Enzyme activities in soils under heavy metal pollution: a case study from the surroundings of a non-ferrous metal plant in Bulgaria

Abstract

Soil enzyme activities are potentially valuable indicators of soil health in cases of heavy metal and metalloid (HM) pollution. However, the extent to which HM pollution affects specific enzyme activities remains unclear. In this study we assessed the level of HM pollution and its effects on soil enzyme activities to select the most reliable biochemical indicators of soil health under HM stress. The initial hypothesis was that enzyme responses would be site-specific, depending not only on the level of pollution but also on soil abiotic and biotic properties. The Nemerow pollution index (NPI) and the enzyme activity index (EAI) were calculated. EAIs were based on the activities of dehydrogenase (Dha), β-glucosidase (BGl), acid (AcP) and alkaline (AlP) phosphatases, arylsulphatase (Ars) and urease (Ur). NPI indicated slight (1.0) to serious (67.46) soil pollution. The EAI varied from 4.50 to 11.54. The overall functional dissimilarity between soils was around 26%. Cluster and SIMPER analyses both indicated that the activities of Dha, BGl and AlP grouped together with Cu, Zn and Pb, and accounted for 85% of the between-soil functional dissimilarity. The results indicated that there was enzyme functional redundancy between soils. Soil enzymes showed a relatively high capacity to tolerate long-term HM pollution. Dha, BGl and AlP were highly sensitive to environmental fluctuations, including HM concentrations. The long-term HM stress on soil enzyme activities calls for further studies of the soil properties and the time scale involved in the evolution of metal tolerant enzyme producers in HM-impacted soils.