Fate of Exogenous Selenium in the Soil–Plant System: Se Accumulation, Translocation, and Effects on Growth in Vegetable, Legume, and Cereal Species

Selenium (Se) is an essential micronutrient that is often deficient in human and animal diets. As primary dietary components, cereals, legumes, and vegetables present viable targets for Se biofortification. However, systematic comparisons of Se behavior in the soil environment and across diverse crops remain limited. To address this, soil incubation and pot experiments were conducted to investigate the fate of Se in soil and various crops, including Brassicaceae (pakchoi), Asteraceae (lettuce), Poaceae (wheat and maize), and Fabaceae (common beans). The evolution of bioavailable Se content in soil pore water and its uptake by the plants were monitored. Selenium accumulation, translocation patterns, and crop responses based on growth parameters were also comprehensively compared among crops through Mantel test, principal component analysis (PCA), and partial least-squares path modeling (PLS–PM). The results demonstrated that the dynamics of Se contents in soil pore water followed a pseudo-second-order reaction model (R² > 0.95). Pakchoi exhibited the highest Se concentration in the crops, reaching up to 732 ± 137 mg/kg in shoots, which are 2.0 to 4.5 times greater than that of wheat, the crop with the lowest Se concentration. In pakchoi and beans, the Se taken up by the crop was primarily translocated to the aboveground parts, with the translocation factor (TF) ranging from 1.5 to 4.6, whereas it was mainly retained in the roots of other crops. High Se application (≥3 mg/kg) significantly reduced the total dry biomass of pakchoi and lettuce by 38–50% and 22–54%, compared to untreated soil, while it markedly increased beans and wheat biomass by 14–30% and 3–29%, respectively. Mantel test and PCA analysis indicated that Se mainly affected the growth of the vegetables, which showed a different response to Se application compared to the other crops. PLS–PM evidenced that soil Se application indirectly affected shoot Se through regulating root Se (path coefficient >0.76). The high Se accumulation and translocation indicated that pakchoi could be a preferable crop for Se biofortification. Additionally, this study provides a reference for understanding Se fate in the soil–plant system, consequently contributing to the enhancement of Se biofortification strategies.