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.

The goal of the present study is to assess the soil quality in Bulgaria using (i) an appropriate set of soil quality indicators, namely primary nutrients (C, N, P), acidity (pH), physical clay content and potentially toxic elements (PTEs: Cu, Zn, Cd, Pb, Ni, Cr, As, Hg) and (ii) respective data mining and modeling using chemometrical and geostatistical methods. It has been shown that five latent factors are responsible for the explanation of nearly 70% of the total variance of the data set available (principal components analysis) and each factor is identified in terms of its contribution to the formation of the overall soil quality—the mountain soil factor, the geogenic factor, the ore deposit factor, the low nutrition factor, and the mercury-specific factor. The obtained soil quality patterns were additionally confirmed via hierarchical cluster analysis. The spatial distribution of the patterns throughout the whole Bulgarian territory was visualized via the mapping of the factor scores for all identified latent factors. The mapping of identified soil quality patterns was used to outline regions where additional measures for the monitoring of the phytoavailability of PTEs were required. The suggested regions are located near to thermoelectric power plants and mining and metal production facilities and are characterized by intensive agricultural activity.

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.

Background: Potassium fertilization shows beneficial effect on formation of tomato vegetative biomass and productivity. The purpose of this study was to determine the extent to which split potassium application and seedling temperature regime affects the growth parameters (leaf number, leaf area, fresh leaf weight and LAI) and yield of various tomato cultivars. Methods: A pot experiment was conducted to investigate the impact of single dose and split potassium fertilization treatments, cultivar specifics and seedling temperature regime on growth parameters and yield of tomato. Ten high yielding classic round shape tomato cultivars with determinate growth habitat were planted on Fluvisol. Single and split potassium fertilization treatments were tested. Result: The seedlings growth temperature regime and splitting the potassium fertilization treatment did not have a significant effect on the development of leaf biomass in tomato plants. Results revealed that leaf number, leaf area, fresh leaf weight and LAI of tomato plants was significantly affected by the cultivar genetic factor. Tomato yield were significantly affected by cultivars and split potassium fertilization treatments. Cultivars that measured the highest leaf area, fresh leaf weight and LAI and were also the highest yielding ones. A positive correlation between LAI and tomato fruit yield was observed.

Nematodes were studied at sites with low, medium and high level of soil metal contamination in the area of a non-ferrous metal plant. Soil total metal concentrations ranged from values below their maximum permissible concentrations (MPC) in soil to 48, 44, 16 and 4 times higher than the MPC for Cd, Pb, Zn and Cu, respectively. Nematodes were isolated from uncultivated, monoculture stands of the metal accumulating mustard weed (Lepidium draba L., Brassicaceae) in April, May and June 2015. Overall, 55 nematode genera were identified during the investigation: 50 genera at low, 39 at medium, 37 at high levels of soil metal pollution. Several nematode diversity measures and the specific responses of some nematode genera differentiated the three investigated sites. Both nematode richness (S) and diversity (H') of genera correlated negatively with the soil metal concentrations in the following order: Cd>Pb>Zn>Cu. Several genera (Alaimus, Cylindrolaimus, Paramphidelus, Scutylenchus, Steinernema) were common at the least contaminated site and were either rare or not found at the rest of the sites. Some genera (Clarkus, Filenchus) were most abundant at the medium level of contamination. The abundance of eleven genera correlated negatively with the metal concentrations in the samples. Most of the negative correlations (Acrobeles, Coslenchus, Geomonhystera, Plectus) were in the following order: Cd>Zn>Pb>Cu. The abundance of nine genera correlated positively with the metal concentrations and most of their correlations were in the order: Cu>Pb>Zn>Cd. Faunal profile analyses of the free living nematode assemblages revealed higher values of Structure index (SI) at low (75–91%) and medium (77–87%) levels of contamination in comparison with the most polluted site (35–46%). Nematode structure metabolic footprints (SFP) at low and medium levels of contamination were much higher than at the high pollution level. Both SI and SFP at the medium level of metal contamination indicated low bioavailability and low toxicity of Pb (1290 mg kg^-1soil), Cd (34 mg kg^-1soil), Zn (1779 mg kg^-1soil) and Cu (190 mg kg^-1soil) for the free-living nematodes, isolated from the rhizosphere of the mustard weed at this site. The rhizosphere of the metal accumulating plant might be protective for these nematodes, providing conditions of reduced bioavailable metals in contaminated soil during the vegetation period of the plants.