Distribution characteristics and main influencing factors of selenium in surface soil of natural selenium-rich area: a case study in Langao County, China

Estimated daily intake and health risk assessment of total and organic selenium in crops across areas with different selenium levels

BACKGROUND

The health risk of Se has gained significant attention. Previous studies mainly focused on the health risk of total Se in high-Se area. Less attention has been paid to the health risk of organic selenium in areas with varying selenium levels.

METHODS

A total number of 109 crop samples (edible parts) were collected in Langao County, Shannxi Province, China from 2018 to 2020, including 42 corn, 18 rice, 9 sweet potato, 25 potato, 12 radish, and 3 eggplant samples. The hydride generation atomic fluorescence spectrometry (HG-AFS) method was used to determine the total and organic Se contents.

RESULT AND CONCLUSION

(1) Corn (2.82 mg/kg), rice (0.44 mg/kg), potato (6.56 mg/kg), and eggplant (0.77 mg/kg) in high-Se area, as well as sweet potato (1.07 mg/kg) and radish (4.28 mg/kg) in medium-Se area, exhibited the highest total Se content among all crops in this county, and 5-328 times higher than the values of Se-enriched standard (2) The average daily intake of total/organic Se of residents in high-Se area reached 676/449 μg/day, which was 1-4 times higher than levels observed in medium-Se area (419/257 μg/day) and low-Se area (196/128 μg/day). The organic Se daily intakes from dietary combinations of rice + radish and rice + eggplant in high-Se area lower than 400 μg/day, which could be safely consumed. The organic Se daily intakes from dietary combinations of sweet potato + radish and sweet + eggplant in medium-Se area higher than 400 μg/day, which could not be safely consumed. The total / organic Se daily intakes of all dietary combinations in low-Se area lower than 400 μg/day, which could be safely consumed. (3) The health risk associated with crops might be overestimated due to the higher non-carcinogenic risk attributed to total Se compared to organic Se. The present study demonstrated that daily intake and health risk of total and organic Se in crops across areas with different Se levels varied significantly.

Molecular mechanisms of growth promotion and selenium enrichment in tomato plants by novel selenium-doped carbon quantum dots

Selenium (Se)-doped nanoparticles as novel Se fertilizers have a promising potential in the agricultural application. Here, the effects of two novel Se-doped carbon quantum dots (Se-CQDs1 and Se-CQDs2, prepared using co-cracking and adsorption-reduction methods, respectively) on the growth and Se enrichment of tomato plants were studied, where the promoting molecular mechanisms were explored in terms of the related genes expression and soil microbial composition. The results indicated that the soil application of 2.5 mg kg-1 Se-CQDs1 more significantly increased the root growth, plant biomass, and fruit yield than that of Se-CQDs2 and Na2SeO3 treatments (control). Specifically, Se-CQDs1 treatment was more effective to up-regulate the expressions of aquaporin gene (i.e., PIP) and growth hormone synthesis gene (i.e., NIT) than Se-CQDs1 and Na2SeO3 treatments. The expressions of Se methyltransferase gene (smt) and methionine methyltransferase gene (mmt) induced by Se-CQDs1 were 1.45 and 1.18 times higher than that by Se-CQDs2 as well as 1.82 and 2.17 times higher than that by Na2SeO3. Also, Se-CQDs1 more greatly increased the diversity and relative abundance of soil bacterial communities, especially the Actinobacteria phylum, which was beneficial to increase plant growth-promoting substances. These outstanding promoting effects of Se-CQDs1 were mainly ascribed to its higher hydrophilicity and content of the stable doped-Se. The overall results demonstrated that Se-CQDs would be a promising candidate for nano-fertilizer to increase crop growth and development (e.g., tomato plants), where the synthesis modes of Se-CQDs play a critical role in regulating the utilization efficiency of Se.

Selenite Stable Isotope Fractionation during Abiotic Reduction by Sodium Sulfide

Reduction of Se(IV) by sulfur reducing bacteria (SRB) can remove Se from groundwater either by direct respiration or the production of H2S(g) and subsequent abiotic reduction. This study examined abiotic Se(IV) reduction by H2S(g) to determine the associated Se isotope fractionation. The extent of fractionation was compared to the results with studies of Se(IV) reduction in systems containing microorganisms to assess whether these processes could be distinguished. A solution containing Na2S was added in increasing concentrations to solutions containing Se(IV) as SeO32- to reduce and precipitate Se. Precipitates with three distinct colors were observed. Powder X-ray diffraction (PXRD) results yielded three distinct spectra for each of the three colors of precipitate, which corresponded to SenS8-n (orange) or Se(0) (red) and S(0) (yellow). The δ82Se values of the residual dissolved Se increased as the aqueous Se concentration decreased. The S/Se in solution affected the isotopic fractionation, with an 82ε of 10.1 ± 0.6‰ observed for solutions with S/Se < 1.7, and of 7.9 ± 0.3‰ for solutions with S/Se > 1.7.

