In this study, the accumulation characteristics of As, Hg, Cd, Cr, and Pb in 63 soil samples from 28 organic farms in Beijing, China, were analyzed to investigate the risk of heavy metal pollution in organic agriculture, and the key related factors were evaluated. The results revealed that the As, Hg, Cd, Cr, and Pb concentrations in the soil samples were below the risk screening values and substantially lower than those in the soil under conventional agriculture. However, the coefficients of variation for Hg and Cd were 112.45% and 38.34%, respectively, indicating a notable anthropogenic impact. Notably, 35.92% of the sampling sites had medium to high potential ecological risk values for Cd, and the Cd concentration increased considerably as the number of planting years increased. Different crop types impacted the soil heavy metal concentrations. The concentrations of Cd and As in the soil of Brassica crops were 0.265 and 12.915 mg/kg, respectively, which were substantially higher than those in the soil of other crop types. The Random Forest model indicated that soil nutrients had the most significant impact on soil heavy metal accumulation, particularly phosphorus. In conclusion, compared with conventional agriculture, organic agricultural soils have lower heavy metal concentrations and exhibit lower ecological risks, with no significant heavy metal pollution detected. However, there is a risk of Cd accumulation, and preventive measures should be implemented, especially for soils under prolonged cultivation and with potential sources of heavy Cd inputs.

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.