Selenium and Biostimulant Co-Application Modulates Arsenic Uptake in Rocket Plants Grown on Calcareous Mine-Affected Soil

Abstract Arsenic contamination in agricultural soils poses a major risk to food safety and human health. Selenium has been reported to counteract As toxicity in plants, while biostimulants can enhance plant growth and stress tolerance. This study investigated whether selenate application and biostimulant co-application can reduce As accumulation in rocket plants grown on calcareous mine-affected soil. A pot experiment was conducted on soil collected from the historical mining area of Lavrion. Two selenate rates (3 and 6 mg Se kg⁻¹ soil) were applied with or without biostimulant. Concentrations of Se, As, Zn, P, and S were determined in plant shoots, while plant growth was assessed through dry weight and SPAD measurements. Selenate application significantly increased Se concentration in plant tissues reaching up to 1076.90 mg kg⁻¹ D.W. However, the higher selenate rate negatively affected plant growth, which was partially alleviated by biostimulant application. Biostimulant co-application at the low selenate rate reduced As concentration by 69% relative to the unamended control, while supporting plant growth. In contrast, the high selenate rate combined with biostimulant partially weakened this antagonistic effect, indicating a dose-dependent Se–As interaction. The results demonstrate that Se-based mitigation of As uptake is dose-dependent, underscoring the need to further investigate appropriate selenate application rates for safe and effective use in contaminated agricultural soils.