Shunjiang Li

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.

The detoxification of crops grown in Cadmium (Cd)-contaminated acid soil has been widely studied, but for contaminated alkaline soil, there is still inadequate research or information. In order to investigate the effects of calcium and sulfur fertilizers, accompanied by different side elements, on the growth and Cd uptake of Spinacia oleracea grown in Cd-contaminated alkaline soil, the plants were subjected to five treatments, including calcium silicate (Ca-Si), calcium biphosphate (Ca-P), magnesium sulfate (S-Mg), ferric sulfate (S-Fe), and zinc sulfate (S-Zn), and a control group. The results showed that the S-Fe achieved the highest shoot fresh mass and dry mass and the highest shoot Cd concentration and accumulation, which were 30%, 68%, 4.6%, and 73% higher than the control group, respectively. The Ca-Si, Ca-P, S-Mg, and S-Zn reduced the root Cd concentration by 18%, 42%, 7%, and 49%, respectively, and reduced the shoot Cd concentration by 25%, 36%, 15%, and 27%, respectively, as compared to the control. S-Fe increases plant N uptake and photosynthesis, which is beneficial to biomass accumulation. Ca-P improves soil and plant P nutrition status, as well as plant K and Ca status, and helps alleviate plant Cd stress. Overall, calcium fertilizers accompanied by phosphorus have the potential to reduce plant Cd contamination risk, while sulfur fertilizers accompanied by iron show potential for enhancing Cd extraction.