Geochemical characteristics, source analysis, influencing factors, and reserves of soil Selenium in Wuming, Guangxi, China

Although selenium (Se) reserves are crucial for the development and exploitation of Se-rich resources in karst soil areas, research on these reserves is still limited. A comprehensive study was conducted in a typical karst region known for its Se richness. A total of 12,547 surface soil samples, 134 deep soil samples, and 60 soil profiles from various locations were systematically collected. The findings showed that the Se content in the surface soil ranged from 0.073 to 9.04 mg/kg, with a baseline level of 0.84 mg/kg. This underscores the high background level and moderate variability in the region. Surface soil Se exhibited a notable positive correlation with deep soil Se, and an inverse correlation with pH (p < 0.01). One-way analysis of variance indicated that land formations and soil structure were the primary determinants affecting the concentration of Se in the topsoil (p = 0.000), with parent rock type and land-use type following closely (p = 0.003). In addition, the study included an investigation of soil Se variations with depth using 60 soil profiles. Through this analysis, it was revealed that Se content exhibited an exponential change with depth. Multiple integrations were employed to derive formulas for calculating Se reserves in the 0-200 cm depth range. Following these calculations, the estimations of Se stockpile across diverse types of source materials, varieties of soils, and land management methods were determined, highlighting the findings using a passive construction. This paper lays the groundwork for advancing the extraction and application of Se.

Stratum affects the distribution of soil selenium bioavailability by modulating the soil physicochemical properties: A case study in a Se-enriched area, China

The soil selenium (Se) content and bioavailability are important for human health. In this regard, knowing the factors driving the concentration of total Se and bioavailable Se in soils is essential to map Se, enhance foodstuffs' Se content, and improve the Se nutritional status of humans. In this study, total Se and Se bioavailability (i.e., phosphate extracted Se) in surface soils (0-20 cm) developed on different strata were analyzed in a Se-enriched region of Southwest China. Furthermore, the interaction between the stratum and soil properties was assessed and how did the stratum effect on the concentration and spatial distribution of Se bioavailability in soils was investigated. Results showed that the median concentration of total Se in soils was 0.308 mg/kg, which is higher than China's soil background. The mean proportion of phosphate extracted Se in total Se was 12.2 %. The values of total Se, phosphate extracted Se, and soil organic matter (SOM) in soils increased with the increasing stratum age. In contrast, the coefficient of weathering and eluviation (BA) values decreased. The analysis of statistics and Geodetector revealed that the SOM, stratum, and BA were the dominant controlling factors for the contents and distributions of soil total Se and phosphate extracted Se. This study provided strong evidence that the soil properties that affected the total Se and Se bioavailability were modulated by the local geological background, and had important practical implications for addressing Se malnutrition and developing the Se-rich resource in the study region and similar geological settings in different parts of the globe.

Spatial distribution and main drivers of soil selenium in Taihu Lake Basin, Southeast China

Selenium (Se) is an essential micronutrient that is both hazardous and beneficial to living organisms. However, few studies have examined soil Se distribution and its driving mechanisms on a large basin scale. Thus, multivariate statistics, geostatistics, boosted regression trees, and structural equation models were used to investigate the spatial distribution, driving factors, and multivariate interactions of soil Se based on 1753 topsoil samples (0-20 cm) from the Taihu Lake Basin. The results indicated that the soil Se concentration ranged from 0.12 to 57.26 mg kg-1, with a mean value of 0.90 mg kg-1. Overall, the spatial pattern of soil Se gradually decreased from south to north with approximately 1.06% of the soil contaminated with Se. Moisture index (MI), soil moisture (SM), and ≥ 0 ℃ accumulative temperature (AAT0) were the main determinants of soil Se accumulation. Additionally, the substantial effect of SM∩AAT0 on soil Se concentrations demonstrated that climate-soil interactions largely governed the spatial pattern of soil Se. The Se-enriched and Se-contaminated soils occurred mainly in regions with high precipitation, MI, SM, AAT0, and soil organic matter. This study provides a theoretical basis and practical guidance for the remediation of soil Se contamination and the sustainable development of Se-enriched agriculture.

