and an essential element to humans. Colonization by arbuscular mycorrhizal fungi (AMF) and supply of phosphorus (P) fertilizer may affect the bioavailability of Se in soils and the absorption of Se by plants. To investigate the interaction between AMF and P fertilizer on the transformation of soil Se fractions and the availability of Se in the rhizosphere of alfalfa, we conducted a pot experiment to grow alfalfa in a loessial soil with three P levels (0, 5, and 20 mg kg-1) and two mycorrhizal inoculation treatments (without mycorrhizal inoculation [-AMF] and with mycorrhizal inoculation [+AMF]), and the interaction between the two factors was estimated with two-way ANOVA. The soil in all pots was supplied with Se (Na2SeO3) at 1 mg kg-1. In our results, shoot Se concentration decreased, but plant Se content increased significantly as P level increased and had a significant positive correlation with AMF colonization rate. The amount of total carboxylates in the rhizosphere was strongly affected by AMF. The amounts of rhizosphere carboxylates and alkaline phosphatase activity in the +AMF and 0P treatments were significantly higher than those in other treatments. The concentration of exchangeable-Se in rhizosphere soil had a positive correlation with carboxylates. We speculated that rhizosphere carboxylates promoted the transformation of stable Se (iron oxide-bound Se) into available Se forms, i.e. exchangeable Se and soluble Se. Colonization by AMF and low P availability stimulated alfalfa roots to release more carboxylates and alkaline phosphatase. AMF and P fertilizer affected the transformation of soil Se fractions in the rhizosphere of alfalfa.

DOM derivations determine the distribution and bioavailability of DOM-Se in selenate applied soil and mechanisms

Straw amendment and plant root exudates modify the quality and quantities of soil dissolved organic matter (DOM) and then manipulate the fractions of soil selenium (Se) and its bioavailability. Two typical soils with distinct pH were selected to investigate the effect of different contributors on DOM-Se in soil. The mechanisms relying on the variation in DOM characteristics (quality, quantity and composition) were explored by UV-Vis, ATR-FTIR and 3D-EEM. Straw amendment significantly (p < 0.05) suppressed the selenate bioavailability. The reduction in wheat Se content was greater in krasnozems than in Lou soil, as more HA fraction appeared in krasnozems. The root exudates of wheat mainly elevated the low molecular hydrophilic compounds (Hy) in soil, which contributed to the SOL-Hy-Se fractions and thus grain Se in soils (p < 0.01). However, straw amendment promoted DOM transforming from small molecules (Hy and FA) to aromatic large molecules (HA), when accompanied with the reduction and retention of Se associated with these molecules. As a result, selenium bioavailability and toxicity reduced with DOM amendment and DOM-Se transformation.

Prediction of selenium uptake by pak choi in several agricultural soils based on diffusive gradients in thin-films technique and single extraction

The accurate assessment of soil selenium (Se) bioavailability is crucial for Se biofortification in Se-deficient areas and risk assessment in selenosis areas. However, a universally accepted approach to evaluate Se bioavailability in soil is currently lacking. This research investigated Se bioavailability in six soils treated with selenite (Se(IV)) or selenate (Se(VI)) by comparing diffusive gradients in thin-films (DGT) technique and chemical extraction methods through pot experiments. A bioindicator method was used to evaluate Se concentrations in pak choi and compare the results with the Se concentration measured by other methods. Results showed that chemical extraction methods presented different extraction efficiencies for available Se over a range of soil types, and the same extraction method had various extraction efficiencies for different Se species in the same soil. DGT measured Se concentrations (C_DGT-Se) for Se(VI) treatment were 2.3-34.1 times of those for Se(IV) treatment. KH₂PO₄-K₂HPO₄ and AB-DTPA extractable Se could predict the bioavailability of soil Se, but they were disturbed by soil properties. HAc extraction was unsuitable for evaluating Se bioavailability in different Se(IV)-treated soils. By contrast, DGT technique was preferable for predicting plant uptake of Se(IV) over chemical extraction methods. Although DGT technique was independent of soil properties, KH₂PO₄-K₂HPO₄ extraction provided the best fitting regression equation for Se(VI) when it was dependent on soil organic matter. Thus, KH₂PO₄-K₂HPO₄ extraction may be preferred to assess Se(VI) bioavailability in different soil types on a large scale.

