Evaluation methods of heavy metal pollution in soils based on enzyme activities: A review

Soil enzyme activities have been suggested as suitable indicators for the evaluation of metal contamination because they are susceptible to microbial changes caused by heavy metal stress and are strictly related to soil nutrient cycles. However, there is a growing lack of recognition and summary of the historic advancements that use soil enzymology as the proposal of evaluation methods. Here, we review the most common methods of heavy metal pollution evaluation based on enzyme activities, which include single enzyme index, combined enzyme index, enzyme-based functional diversity index, microbiological stress index, and ecoenzymatic stoichiometry models. This review critically examines the advantages and disadvantages of these methods based on their execution complexity, performance, and ecological implications and gets a glimpse of avenues to come to improved future evaluation systems. Indices based on a single enzyme are variable and have no consistent response to soil heavy metals, and the following three composite indices are characterized by the loss of many critical microbial processes, which thus not conducive to reflect the effects of heavy metals on soil ecosystems. Considering the dexterity of ecoenzymatic stoichiometry methods in reflecting changes in soil functions under heavy metal stress, we propose that microbial metabolic limitations quantified by ecoenzymatic stoichiometry models could be promising indicators for enhancing the reality and acceptance of results and further improving the potential for actual utility in environmental decision-making.