Using machine learning to predict selenium and cadmium contents in rice grains from black shale-distributed farmland area

Cadmium (Cd) and selenium (Se) are widely enriched in soil at black shale outcropping areas, with Cd levels exceeding the standard (2.0 mg/kg in 5.5 < pH ≤ 6.5) commonly. The prevention of Cd hazards and the safe development of Se-rich land resources are key issues that need to be urgently addressed. To ensure safe utilization of Se-rich land in the CdSe coexisting areas, 158 rice samples, their corresponding rhizosphere soils, and 8069 topsoil samples were collected and tested in the paddy fields of Ankang City, Shaanxi Province, where black shales are widely exposed. The results showed that 43 % of the topsoil samples were Se-rich soil (Se > 0.4 mg/kg) wherein 79 % and 3 % of Cd concentrations exceeded the screening value and control value, respectively, according to the GB15618-2018 standard. Meanwhile, 63 % of the rice samples were Se rich (Se > 0.04 mg/kg) and the Cd content exceeded the prescribed limit (0.2 mg/kg) in Se-rich rice by 26 %. There was no significant positive correlation between the Se and Cd contents in the rice grains and the Se and Cd contents in the corresponding rhizosphere soil. The factors influencing Se and Cd uptake in rice were SiO2, CaO, P, S, pH, and TFe2O3. Accordingly, an artificial neural network (ANN) and multiple linear regression model (MLR) were used to predict Cd and Se bioaccumulation in rice grains. The stability and accuracy of the ANN model were better than those of the MLR model. Based on survey data and the prediction results of the ANN model, a safe planting zoning of Se-rich rice was proposed, which provided a reference for the scientific planning of land resources.

Environmental selenium and human longevity: An ecogeochemical perspective

Selenium (Se) has been called the "longevity element" by the scientific community because it has anti-cancer, anti-inflammatory, and anti-oxidant activity in humans. The geochemical properties and biological functions of Se have been widely studied in various fields, including geology, agriculture, and medicine. Bama Yao Autonomous County (Bama), a typical township in China with high longevity among the population, was selected as the research area. The present study organically combines the geological background, dietary structure, absorption and metabolism, and other biogeochemical aspects to comprehensively analyze the anti-aging properties of Se under high-Se conditions and a fiber-rich polysaccharide diet. Biogeochemical samples of surface soil, food, human hair, and urine were systematically collected from the environment and the residents, and the content and speciation of Se were analyzed. Concentrations of Se in the soil were moderate to high for Bama, with high-Se soil in 77.21% of the samples. Water-soluble Se concentrations were also high, 90% of the samples with moderate to high Se, and had a significant positive correlation with Se concentrations in crops. With both high total and high available Se, dietary samples from the study area showed Se-enriched characteristics. Accordingly, Se intake was also high (82.54 μg/d) in the population, strongly maintaining the normal bodily functions of the elderly. Accumulation and metabolism of Se in the population were assessed based on concentrations of Se in the hair and urine of residents over 60 years old. Continuous accumulation of Se was found to occur from 71 to 80 years of age. Concentrations of Se in residents under 71 years old and over 80 years old were in a state of loss. Overall, Se absorption and metabolism are maintained at the optimal physiological state, which is one of the primary factors maintaining the health and longevity of the elderly people in the study area.

Nitrogen Application Can Optimize Form of Selenium in Soil in Selenium-Rich Areas to Affect Selenium Absorption and Accumulation in Black Wheat

The composition and form of selenium in the soil have significant effects on the selenium content of crops. In this study, we investigated the selenium absorption pathway in plants by studying the interaction between nitrogen fertilizer and soil selenium. Our results showed that the selenium concentration enrichment factors (CEF) varied within the same region due to nitrogen fertilizer application, where they ranged from 1.33 to 5.02. The soil selenium flow coefficient (mobility factor, MF) increased with higher nitrogen application rates. The sum of the MF values for each soil layer treated with nitrogen application rates of 192 kg hm-2 and 240 kg hm-2 was 0.70, which was 64% higher than that for the control group with no nitrogen application. In the 0-20 cm soil layer, the highest summed water-soluble and exchangeable selenium and relative percentage of total selenium (12.45%) was observed at a nitrogen application rate of 240 kg hm-2. In the 20-40 cm soil layer, the highest relative percentage content of water-soluble and exchangeable selenium and total selenium (12.66%) was observed at a nitrogen application rate of 192 kg hm-2. Experimental treatment of black wheat with various concentrations of sodium selenite showed that selenium treatment at 50 μmol L-1 significantly increased the reduced glutathione (GSH) levels in the leaves and roots of seedlings, where the GSH contents increased by 155.4% in the leaves and by 91.5% in the roots. Further analysis of the soil-black wheat system showed that nitrogen application in selenium-rich areas affected the soil selenium flow coefficient and morphological composition, thereby changing the enrichment coefficient for leaves (0.823), transport capacity from leaves to grains (-0.530), and enrichment coefficient for roots (0.38). These changes ultimately affected the selenium concentration in the grains of black wheat.

