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Cadmium (Cd) contamination of rice fields is a global agro‐environmental issue. Rapeseed (canola) residue organic fertilizer (RROF) as a potential strategy to minimize Cd accumulation in rice planted in acidic soil. The results showed that application of 7.5–30.0 g kg−1 RROF increased the soil pH value by 0.01–0.55 units, increased soil organic matter (OM) by 7.8–25.0%, increased soil microbial biomass carbon (MBC) and nitrogen (MBN) by 0.0–89.7% and 0.0–47.5%, respectively, and increased soil dehydrogenase (DH), acid phosphatase (ACP), and urease (UA) activities by 14.3–344.0%, 0.0–127.1%, and 3.5–14.3%, respectively. The application of RROF treatments reduced the soil acid‐extractable Cd (ACI–Cd) content by 8.1–36.4% and increased the reducible Cd (RED–Cd) and oxidizable Cd (OXI–Cd) by 1.3–36.4% and 16.2–47.9%, respectively. The path analysis demonstrated that soil OM, ACP activity, Mn, and Ca ions had a significant negative indirect effect on the decrease in soil ACI‐Cd (rOM = −0.628, rACP = −0.582, rMn = −0.627, rCa = −0.341), while Zn ions showed a strong direct effect (bZn = 0.567) and indirect positive effect (rZn = 0.580). In rice plants, the contents of Cd, Fe, K, Mg, P, and Zn in rice tissues increased, and the Cd content in rice grains decreased by 3.1–31.3% under RROF application. The results of the correlation analysis found that K and Zn in rice husks and Mg in grains were significantly and negatively correlated with the husk–grain translocation factor of Cd (r = −0.88** for Zn, r = −0.81** for K, and r = −0.59* for Mg). In addition, a significant negative correlation of root–straw translocation factors of Cd with Ca and Fe ions in the roots was observed (r = −0.63* and r = −0.60* for Ca at the filling and maturity stages of rice, r = −0.61* for Fe at the tillering stage of rice). RDA showed that the distribution fraction of soil Cd controlled the accumulation of Cd in various parts of rice, with soil ACI–Cd content explaining 22.4% of the total eigen values at a significant level, followed by 14.1% for OXI–Cd. This means that ACI–Cd was the primary factor that controlled Cd accumulation in rice grain, followed by OXI–Cd. [ABSTRACT FROM AUTHOR]

Cadmium (Cd) is considered the primary dietary toxic element. Previous studies have demonstrated significant differences in heavy metal accumulation among crop species. However, this information in karst areas with low heavy metal activity is missing. In this study, the uptake and accumulation characteristics of cadmium in soil–crop samples of group 504 in the core karst region of East Asia were analyzed. Cadmium low-accumulating maize and rice were screened using cluster and Pareto analytic methods. In addition, a new method, the species–sensitive distribution model (SSD), was proposed, which could be used to estimate the environmental threshold for cadmium in regional cropland. The results showed that both maize and rice soils in the research area were contaminated with varying degrees of cadmium. The total concentrations of cadmium ω (T–Cd) in maize and rice fields are 0.18–1.32 and 0.20–4.42 mg kg–1, respectively. The ω (T–Cd) of heavy metals in maize kernels and rice grains is 0.002–0.429 and 0.003–0.393 mg kg–1, respectively. The bioaccumulation factor (BCF) of cadmium in maize ranged from 0.0079 to 0.9701, with a coefficient of variation of 1.71; the BCF of cadmium in rice ranged from 0.0074 to 0.1345, with a coefficient of variation of 0.99. According to cluster and Pareto analyses, the maize crop varieties with low cadmium accumulation suitable for local cultivation were screened as JHY809, JDY808, AD778, SN3H and SY13, and the rice varieties were DMY6188, GY725, NY6368, SY451 and DX4103. In addition, the environmental cadmium threshold ranges of 0.30–10.05 mg kg–1 and 0.89–24.39 mg kg–1 for maize and rice soils, respectively, were deduced in this study. This threshold will ensure that 5–95% of maize and rice will not be contaminated with cadmium in the soil. [Display omitted] • Differences in cadmium accumulation among different cereal varieties in karst areas were investigated. • Five rice and maize varieties with low cadmium accumulation were separately screened. • The SSD model is a new method for determining soil environmental thresholds. • Environmental thresholds of different grains help to reduce over- or underprotection of soils. [ABSTRACT FROM AUTHOR]