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.

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 Se2- 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 CDGT to soil solution Se (Csoln) 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.

Effects of selenium application on Se content and speciation in Lentinula edodes.

Edible fungi have strong ability to transform inorganic Se into organic forms. Therefore, different concentrations of selenite, selenate and Se-yeast were injected as Se-supplements into substrates to produce Se-enriched Lentinula edodes. The Se content and its speciation distribution in the fruit bodies of L. edodes were analysed at different harvest times. Results indicate that Se concentrations of L. edodes increased first and then decreased over time. Based on Se accumulation in L. edodes, selenium use efficiency was ranked as selenite > selenate > Se-yeast. SeMet was the predominant Se speciation in the fruit bodies of L. edodes. SeMet made up the biggest proportion of total Se content and increased with application time for selenite and selenate treatments, whereas no significant change was found for Se-yeast treatment. This study demonstrates that Se-enriched L. edodes is a good source of dietary Se.

Selenium distribution in the Chinese environment and its relationship with human health: A review.

This paper reviewed the Se in the environment (including total Se in soil, water, plants, and food), the daily Se intake and Se content in human hair were also examined to elucidate Se distribution in the environment and its effects on human health in China. Approximately 51% of China is Se deficiency in soil, compared with 72% in the survey conducted in 1989. Low Se concentrations in soil, water, plants, human diet and thus human hair were found in most areas of China. The only significant difference was observed between Se-rich and Se-excessive areas for Se contents in water, staple cereal, vegetables, fruits, and animal-based food, no remarkable contrast was found among other areas (p>0.05). This study also demonstrated that 39-61% of Chinese residents have lower daily Se intakes according to WHO/FAO recommended value (26-34µg/day). Further studies should focus on thoroughly understanding the concentration, speciation, and distribution of Se in the environment and food chain to successfully utilize Se resources, remediate Se deficiency, and assess the Se states and eco-effects on human health.

Risk assessment for human health in a seleniferous area, Shuang'an, China.

Shuang'an in Ziyang is a typical high-selenium (Se) area in China, where human selenium (Se) poisoning was reported 30 years ago. To assess the risk of Se poisoning in the area, the Se content in agricultural soil, plant, and water in Naore, Shuang Hekou, and Lin Benhe villages of Ziyang was systematically investigated. The probable daily intake (PDI) was calculated on the basis of Se contents in food, water, and dietary habits to evaluate the risk of selenosis. Se content in hair from local habitants (32 men and 34 women) was determined to further verify the risk. Results showed that Se content in the soil ranged from 0.21 to 36.07 mg/kg, with a geometric means of 3.02 ± 5.16 mg/kg, respectively. Approximately 60% of soil reached the Se toxicity threshold (>3 mg/kg). The Se content in plants ranged from 0.02 to 17 mg/kg, with an average of 0.76 ± 2.51 mg/kg. Approximately 40% of the plant reached the toxicity standard (1 mg/kg). Se content in soil and plant of Naore village was significantly higher than that from two adjacent villages. The Se content in vegetables (on fresh weight basis) was higher than that in cereals. Se contents were 3.73 ± 9.08 and 1.32 ± 3.50 mg/kg in eggplant and pepper, respectively. The Se content in drinking water was 7.85 ± 6.04 µg/L, lower than the upper tolerable limit (40 µg/L) set by WHO. The Se content in stream water (18.5 µg/L) was significantly higher than that in drinking water because of soil erosion. The calculated PDI of habitants in Naore village (1801 µg/day) was significantly higher than that in Lin Benhe (666 µg/day) and Shuang Hekou (686 µg/day), all of which was higher than the tolerable upper Se intake (400 µg/day) set by WHO. The calculated blood Se content was relatively high, especially for habitants of Naore village (2783-2824 µg/L). Moreover, the Se content in the hair of local habitants reached as high as 12.72 ± 13.81 mg/kg, and 78.79% exceeded hair Se toxicity threshold (>3 mg/kg), which further verified the potential selenosis risk. Our study provides significant implications of the potential Se intoxication of local residents. Therefore, governments and other institutions should implement various measures to reduce the daily Se intake and to mitigate the associated health risks.

Assessment of bioavailability of selenium in different plant-soil systems by diffusive gradients in thin-films (DGT).

