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.

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 (BCFgrain) 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 BCFgrain of maize, whereas BCFgrain 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.

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.

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.