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.

Selenium-Containing Organic Fertilizer Application Affects Yield, Quality, and Distribution of Selenium in Wheat

This study was designed to investigate the effect on wheat yield of applying organic fertilizers (OF) with five different selenium (Se) concentrations. The mineral nutrients, cadmium (Cd) content, and the distribution of Se in wheat plants were also measured. The results showed that wheat yields reached a maximum of 9979.78 kg ha-1 in Mengcheng (MC) County and 8868.97 kg ha-1 in Dingyuan (DY) County, Anhui Province, China when the application amount of selenium-containing organic fertilizer (SOF) was up to 600 kg ha-1. Among the six mineral nutrients measured, only the calcium (Ca) content of the grains significantly increased with an increase in the application amount of SOF in the two regions under study. Cd content showed antagonistic effects with the Se content of wheat grains, and when the SOF was applied at 1200 kg ha-1, the Cd content of the grains was significantly reduced by 30.1% in MC and 67.3% in DY, compared with under the Se0 treatment. After application of SOF, the Se content of different parts of the wheat plant ranked root > grain > spike-stalk > glume > leaf > stem. In summary, SOF application at a suitable concentration could increase wheat yields and significantly promote the Ca content of the grains. Meanwhile, the addition of Se effectively inhibited the level of toxic Cd in the wheat grains.

Abnormal Phenylalanine Metabolism of Procapra przewalskii in Chronic Selenosis in Selenium-Enriched Habitats

Selenium (Se)-enriched habitats have led to chronic selenosis, seriously affecting the health and survival of Procapra przewalskii (P. przewalskii). Our targets were to explore the molecular mechanisms of chronic selenosis and to look for a new way to protect endangered species. The mineral contents of soils, grass, blood, and muscle were analyzed. The biochemical indices, antioxidant capability, and immune function were also investigated. The analyses of proteomics and metabolomics were also carried out. The results showed that the Se contents in the muscle and blood of P. przewalskii, and the soil and grass in the Se-enriched habitats were significantly higher than those in healthy pastures. The P. przewalskii in the Se-enriched habitats showed symptoms of anemia, decreased antioxidant capability, and low immune function. A total of 44 differential proteins and 36 differential metabolites were screened by analyzing their proteomics and metabolomics. These differential proteins and metabolites were involved in glycolysis pathway, amino acid biosynthesis, carbon metabolism, phenylalanine metabolism, and energy metabolism. In particular, phenylalanine metabolism was the common pathway of proteomics and metabolomics, which was an important finding in studying the mechanism of chronic selenosis in animals. This study will help us to further understand the mechanism of chronic selenosis in P. przewalskii, and it provides a scientific basis for the protection of endangered species in Se-enriched habitats.

Use of Microbial Consortia in Bioremediation of Metalloid Polluted Environments

Metalloids are released into the environment due to the erosion of the rocks or anthropogenic activities, causing problems for human health in different world regions. Meanwhile, microorganisms with different mechanisms to tolerate and detoxify metalloid contaminants have an essential role in reducing risks. In this review, we first define metalloids and bioremediation methods and examine the ecology and biodiversity of microorganisms in areas contaminated with these metalloids. Then we studied the genes and proteins involved in the tolerance, transport, uptake, and reduction of these metalloids. Most of these studies focused on a single metalloid and co-contamination of multiple pollutants were poorly discussed in the literature. Furthermore, microbial communication within consortia was rarely explored. Finally, we summarized the microbial relationships between microorganisms in consortia and biofilms to remove one or more contaminants. Therefore, this review article contains valuable information about microbial consortia and their mechanisms in the bioremediation of metalloids.

