Synergistic Inhibition Effect of Cd in Soil–Rice System from a Ca and Zn Enriched Karst Paddy: A Field-Based Study

The geochemistry of karstic paddy soils significantly influences Cd bioavailability and its accumulation by rice. However, there is limited research on Cd translocation and accumulation in rice, grown in paddy soil with elevated CaO and Zn levels. Addressing this, a field-based study in Hanyuan County, Sichuan Province, China, examined the Cd distribution and translocation patterns across the soil–root–straw–grain continuum, aiming to elucidate how elevated CaO and Zn levels regulate Cd accumulation in grains. The geochemical characterization of the soil revealed elevated concentrations of cadmium (Cd), calcium oxide (CaO), and zinc (Zn), with mean values of 2.12 ± 2.93 mg/kg, 5.77 ± 5.80 wt.%, and 359 ± 472 mg/kg, respectively. The sequential extraction results (by Tessier) demonstrated that the distribution of Cd followed the general order: residual > Fe-Mn oxide-bound ≈ exchangeable ≈ carbonate-bound > organic matter-bound. The exchangeable Cd content ranged from 0.063 to 0.93 mg/kg, accounting for up to 21.4% of the total Cd on average. The correlation analysis, Bioconcentration Factor (BCF), and Transfer Factor (TF) of Cd in different rice tissues, and structural equation modeling (SEM) revealed that elevated CaO and Zn levels in soil exhibit synergistic effects in inhibiting Cd accumulation in grains. Specifically, the CaO in soil reduces the Cd bioavailability by the formation of carbonate-bound specie (CdF2), thereby inhibiting Cd uptake by roots. In rice tissues, Zn could limit Cd translocation, particularly from roots to straws. Consequently, the regional rice Cd exceedance rate (25.8%) was significantly lower than that reported for other similar karst soils in China (over 50%).