Intercropping Pennisetum giganteum and Medicago sativa for Iron-manganese mitigation and control in coal gangue: root system dynamics and phytoremediation potential

Abstract Background and aim The diffusion of iron (Fe) and manganese (Mn) pollution caused by coal gangue accumulation has garnered significant attention. While intercropping is recognized as a promising remediation strategy, its efficacy and underlying mechanisms require further investigation. Methods This study employed a leaching column experiment to evaluate the effects of intercropping Pennisetum giganteum Z.X.Lin and Medicago sativa L. on Fe/Mn concentrations, chemical fraction in coal gangue-contaminated soil and leachate, and plant root responses. Results Results demonstrated that: (1) The P. giganteum / M. sativa intercropping system exerted synergistic regulatory effects on Fe/Mn contents and chemical fraction in coal gangue soils, reducing deep-layer (20–40 cm) soil Fe by 4.55%-5.04% and Mn by 12.01%-14.10%. It increased the residual Fe proportion by 8% and acid-extractable Mn proportion by 7%, while enhancing bioavailable Mn by 14.61%-21.75% at 20–40 cm depth. (2) Intercropping optimized root spatial distribution and Fe/Mn enrichment capacity, significantly increasing specific root length and root mass density. M. sativa root Fe and shoot Mn contents rose by 78.96% and 23.51%, respectively, while P. giganteum root Fe/Mn increased by 21.99% and 23.81%. Intercropping facilitated the conversion of Fe/Mn into residual fractions within plants, promoting their immobilization. (3) The intercropping system reduced Fe/Mn leaching outputs by 34.80% and 11.75%, respectively. Cropping pattern and root iron plaque Mn content were identified as primary drivers for Fe and Mn leaching, respectively. Conclusions P. giganteum / M. sativa intercropping reduces Fe/Mn diffusion via complementary root vertical zonation, providing an effective technical approach for ecological restoration of coal gangue dumps.