Vertical distribution and major influencing factors of soil selenium in tropical climate: A case study of Chengmai County, Hainan Island

Soil selenium is of great significance to human health. Soil-forming parent rocks are the most critical factor that influences soil Se levels. Chengmai County, Hainan Island, has a tropical climate and diverse types of parent rocks, in which soil Se content is high. This study investigated the vertical distribution of soil Se from various parent rock substrates under tropical climatic conditions, and the factors that influence these soil Se contents, with 69 vertical soil profiles covering Chengmai County. The vertical distribution of soil Se and correlations with CIA (chemical alteration index), Al₂O₃, TFe₂O₃ (total iron oxide expressed as Fe₂O₃), total iodine, SOC (soil organic carbon), and pH were analysed. As per the results, the mean ± standard error of Se content in the A, B, and C horizons was 0.88 ± 0.13 mg/kg, 0.77 ± 0.08 mg/kg and 0.45 ± 0.05 mg/kg, respectively. The parent rock strictly controlled the horizon distribution of Se in the A-horizon. Soil Se showed A-B-horizons-enrichment in the vertical profile, especially in soil profiles overlying granite and basalt. It is hypothesised that the Se enriched in soils developed from the Tuolie Formation due to the release of Se from the weathering process of Se-rich rocks. Meanwhile, Se in soils developed from granite and basalt is more closely associated with exogenous input. Another crucial factor for the high level of Se in Chengmai County is the tropical climate, which has led the rocks to generally undergo intense chemical weathering. This results in soils rich in clay minerals and Fe/Al oxyhydroxides, which easily absorb and retain Se. Furthermore, the Se content of the B-horizon was generally higher than that of the A-horizon due to leaching. These results provide further knowledge and understanding of the geochemical behaviour of soil Se and guide the evaluation of Se-rich land resources under tropical climatic conditions.

Distribution Pattern and Enrichment Mechanism of Selenium in Topsoil in Handan Se-Enriched Belt, North China

Selenium (Se) is an essential trace element for human health with crucial biological functions. In this study, Se concentrations and physicochemical properties of soils in central and western Handan were determined to investigate their spatial distribution, enrichment degree, influencing factor, and geological source. The results show that: (1) Se concentrations vary from 0.00 to 1.95 μg/g, with an average of 0.45 μg/g, which exceed the mean of Se in soils in China (0.29 μg/g) and Hebei Plain (0.21 μg/g). (2) A continuous and irregular ring-like area showing significant enrichment of Se could be identified in Handan city, Yongnian District, Wu’an City, and Fengfeng Mining District. It can be defined as a positive abnormal Se zone, which is mainly located in the hilly area in the west of Handan City and east of Taihang Mountains, and the plains near Handan City. (3) Comprehensively, Se enrichment in the soil is principally affected by rock weathering, mining activities, and coal combustion. (4) As far as the single-factor pollution index (SFPI) is concerned, most of the study areas are in the safety domain and slightly polluted domain and are at low ecological risk. According to the Nemerow integrated pollution index (NIPI), the moderately and seriously polluted domain are distributed in Handan City, Fengfeng Mining District, and other central areas.

The Effects of Selenium on Bone Health: From Element to Therapeutics

Osteoporosis, characterized by low bone mass and a disruption of bone microarchitecture, is traditionally treated using drugs or lifestyle modifications. Recently, several preclinical and clinical studies have investigated the effects of selenium on bone health, although the results are controversial. Selenium, an important trace element, is required for selenoprotein synthesis and acts crucially for proper growth and skeletal development. However, the intake of an optimum amount of selenium is critical, as both selenium deficiency and toxicity are hazardous for health. In this review, we have systematically analyzed the existing literature in this field to determine whether dietary or serum selenium concentrations are associated with bone health. In addition, the mode of administration of selenium as a supplement for treating bone disease is important. We have also highlighted the importance of using green-synthesized selenium nanoparticles as therapeutics for bone disease. Novel nanobiotechnology will be a bridgehead for clinical applications of trace elements and natural products.

Differences of selenium and other trace elements abundances between the Kaschin-Beck disease area and nearby non-Kaschin-Beck disease area, Shaanxi Province, China

In order to find out the environmental geochemical characteristics of KBD affected area, KBD and non-KBD villages in KBD Counties, Weibei KBD area, Shaanxi Province, China were studied. Contents of Se and other elements in soil, wheat, maize and drinking water were analyzed. The results show that soil of Weibei KBD area is deficient in trace elements comparing with BSC and non-KBD area. Wheat of Weibei KBD area is deficient both in major elements and trace elements comparing with REC. Wheat of KBD village is especially deficient in Cr, Co, Se, Mo and Mg. Contents of Sr, Li and Mo in wheat of KBD village are significantly lower than nearby non-KBD village, but the average Se content is slightly higher. The deficiency of trace elements, especially Se, Sr, Li and Mo in staple food and trace elements in soil, is the main characteristic for the environment of KBD area.