Sub-micrometer oyster shell powder mitigates cadmium phytotoxicity through soil and root barrier reinforcement

Oyster shells are abundant marine biogenic wastes with potential for reuse as soil amendments, but the limited reactivity of conventional granular powders constrains their Cd-remediation efficiency. Here, we developed a scalable mechanical micronization strategy to produce micronized oyster shell powder with a sub-micrometer mean diameter (SOSP) from granular oyster shell powder (GOSP) and evaluated whether particle-size refinement improves its capacity to mitigate Cd phytotoxicity. The mitigation efficiency increased with amendment dose, with the most pronounced effects at 1000 mg kg⁻¹. At this level, SOSP increased shoot and root biomass by 37% and 55% and reduced shoot and root Cd concentrations by 15% and 54%, relative to GOSP. This mitigation was driven by dual mechanisms across the soil-root system. In soil, SOSP increased pH (+2%) and Ca availability (+4%), reducing exchangeable Cd by 58% relative to GOSP. In roots, SOSP increased the cell-wall-associated Cd proportion from 58% with GOSP to 72%, accompanied by activation of the phenylpropanoid pathway (4CL-CCR-CAD), greater lignin accumulation, and cell-wall thickening. These findings show that particle-size refinement couples enhanced soil Cd immobilization with root apoplastic barrier reinforcement, supporting the development of high-reactivity oyster-shell-derived amendments for Cd-risk mitigation.