Abstract: Soil Se is primarily derived from parent rocks. However, the mechanisms of Se migration and transformation during weathering and soil development, as well as their effects on crop accumulation, remain unclear. Soils derived from basalt (Xuwen, Zhanjiang) and granite (Yamen, Jiangmen) weathering in the red soil region of South China both exhibit Se enrichment (>0.4 mg·kg^-1). We analyzed the geochemical distribution of Se in the “parent rock-soil-crop” system. The results showed that the Se content in basaltderived soils (1.04 mg·kg^-1) was higher than that in granite-derived soils (0.90 mg·kg^-1), which was closely related to differences in parental rock composition and weathering processes. The migration and transformation of Se at the rock-soil interface were mainly influenced by soil Al, Fe, S, and pH. The Se accumulation capacity in crops was similar in both regions. In the basalt region, 25% of rice samples and all vegetable samples met the Se-rich criteria, while31.25% of rice samples and 77.14% of vegetable samples reached the Se-rich standard in the granite region. Furthermore, the Se content in crops was significantly positively correlated with the Ca content and soil pH, indicating that Ca affects the bioavailability absorption and accumulation of Se in crops mainly by regulating soil acidity and ion balance. These findings provide new insights into the biogeochemical behavior of Se in geological high-background regions and offer a scientific basis for evaluating the mechanisms of natural Se enrichment in soils and its rational utilization.

Key words: parent rock, weathering process, crop, selenium, migration and transformation