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Article

Analysis of Changes in the Microbial Biodiversity of Soil Contaminated with Cr(III) and Cr(VI)

by
Edyta Boros-Lajszner
,
Jadwiga Wyszkowska
*,
Małgorzata Baćmaga
and
Jan Kucharski
Department of Soil Science and Microbiology, Faculty of Agriculture and Forestry, University of Warmia and Mazury in Olsztyn, Plac Łódzki 3, 10-727 Olsztyn, Poland
*
Author to whom correspondence should be addressed.
Appl. Sci. 2025, 15(20), 10951; https://doi.org/10.3390/app152010951
Submission received: 25 September 2025 / Revised: 8 October 2025 / Accepted: 11 October 2025 / Published: 12 October 2025
(This article belongs to the Special Issue Degraded Soil Treatment and Influence on Biodiversity)
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Abstract

Contamination with heavy metals, including chromium that exists in two oxidation states—Cr(III) and Cr(VI)—poses a significant challenge for the soil environment. Both chemical forms of chromium can exert toxic effects on microorganisms that play a key role in maintaining soil fertility and plant health. The aim of the study was to compare the selective toxic effects of Cr(III) and Cr(VI) ions on soil bacterial and fungal taxonomic diversity using NGS technology. The data obtained enabled a comprehensive characterisation of the taxonomic profile of the soil microbiome exposed to both forms of chromium, providing a basis for further research into the adaptation and resistance mechanisms of microorganisms. The calculated diversity indices, in particular the Shannon-Wiener index, suggest that Cr(VI) is more toxic to bacteria than Cr(III). In soil contaminated with chromium, the relative abundance of chromium-resistant bacteria of the phylum Actinobacteriota increased to the detriment of chromium-sensitive Acidobacteriota and Proteobacteriota. The abundance of Ascomycota, the dominant fungal phylum, increased in soil with Cr(III) and decreased in soil with Cr(VI). Cr(III) promoted the growth of bacteria of the genera Phycicoccus and Arthrobacter and Penicillium fungi. In turn, Cr(VI) stimulated the growth of bacteria of the genera Mycoplana and Cellulosimicrobium, and Trichoderma fungi. The study demonstrated that microbial resistance mechanisms are influenced by the chemical form of chromium. In addition, the increased abundance of chromium-resistant taxa highlights their potential for the bioremediation of soils contaminated with this element.

