Engineered Nanomaterials, Microbial Community Responses, and Fe-Mediated Regulation of As and Cd Fate in the Flooded Rice Rhizosphere: A Mechanistic Synthesis

The flooded rice rhizosphere is a continuous reactive interface composed of sediment, porewater, root-surface oxic microdomains, and iron plaque, where redox processes and Fe cycling regulate Cd/As speciation, bioavailability, and plant accumulation. Engineered nanomaterials (ENMs) have shown potential for reducing Cd/As uptake in rice, but the coupled roles of microbial community responses, iron-plaque gating, and cross-interface elemental migration remain insufficiently integrated. This review synthesizes the current evidence on ENM transformation and partitioning at flooded rhizosphere microinterfaces, focusing on front-end speciation changes, root-surface retention, microbial functional regulation, and plant sequestration or transport. Correlative evidence suggests that rhizosphere microorganisms are associated with altered redox conditions, Fe cycling, As methylation potential, and metabolite secretion, which may influence Cd/As partitioning and cross-interface migration. However, direct causal validation of the complete ENM transformation–microbial response–Fe cycling–Cd/As flux–grain accumulation sequence within a single integrated system remains lacking. We further discuss how elevated CO2, micro-/nanoplastics, Fe/DOM dynamics, and water management regimes may modify this framework, and we identify Sb as a theoretical boundary case because direct ENM–rice evidence remains limited. Finally, we highlight the need to integrate spatial tracing and imaging methods, including persistent luminescence tracing, LA-ICP-MS, NanoSIMS, and µ-XRF/µ-XANES, with metaomics to connect particle localization, microbial function, and contaminant fate.