Responses of Pb and Cd extractability, sorghum accumulation, and rhizosphere microbial communities to EDDS under PBS microplastic co-contamination

Microplastics can modify the behavior and ecological effects of coexisting heavy metals in soil, but the response of microplastic–heavy-metal systems to [S,S]-ethylenediamine disuccinic acid (EDDS) remains poorly understood. A pot experiment was conducted using sorghum in soil co-contaminated with cadmium (Cd), lead (Pb), and polybutylene succinate (PBS). Different cumulative EDDS doses were applied to evaluate treatment-associated changes in plant growth, metal accumulation, soil properties, and rhizosphere bacterial and fungal communities. Pb-Cd co-contamination significantly inhibited sorghum growth, and, under the tested heavy-metal condition, PBS addition was associated with greater growth inhibition. EDDS slightly increased shoot length in some treatments but did not consistently improve biomass. EDDS treatments generally increased DTPA-extractable Cd and Pb, but these increases did not consistently translate into greater plant metal accumulation because biomass was not simultaneously improved. Increasing PBS loading under the fixed EDDS condition was associated with greater metal extractability but progressively lower sorghum biomass, indicating a trade-off between metal mobilization and plant performance. Treatment groups differing in PBS and EDDS levels also exhibited distinct rhizosphere soil properties and microbial community structures. Bacterial community differentiation was more pronounced among treatments, whereas changes in fungal richness indices were relatively prominent. DTPA-extractable Pb and Cd, available potassium, and available phosphorus were significantly associated with rhizosphere microbial community variation. Variations in DTPA-extractable metals were also associated with plant metal accumulation patterns. These findings characterize treatment-associated plant–soil–microbial responses to EDDS under the tested PBS–Pb–Cd co-contamination conditions and highlight a trade-off between metal mobilization and maintenance of plant performance during chelator-assisted phytoextraction.