Soil heavy metal contamination has emerged as a global environmental and public health challenge. Among them, cadmium (Cd) is of particular concern due to its high mobility and ecotoxicity. To identify the key limiting factors and their nonlinear threshold effects for Cd accumulation in maize grains (Grain-Cd) in heterogeneous soil environments, a coordinated sampling campaign of soil and maize was conducted at the municipal scale in Chongqing, China. A total of 499 paired soil–maize samples were obtained, and the correlations between Grain-Cd concentrations and soil physicochemical properties, as well as soil Cd pollution characteristics, were quantitatively evaluated using the integrated Random Forest (RF) model and SHAP (SHapley Additive exPlanations) algorithm instead of traditional linear statistical methods. The results showed that the average Cd content in the soil of maize-growing areas in Chongqing City was 0.30 mg·kg−1, with a variation coefficient (CV) of 53%, and the spatial heterogeneity was significant. The average Cd content in maize grains was 0.03 mg·kg−1, with an exceedance rate of 9.6% over the Chinese National Standard (0.10 mg·kg−1), indicating a certain food safety risk. The RF model achieved a high predictive accuracy for Grain-Cd (R2 = 0.815, RMSE = 0.028 mg·kg−1, MAE = 0.013 mg·kg−1), which was significantly superior to the traditional linear regression model (R2 = 0.526, RMSE = 0.0459 mg·kg−1). The available Cd (avlCd) in the soil was identified as the core controlling factor for the Grain-Cd content, while total soil Cd (SCd) only showed its positive contribution at contents higher than 0.5 mg·kg−1. Soil pH, CEC (cation exchange capacity), and total phosphorus (TP) exerted significant influences on the Grain-Cd by regulating soil avlCd. The dependence of Grain-Cd on these soil factors was typically nonlinear, and an obvious turning point (threshold) existed for each factor with its occurring level in soil, determined by SHAP analyses as avlCd: 0.29 mg·kg−1, pH: 6.58, CEC: 18.9 cmol (+)/kg, and TP: 0.5 g·kg−1, respectively. This study clarifies the nonlinear regulatory mechanisms of key soil factors on Cd accumulation in maize grains in Chongqing, and the established RF-SHAP framework and identified soil factor thresholds lay a scientific foundation for the interpretable quantification of the soil–maize Cd system, while providing a scientific basis for the precise, targeted remediation of Cd-contaminated dryland farmland and the assurance of regional maize production safety.
Zhang et al. (Thu,) studied this question.