The toxic factor (TF) is a key parameter commonly used to evaluate the potential ecological risk index (RI) of heavy metals. However, it remains largely unknown whether this traditional TF is applicable to the ecological risk assessment of heavy metal to soil microorganisms, which are essential to ecological functions and ecosystem sustainability. Here, based on the TF values of 5 and 30 for copper (Cu) and cadmium (Cd), respectively, we constructed soil microcosms to establish a gradient of ecological risk levels, and different combinations of Cu and Cd concentrations were used at each ecological risk level. We found that bacterial abundance and functional diversity significantly decreased and the metal resistance gene (MRG) abundances increased with the increasing RI level. At the same RI level, the bacterial abundance and functional diversity decreased while MRG abundances increased with increasing Cu concentrations, suggesting that the ecological risk to soil bacterial community was more relevant to Cu, rather than Cd. The traditional TF of Cu used to calculate the RI might be underestimated if it is used for evaluation of the risk to soil bacteria. Our calibration analysis revealed that the TF of Cu should be adjusted to 6 during the assessment of the RI for soil bacterial community in heavy-metal contaminated area.

毒性因子（TF）是通常用于评估重金属潜在生态风险指数（RI）的关键参数。但是，这种传统的TF是否适用于重金属对土壤微生物的生态风险评估仍是一个未知数，这对于生态功能和生态系统的可持续性至关重要。在这里，我们分别基于铜（Cu）和镉（Cd）的TF值分别为5和30，我们构建了土壤微观世界以建立生态风险等级的梯度，并且在每种生态环境中使用了不同的Cu和Cd浓度组合风险等级。我们发现，随着RI的增加，细菌的丰度和功能多样性显着降低，金属抗性基因（MRG）的丰度增加水平。在相同的RI水平下，细菌含量和功能多样性随Cu浓度的增加而降低，而MRG含量则随Cu浓度的增加而增加，这表明对土壤细菌群落的生态风险与Cu而不是Cd更为相关。如果将铜用于计算RI的传统TF用于评估土壤细菌的风险，则可能会低估它。我们的校准分析表明，在重金属污染区土壤细菌群落的RI评估中，应将Cu的TF调整为6 。