Application of stress index in evaluating toxicological response of soil microbial community to contaminants in soils

•The use of SI in ecological risk assessment is indicative and helpful.•Heavy metal contaminated soils had higher SIs relative to the reference site.•N and C mineralization and alkaline phosphatase were sensitive to Siduron addition.•Heavy metal contaminated soil showed more severe impairment when added with Siduron. Toxic effects of chemical contaminants on soil microbial community structure and function have been widely studied. However, it is difficult to assess risks regarding soil microbial toxicity. In the ratio to reference approach, the stress index (SI) is used to indicate the relative change of biological response of organisms compared to a reference. In this study, the SI approach was used to assess the soil microbial stress levels of multiple heavy metal contaminated urban soil in three sites. Soil microbial community functional parameters suggested that two heavy metal contaminated sites (G and D) had apparently higher stress indexes relative to the reference site, N. Use of those similar microbial community functional parameters revealed that parameters such as N and C mineralization and alkaline phosphatase activity were sensitive to the addition of the herbicide Siduron. Soil G, which had the highest microbial stress index, showed more severe impairment when amended with Siduron, while soil N and soil D showed dose-dependent responses to Siduron. Overall, the results of this study indicated that, when compared with direct use of real data describing microbial parameters, use of SI in ecological risk assessment is more indicative and helpful.