Mitigating cadmium-phenanthrene co-contamination in rice via rhizosphere tube-insertion: The dual role of iron plaque and microbial regulation.

Cadmium (Cd) and phenanthrene (Phen) co-contamination threatens the safety of rice production in paddies. This study introduces a novel rhizosphere regulation technique-plastic tube insertion into the rhizosphere soil (PIRS)-which involves placing adsorbent packed plastic dropper tubes into the contaminated rice rhizosphere. This approach combines adsorbent-packed tubes with the in-situ formation of iron oxide coatings on tube walls and enrichment of functional microorganisms for pollutant mitigation. Field experiments demonstrated that PIRS reduced Cd in brown rice by 50.3-96.8% (below the Chinese limit) and Phen by 11.5-22.1%, while shifting bioavailable pollutants in rhizosphere soil toward more stable fractions. PIRS enhanced iron plaque on roots and iron oxide accumulation on tubes, enriched bacterial genera with Fe(II) oxidation, Cd resistance, and Phen degradation capabilities (e.g., Sideroxydans and Sphingomonas), and up‑regulated related functional genes (e.g., czcC and pahE4). Tube removal enabled the direct pollutant removal from rhizosphere, corresponding to a removal of 39.9-65.0 ng Cd and 6.7-91.1 ng Phen per cm3 of inserted tube volume. Combined with straw removal, this strategy removes 3.4 g Cd and 688.6 mg Phen/ha/season, assuming no exogenous input, at an estimated material cost of 1250 USD/ha/season. This work provides an in-situ, controllable, and low‑impact rhizoremediation strategy for Cd-Phen co-contaminated paddies.