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.

Soil pollution by heavy metals (HMs) poses a major threat to soil quality and human health, with mining and smelting industries identified as key sources. Soils around smelters are often considered polluted hotspots, being generally unsuitable for agricultural activities. Although many studies have identified microbial taxa able to survive in such environments, most have focused on relatively low HM concentrations. The purpose of the study was to assess the ecological risk and to evaluate the diversity and structural shifts in microbial communities, as well as to predict key metabolic pathways associated with HM resistance in soils near Pb–Zn smelter in Bulgaria. The soils ranged from low-risk to disastrous, with cadmium (Cd) identified as the primary contributor to soil toxicity. High-throughput sequencing of 16S rRNA and ITS amplicons revealed widespread dominance of the phyla Proteobacteria, Actinobacteriota and Acidobacteriota, and Ascomycota, with the prevailing classes Acidobacteriae, Chloroflexia, and Eurotiomycetes, indicating their high tolerance to HMs. Functional predictions suggested enrichment of key pathways in the most polluted soils related to HM resistance, including efflux systems and detoxifying enzymes. These results highlight the necessity of integrating soil microbial indicators into agricultural management strategies to ensure safe food production.

Uranium mining and milling activities adversely affect the microbial populations of impacted sites. The negative effects of uranium on soil bacteria and fungi are well studied, but little is known about the effects of radionuclides and heavy metals on archaea. The composition and diversity of archaeal communities inhabiting the waste pile of the Sliven uranium mine and the soil of the Buhovo uranium mine were investigated using 16S rRNA gene retrieval. A total of 355 archaeal clones were selected, and their 16S rDNA inserts were analysed by restriction fragment length polymorphism (RFLP) discriminating 14 different RFLP types. All evaluated archaeal 16S rRNA gene sequences belong to the 1.1b/Nitrososphaeracluster of Crenarchaeota. The composition of the archaeal community is distinct for each site of interest and dependent on environmental characteristics, including pollution levels. Since the members of 1.1b/Nitrososphaeracluster have been implicated in the nitrogen cycle, the archaeal communities from these sites were probed for the presence of the ammonia monooxygenase gene (amoA). Our data indicate thatamoA gene sequences are distributed in a similar manner as in Crenarchaeota, suggesting that archaeal nitrification processes in uranium mining-impacted locations are under the control of the same key factors controlling archaeal diversity.

This study presents the distribution of bacterial and fungal abundances in long-term copper (Cu) contaminated soils in Topolnitsa-Pirdop valley – a highly industrialized zone with a number of mines and processing plants for copper and other non-ferrous metals. The bacterial (16S rRNA gene copies) and fungal (ITS rRNA gene copies) were estimated using quantitative PCR technique in five topsoils, differently Cu contaminated (ranging from 28.05 to 198.9 mg kg-1). Bacterial abundance varied in a range of 1.68 × 1011 to 3.24 × 101116S rRNA genes, whereas fungi amounted from 1.95 × 108 to 6.71 × 108 ITS rRNA genes. Fungal and bacterial abundances were significantly (fungi) and insignificantly (bacteria) influenced by Cu contamination. The fungal/bacterial ratio related negatively with soil Cu, which shifted microbial communities’ structure towards bacterial dominance. Since the ratio between bacteria and fungi are vital in explaining many soil functions, the calculated changes in this ratio indicated deterioration in soil quality, being of primary importance for plant production.