Dynamics of Selenium uptake, speciation, and antioxidant response in rice at different panicle initiation stages

Selenium (Se) is an essential element in animals and humans, and its deficiency may cause conditions such as cardiac disease. The production of Se-enriched rice is one of the most important ways to supply Se in the human body, and thus, understanding of the mechanisms of Se-enriched rice is of great significance. A pot experiment was conducted to study the effects of Se addition on the growth, antioxidation, Se uptake and distribution, and Se speciation in three different stages of panicle initiation stage (i.e., pistil and stamen formation stage, pollen mother cell formation stage, pollen mother cell meiosis stage) and the maturity stage. The results showed that soil Se application significantly increased Se uptake in rice. Low rates of Se (<5 mg kg^-1) application enhanced the plant growth and rice yield. Se speciation assays showed that SeCys and SeMet were the two main forms found in rice, of which SeMet accounted for 65.5%-100% in the ears and leaves, while SeCys accounted for 61.4%-75.6% in brown rice. SeMet was also the main Se-species found in different subcellular parts at the panicle initiation stage. However, inorganic Se was present in brown rice, mainly as Se(VI), when the soil Se addition exceeded 5 mg kg^-1. Lower rates of Se (<5 mg kg^-1) promoted the antioxidant capacity, while high levels of Se (≥5 mg kg^-1) reduced the antioxidant capacity of rice. The results indicate that Se effects are dose dependent, and the suitable amount of soil Se application for Se-enriched rice production would be <5 mg kg^-1.

Assessing the uptake of selenium from naturally enriched soils by maize (Zea mays L.) using diffusive gradients in thin-films technique (DGT) and traditional extractions

A generally accepted method to predict selenium (Se) bioavailability of long-term contaminated soils has not yet been established, even if risk assessments in selenosis areas are crucial. In this study, a set of methods were tested to assess the bioavailability of Se to field maize. Fifty maize (Zea mays L.) samples and corresponding soils were collected from a selenosis area (Ziyang, China). The diffusive gradients in thin-films (DGT) technique and the traditional chemical extraction methods, including seven single-step extraction procedures and a five-step sequential extraction were used to predict the bioaccumulation of Se in plant. The result verified the presence of 50% of total Se in the form of residual Se fraction, followed by organic-bound and Fe-Mn oxide-bound Se fractions in soil. In addition, Se⁶⁺, Se⁴⁺, and Se^2- were all detected in the solution extracted by H₂O, KCl, phosphate-buffered solution (PBS), NaHCO₃, ethylenediaminetetraacetic acid-2Na (EDTA-2Na) and ammonium bicarbonate-diethylenetriaminepentaacetic acid (AB-DTPA), but Se⁶⁺ was not extracted by NaOH. The Se extracted by single-step extraction methods was weakly correlated with the Se uptake by plants with relatively high Se concentration (>3 mg·kg^-1). The abilities of the tested methods to predict Se bioavailability in naturally Se-enriched soils declined in the following order: DGT > soil solution > PBS > KCl > H₂O > NaHCO₃ > EDTA > DTPA > NaOH. The ratio of C_DGT to soil solution Se (C_soln) totaled 0.13, indicating an extremely low Se supply from the soil solid phase to the soil solution. Se measured by DGT was mainly derived from the soluble and exchangeable Se fractions that can accurately reflect the plant-absorbed Se pool. Therefore, the DGT technique is highly applicable in the simultaneous prediction of Se bioavailability in naturally Se-enriched soils.