[Threshold of Se-rich Soil Based on Available-Se and Influencing Factors of Available-Se]

Available selenium (Se) in soil was the predominant factor affecting the content of Se in crops. In order to reasonably delineate the Se-rich soil range and propose theoretical guidance for the cultivation of natural Se-rich crops in a region where the surface soils had a high level of available-Se and a low level of total-Se, 8814 samples in surface soil and 195 root-crop matching samples were collected in Shizuishan in northern Ningxia. On the basis of the main line of analysis of available-Se, the following research was conducted: by synthetically studying the total-Se and available-Se in surface soil and root soil, the morphology of Se in surface soil, as well as Se in crops, deep and coordinated analyses of content among total-Se, available-Se, and Se in root-crop matching samples were carried out, and the suitable threshold for Shizuishan was confirmed. A multiple regression model of available-Se was established to determine the main physical and chemical indexes affecting available-Se, which were expected to improve the Se enrichment rate of crops through the enhancement of available-Se. The results demonstrated that ω(Se) and ω(Se_available)in the surface soil in Shizuishan were 0.26 mg·kg^-1 and 12.85 μg·kg^-1, respectively, and the characteristics of Se and available-Se in root-crop matching samples could represent those in surface soil. Thus, it was recommended to use 0.24 mg·kg^-1 as the suitable threshold of Se-rich soil. The multiple regression model of available-Se showed that the increase in total-Se and soil elements affecting soil fertility could promote the enrichment of available-Se.