Uptake of selenium (Se) by plants largely depend on the availability of Se in soil. Soils and plants were sampled four times within 8 weeks of plant growth in pot experiments using four plant species. Sequential extraction and diffusive gradients in thin-films (DGT) method were employed to measure Se concentrations in potted soils in selenite- or selenate-amended soils. Results showed that DGT-measured Se concentrations (CDGT-Se) were generally several folds higher for selenate than selenite amended soils, which were obviously affected by the plant species and the duration of their growth. For example, the folds in soil planted with mustard were 1.49-3.47 and those in soils planted with purple cabbage and broccoli, which grew for 3 and 4 weeks after sowing, were 1.06-2.14 and only 0.15-0.62 after 6 weeks of growth. The selenate-amended soil planted with wheat showed an extremely high CDGT-Se compared with selenite-amended soil, except the last harvest. Furthermore, minimal changes in CDGT-Se and soluble Se(IV) were found in selenite-amended soils during plant growth, whereas significant changes were observed in selenate-amended soils (p < 0.05). Additionally, Se distribution in various fractions of soil remarkably changed; the soils planted with purple cabbage and broccoli showed the most obvious change followed by wheat and mustard. Soluble Se(VI) and exchangeable Se(VI) were likely the major sources of CDGT-Se in selenate-amended soils, and soluble Se(IV) was the possible source of CDGT-Se in selenite-amended soils. In selenate-amended soils, soluble Se(VI) and exchangeable Se(VI) were significantly correlated with Se concentrations in purple cabbage, broccoli, and mustard; in wheat, Se concentration was significantly correlated only with soluble Se(VI) but not with exchangeable Se. CDGT-Se eventually became positively correlated with Se concentrations accumulated by different plants, indicating that DGT is a feasible method in predicting plant uptake of selenate but not of selenite.

Effects of selenite and selenate application on distribution and transformation of selenium fractions in soil and its bioavailability for wheat (Triticum aestivum L.).

This study investigated the effect of selenate and selenite application on the distribution, transformation of selenium (Se) fractions in soil, as well as the accumulation and availability of Se in each part of wheat plants. A pot experiment was conducted using different concentrations of exogenous selenite or selenate (0.5, 1, 2.5, 5, and 10 mg Se kg-1 soil). Sequential extraction was used to determine the Se fractions in soil, and different models were used to study the behavior of Se in soil and its availability to wheat. Results showed that the distribution and availability of Se in soil and its accumulation in wheat affected both by Se concentrations and forms of exogenous Se. In selenite-treated soil, the proportion of exchangeable and carbonate-bound Se (EXC-Se) (21-42%) fraction increased compared to that in control (12%), while organic matter-bound Se (OM-Se) (23-33%) and Fe-Mn oxide-bound Se (FMO-Se) (11-15%) fractions decreased compare with those in control (37 and 32%, respectively). In selenate-treated soil, soluble-Se (SOL-Se) fraction (30-54%) increased and the OM-Se (9.8-20%) and FMO-Se (4.7-14.2%) fractions decreased compared with those in control. Residual Se (RES-Se) fraction was increased for selenite (7.4-13.4%) and selenate (12-20%) treatments compared with that in control (6.5%). In comparison with control, the available Se (SOL-Se + EXC-Se) fraction increased for both selenite (32-47%) or selenate (54-72%) treatments. Moreover, at the same rate of Se application, Se availability was higher in wheat grown in selenate-treated soils than that in selenite-treated soils. The redistribution index (U ts) of Se increased from 1 (in control) to 1.2-1.9 and 1.5-2 for selenite and selenate treatments, respectively; additionally, the mobility factor (MF) in selenate-treated soil was 40-90% higher than that in selenite-treated soil. Furthermore, relative bonding intensity (I R ) for both selenite (0.38-0.45) and selenate treatment (0.33-0.41) decreased compared with that in control (0.55). These differences indicated that selenite and selenate varied in terms of fixation capacities in soil, in transformation and distribution of Se in soil fractions, and in their availability to plants. The results of Michaelis-Menten equation demonstrated the high affinity of leaf to selenate, and the high affinity of roots and grains to selenite. Selenate was dominant in nearly all parts of wheat plants and in each application level. However, the affinity of selenite to wheat grains suggests that selenite is a useful Se fertilizer that must be considered in biofortification programs. In-depth studies at the pot and field scales by using different wheat varieties and application methods of Se in different ecological zones must be conducted to elucidate the mechanism and biochemical properties of Se in soil-plant system and ultimately produce Se-rich staple foods.