[Selenium Occurrence Characteristics and Bioavailability of Soil in the Hinterland of the Hetao Plain]

In order to find out the influencing factors of soil selenium （Se） and bioavailability of Se, 1985 soil samples and 120 sets of crop and root soil samples were collected in the hinterland of the Hetao Plain, and the characteristics of soil Se, different forms of Se, and crop Se and heavy metal content were analyzed. The results showed that approximately 5.59% of soil samples exceeded the risk screening values for soil contamination of agricultural land of GB15618-2018 standard, but the heavy metal content in crops was much lower than the standard value, suggesting that the overall risk of heavy metal pollution was low. Se had significant autocorrelation in soil and appeared enriched locally, which were mainly related to natural factors. The content of different forms of Se in root soil was ranked as： residue state > humic acid state > water soluble state ≈ strong organic state > carbonate state > ion exchange state > iron and manganese oxidation state, this result showed that the characteristics of Se in root soil were high availability state, high potential availability state, and low unavailability state compared with those of most areas in China. At the same time, the results showed that water-soluble, ion exchange, and carbonate Se of root soil were significantly correlated with Se of crops,indicating that they were effective forms that could be absorbed and utilized by plants. According to the Se content and bioconcentration factor, different crops were ranked as： white melon seeds > sunflowers > corn. By means of cluster analysis and correlation analysis, the factors affecting the total Se and bioavailability of Se in soil were studied. It was concluded that soil texture, organic matter, and pH had opposite effects on Se and bioavailability of Se in soil. Factors such as clay soil, high organic matter, and low pH degree could promote the accumulation of Se in soil but also limited the activity of available Se. Based on crop and root soil sample data, the soil Se enrichment threshold was discussed. The results demonstrated that using "ω（total Se）≥0.222 mg·kg^-1" as the threshold, the proportion of soil Se enrichment was closer to the Se enrichment level of crops than others.