24-Epibrassinolide Enhances Pb-As Phytostabilization in Willow-Vetiver Intercropping Systems: Plant Accumulation Patterns and Soil Biogeochemical Perspectives

Soil co-contamination with lead (Pb) and arsenic (As) poses severe environmental risks. This study investigated the effects of 24-epibrassinolide (24-Brs) on Pb and As accumulation in willow (Salix babylonica × S. × leucopithecia ‘NJU513’) and vetiver grass (Chrysopogon zizanioides) under monoculture and intercropping in naturally contaminated soil (215.48 mg Pb kg−1, 120.16 mg As kg−1). Plant biomass, metal accumulation, soil physicochemical properties, metal fractionation, and mineral nutrient dynamics were evaluated. Exogenous 24-Brs significantly promoted plant biomass under Pb-As stress (p < 0.05), with the greatest increase up to 114.81% in vetiver monoculture (vs. untreated stress control, NBrs). 24-Brs increased root Pb concentrations in willow (14.23 to 24.55 mg kg−1) and intercropped vetiver (44.51 to 68.58 mg kg−1), and root As in willow (10.80 to 13.55 mg kg−1, not significant). Soil fractionation revealed that 24-Brs shifted Pb and As toward more bioavailable fractions, with exchangeable Pb increasing by 63.22% in willow-planted soil. 24-Brs also reshaped essential mineral nutrient (Zn, Cu, Ni, Mn) fractionation and uptake patterns. Intercropping combined with 24-Brs achieved the lowest As leaching (1.50 μg L−1, a 90.20% reduction compared with the unplanted control at 15.31 μg L−1), although the difference between 24-Brs and NBrs treatments within each planting pattern was not statistically significant. These findings indicate that 24-Brs enhances Pb–As phytostabilization through combined effects on plant metal accumulation and soil biogeochemistry, although the underlying plant physiological mechanisms require further direct investigation.