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.

The goals of the study were to assess the diversity and structure of the bacterial communities within the soil depth along a gradient of heavy metal (HM) contamination and to identify indigenous bacterial species in the agricultural area of a non-ferrous metal plant KCM 2000 Group (Plovdiv) by using 16S rRNA gene retrieval. 16S rRNA gene clone libraries were constructed for nine samples, which were collected from two soil depths in June 2020. From each library, up to 100 clones were analysed and grouped into operational taxonomic units (OTUs) by restriction fragment length polymorphism (RFLP). The representatives of the OTUs were sequenced, followed by phylogenetic analysis. The results revealed that phyla Proteobacteria (11.11–71.43%) and Actinomycetota (3.57–33%) were the most abundant. Surface soils (12 phyla and 15 classes) were more diverse than subsurface ones (7 phyla and 12 classes). The lowest diversity at both phylum and class levels was calculated for the moderately contaminated soils from the two studied depths. Thirteen 16S rDNA sequences were identified to a species level, and they belonged to Proteobacteria, Actinomycetota and Firmicutes. This study highlighted that both HM contamination and soil depth caused shifts in diversity and structure of soil bacterial communities.

The aim of the study was to evaluate the long-term impact of heavy metals on soil bacterial metabolism by assessing their capacity to utilize a set of carbon sources. The study area is in the vicinity of the Pb/Zn smelter KCM 2000, South-Central Bulgaria, with considerable Pb, Zn, Cd, Cu, and As pollution. The Biolog Ecoplate approach was used to evaluate the bacterial metabolic activity, expressed by the average well colour development; the bacterial metabolic fingerprints expressed by community-level physiological profiling, and Shannon bacterial functional diversity. The results revealed reduced metabolic activity recorded for bacterial communities inhabiting the heavily polluted soils. The most preferred carbon groups utilized by the bacterial communities were the polymers, followed by carbohydrates and amino acids. Local variability of soil properties, such as inorganic ions, total organic carbon, pH, soil moisture, and soil texture, could modify the effects of heavy metals. This study highlighted that Biolog EcoplateTM is a suitable approach for assessing bacterial functional shifts in relation to long-term soil pollution stress.