Optimal Timing of Lime Application for Reducing Cadmium Accumulation in Rice: A Growth-Stage-Dependent Study

Soil cadmium (Cd) pollution poses a significant threat to rice production and food safety. Although lime amendment is known to reduce Cd bioavailability in soils, the optimal growth stage for its application remains unclear. This study employed pot experiments with the rice cultivar Wuyouhuazhan as the test material to investigate the effects of lime (Ca(OH)2) application during four critical rice growth stages, namely seedling (LS), tillering (LT), booting (LB), and filling (LF), on Cd availability, soil properties, and Cd accumulation in rice. Results showed that lime application at all stages significantly reduced soil-available Cd by 53–63%, primarily by promoting the transformation of exchangeable Cd into more stable residual forms. Lime also increased biomass across rice tissues by 1–153%, with the most pronounced effects observed when applied at the seedling stage. Following lime application at different stages, Cd concentrations in all rice tissues showed a decreasing trend. Compared to CK (without lime application), Cd concentrations decreased by 2–26% in roots, 33–80% in stems, and 8–62% in grains. Among the treatments, LS was the most effective in reducing Cd levels, while LT, LB, and LF exhibited progressively weaker reductions. Structural equation modeling indicated that soil pH and stem Cd concentrations were key factors influencing grain Cd accumulation. These findings demonstrate that lime application at the early seedling stage is most effective in mitigating Cd uptake by rice, providing a practical strategy for safe rice production in Cd-contaminated soils.