Cd Is a Heavily Enriched Heavy Metal Controlled by Organic Fertilizer Use in Facility Farmland Soils

Heavy metals are a common type of contaminant found in soil. However, knowledge gaps still exist in the characteristics of the heavy metals in facility farmland soils. Therefore, a differential analysis was conducted on the heavy metal (Cd, Cr, Pb, As, Cu, Zn, and Ni) content in soils that were amended with different types of fertilizers (T1, organic fertilizer; T2, organic fertilizer and chemical fertilizer). Regression analysis was employed to demonstrate the trends of metal content changes with cropping durations (5, 10, 15, and 20 years). The Pearson correlation analysis was applied to investigate the correlation between soil nutrients and soil metals, and to determine the key factors influencing heavy metal concentration through the random forest model. The results indicated that the average concentration of Cd (0.148 mg·kg−1) was higher than the soil background value but lower than the potential ecological risk threshold (0.3 mg·kg−1, 6.5 < pH ≤ 7.5). The Cd content in soil where only organic fertilizer was applied was 0.118 mg·kg−1, while the Cd content in soil where both organic fertilizer and chemical fertilizer were applied was 0.148 mg·kg−1. There was no significant difference in Cd content between the two fertilization types. The regression analysis indicated a linear increasing trend in Cd content as the cropping duration increased. The Pearson correlation analysis showed that there were also significant correlations between pH and both As and Ni; between SOM (soil organic matter) and Cr, Cu, and Zn; as well as between Cd and EC (electrical conductivity). The random forest model exhibited high prediction accuracy when explaining the most significant factors influencing the concentrations of Cd, Cr, Pb, As, Cu, Zn, and Ni, with overall correlation coefficients (r) of 0.009, 0.44, 0.33, 0.04, 0.32, 0.61, and 0.65, respectively. Using Hakanson’s method to conduct an ecological risk assessment on the soil in the study area, it was found that the overall potential ecological risk level was relatively low. However, the proportion of moderate to high ecological risk associated with Cd elements was close to 40%, which requires special attention.