Cd toxicity thresholds in response to freeze-thaw and wetting-redrying cycles in various parental paddy soils

In order to clarify the stabilization characteristics of cadmium (Cd) after different aging methods, soil culture tests of freeze-thaw, wetting-redrying cycles, and natural aging affecting on Cd phytotoxicity to rice were conducted. According to the national food safety standard GB2762-2022, the toxicity thresholds (RT) of Cd under different aging modes were derived. The results showed that: as the aging time increased to 360 days, the RT of different parent soils increased by 28.49 %-204.86 % (freeze-thaw), 37.80 %-192.55 % (natural), 34.45 %-205.87 % (wetting-redrying), compared with 14d' aging. Compared with natural aging: The RT of granite soil (S_G), river sandy mud (S_R), stucco field (S_S) increased by 99.46 % under freeze-thaw aging, decreased by 66.68 % under wetting-redrying aging; The RT of purple sandy shale (S_P), quaternary red clay soil (S_Q) decreased by 20.32 % under freeze-thaw aging, increased by 70.99 % under wetting-redrying aging. RT of yellow mud soil (S_Y) showed a decreasing trend under unnatural aging. The prediction models of RT based on different parental characteristics under different aging methods were established: log₁₀(AF₃₆₀)=1.038AG-0.251Chaol-1.052, log₁₀(AF₃₆₀)=0.481pH+1.492CEC+1.223MW-6.375, log₁₀(AF₃₆₀)=0.119pH+0.264CEC+1.091AD+0.263. The regression coefficients of aging factors (AF₃₆₀), which were 1.04, 1.49, and 1.09 respectively, provided that water-stable aggregates (AG), cation exchange capacity (CEC), and the adhesive film (AD) were the primary controlling factors influencing RT with positive correlations. This study provided an important basis for evaluating different long-term aging modes affecting on soil Cd toxicity behavior and RT of Cd on rice.