Quantification of the Kinetics of Soil Selenium Diffusive Gradients in Thin-Films Process under Long-Term Moisture Changes

The effects of long-term moisture changes on the migration, release, and bioavailability of selenium in soil are complex. Due to the lack of effective monitoring methods for precise quantification, its dynamic behavior is still unclear. Based on the DGT (Diffusive Gradients in Thin-films) technology, this study sets up three moisture control scenarios: continuous wet, wet-dry alternating, and continuous dry, and carries out a 6-month soil moisture control experiment. In the experiment, the DGT device collected the diffusion gradient data of soil selenium under different scenarios, and analyzed the migration characteristics of selenium in combination with the adsorption isotherm. Meanwhile, the release rate, migration coefficient, and bioavailability parameters of selenium are calculated by fitting the first-order kinetic model, further verifying the reliability and applicability of the DGT data. The experimental results demonstrate that under continuous wet conditions, the release rate of soil selenium reaches 1.85 µg·cm⁻²·h⁻¹, with a migration coefficient of 0.012 cm²·h⁻¹ and a bioavailability parameter of 0.74; under wet-dry alternating conditions, they are 1.42 µg·cm⁻²·h⁻¹, 0.01 cm²·h⁻¹, and 0.68, respectively; under continuous dry conditions, the release rate of soil selenium is the smallest, at 0.88 µg·cm⁻²·h⁻¹, with a migration coefficient of 0.004 cm²·h⁻¹ and a bioavailability parameter of 0.5. The results of this experiment reveal the dynamic behavior of soil selenium under different moisture conditions and reflect the high efficiency of DGT technology in dynamic monitoring and quantitative analysis of soil selenium behavior, providing a scientific basis for the optimal management of rhizosphere soil selenium.