In order to determine the distribution characteristics of Se in soil-crop systems, we carried out a study on the Se-rich soil threshold by collecting 8789 surface soils and 155 deep soils in the Qianjiang District of Chongqing City, China, and 141 corn seeds and 159 rice seeds (simultaneously collecting 141 and 159 corresponding root soil samples, respectively). We then analyzed the Se content, organic matter, S, Mn, TFe2O3, Al2O3, and K2O in soils and crops, and soil pH. We also analyzed the surface layer using geostatistical methods and the distribution characteristics of Se in deep soils using multiple regression analysis to study the factors influencing the bioavailability of Se. Based on the contents of each component of root soil and the Se contents of crops, the Se rich threshold was examined. The results showed that the high-Se soils in the study area account for 32.72% of the total area; the distribution of Se contents in the surface and deep soils is mainly controlled by the parent material, the source of soil Se is stable, and the surface enrichment is obvious. The Se-rich rates of corn and rice were 75.35% and 46.81%, respectively, and soil organic matter and S content will limit the bioavailability of Se. If the planted crop is corn, it is recommended to use 0.3 mg·kg-1 as the Se-rich soil threshold; if the planted crop is rice, when the soil pH is ≤ 7.5, it is recommended to use 0.3 mg·kg-1 as the Se-rich soil threshold, while at a soil pH>7.5, it is recommended to use 0.4 mg·kg-1 as the threshold. Similarly, if other large crops are planted in the study area, this method can also be used to carry out a study on the proposed Se-rich soil threshold.
Keywords: Se-rich soil threshold; Selenium; bioavailability; crops; influencing factors; soil.