Rhizosphere Transformation and Plant Responses of Cerium Oxide Nanoparticles: Mechanistic Insights and Boundaries Compared with Other Rare-Earth Nanomaterials

The increasing use of cerium oxide nanoparticles (CeO2-NPs) has raised concerns regarding their environmental fate and biological effects in soil–plant systems. This review uses cerium oxide nanoparticles (CeO2-NPs) as a mechanistically well-characterized case study to examine relationships among dose, chemical speciation, and plant responses, with emphasis on Ce3+/Ce4+ redox cycling and condition-dependent, nanozyme-like activity. We discuss how particle size, surface charge, Ce3+/Ce4+ ratio, and coating interact with rhizosphere processes, including organic acid complexation, phosphate-mediated CePO4 immobilization, and potentially microbially mediated transformation, to influence effective root-surface exposure. The resulting particulate, ionic, and secondary transformation-product pools affect root uptake, vascular transport, redox homeostasis, metabolism, and hormone-associated signaling. Major limitations include weak causal links between rhizosphere speciation and bioavailability, insufficient separation of particulate and ionic contributions, limited integration of multi-omics with speciation data, and uncertain extrapolation from hydroponic to soil systems. Standardized exposure protocols, integrated XANES, spICP-MS, and imaging workflows, and long-term soil studies are therefore required. Other rare-earth nanomaterials are discussed only briefly to define the material-specific boundaries of the CeO2-centered framework.