1. Introduction

Chromium is widely used in many industries, including in the production of stainless steel, leather tanning, and the production of dyes and corrosion inhibitors [1,2,3]. However, industrial activities contribute to the contamination of the soil environment and poses a significant threat for ecosystem functions [4,5]. Soil contamination with chromium is a global and persistent issue. Studies conducted in China have shown that long-term industrial processes, such as tanning, lead to the immobilisation of chromium in soils, demonstrating the persistence and chronic nature of the problem [6]. Similar challenges related to widespread heavy metal contamination, including chromium, and the need for policy intervention are observed in Africa, as confirmed by a systematic review of contamination in Nigeria [7]. The scale of the problem is closely linked to the increasing global production of chromium, which rose from 23.7 to 41 million Mg between 2010 and 2021. The main producers of chromium are South Africa, India, Kazakhstan, and China [8]. In 2022, the EU Member States with the largest share of total chromium (Cr) emissions (above 10%) were Germany, Poland, and Italy [9]. Cr(III) and Cr(VI) are the two most prevalent chemical forms of chromium in the environment that differ considerably in toxicity and their effects on microorganisms [10,11]. Cr(III) is regarded as relatively stable and has a smaller impact on the soil microbiome, although it may become toxic at higher concentrations. In turn, Cr(VI) is highly toxic and causes serious structural and functional damage to cells. It exacerbates oxidative stress, leads to DNA damage, and disruptions in enzyme activity, thus directly compromising the survival of soil-dwelling microorganisms [12,13,14]. Enzyme activity is reduced in soils contaminated with Cr(VI) [15,16], which exerts a negative effect on the key biogeochemical processes such as the carbon cycle and the nitrogen cycle. The above modifies the physical and chemical properties of soil and inhibits the proliferation of bacterial and fungal populations [17,18].
Soil microorganisms, including bacteria and fungi, play fundamental roles in ecosystems by participating in organic matter decomposition, soil renaturation, and the biogeochemical cycles of elements [19]. Their adaptation to environments contaminated with heavy metals, including chromium, is a complex process involving the development and acquisition of various resistance mechanisms [20]. The primary microbial defense strategies against heavy metals include bioaccumulation, biosorption, bioprecipitation, and bioreduction [21,22].
Bacteria of the phyla Proteobacteriota (Gram-negative) and Firmicutes (Gram-positive) exhibit diverse resistance mechanisms that increase their chances of survival under unfavorable environmental conditions. Gram-negative bacteria often harbor genes encoding efflux proteins, which remove heavy metals from cells and reduce their toxic effects [23,24]. Bacteria of the genera Pseudomonas, Bacillus, and Azotobacter are particularly resistant to chromium. These microorganisms reduce toxic Cr(VI) to the more stable Cr(III) via reducing enzymes, which is an important part of their survival strategy. The bioremediation potential of these bacteria and their adaptation to metal-contaminated soils are determined by the presence of specific enzymes and regulator genes [25].
The fungi of the genera Aspergillus and Penicillium have also developed mechanisms for chromium tolerance, including the ability to bioaccumulate metals in hydrophobic structures. The incorporation of metal ions into cell wall components reduces their toxic effects in the environment. This process may be supported by extracellular polymers, such as chitin, which effectively bind heavy metals [26,27,28]. Chromium resistance mechanisms have also been studied at the molecular level. Plasmids containing genes that encode resistance to metals have been identified in bacterial genomes. Such examples include CHR genes encoding proteins involved in chromium detoxification and efflux pump genes such as P-type ATPase that actively remove chromium ions from cells [29,30]. Similar mechanisms have been identified in fungi, where genes encoding enzymes that reduce Cr(VI) to Cr(III), such as dehydrogenases and oxidases, play a key role in microbial adaptation to contaminated environments. The expression of these genes is regulated by both environmental factors and internal cell mechanisms, ensuring a flexible microbial response to fluctuating conditions [31,32].
Research on microbial resistance to chromium plays a particularly important role in the bioremediation of heavy metal-contaminated soils [33]. These mechanisms should be further investigated and harnessed to design effective environmental decontamination strategies. The use of microorganisms capable of bioreducing Cr(VI) could offer an effective and environmentally friendly approach to developing new technologies for mitigating metal toxicity in ecosystems [34,35].
In light of these considerations, the present experiment was conducted to test two research hypotheses: (i) soil contamination with Cr(VI) leads to a greater decline in microbial biodiversity than Cr(III) contamination, and (ii) both chemical forms of chromium (Cr(III) and Cr(VI)) modify the composition of soil bacterial and fungal communities, increasing the relative abundance of microorganisms that are tolerant to this metal.
Despite numerous studies [36,37,38] investigating the impact of chromium on soil microorganisms, a comprehensive metagenomic analysis (NGS) comparing the selection mechanisms of Cr(III) and Cr(VI) across entire bacterial and fungal communities is still lacking. This limits the potential for precise use of microorganisms in bioremediation, as developing optimal strategies requires consideration of the specific responses of microorganisms to each form of chromium. The scientific value of this work lies in the development of a thorough taxonomic characterization of bacteria and fungi that can act as bioindicators of chromium tolerance, as well as potential biological tools to support the remediation of soils contaminated with Cr(III) and Cr(VI) ions. Based on the above, two research hypotheses were formulated: (i) soil contamination with Cr(VI) ions results in a greater reduction in microbial biodiversity than exposure to Cr(III); and (ii) the presence of both chromium forms causes significant changes in soil bacterial and fungal community composition, leading to an increased proportion of metal-resistant microorganisms. This study aimed to compare the selective toxicity of Cr(III) and Cr(VI) in relation to the functional structures of microbial communities using NGS analysis. The analysis sought to identify differences in the functional structures of the soil microbiome and to indicate potential taxa key to developing sustainable bioremediation strategies for chromium-contaminated soils.

2. Materials and Methods

2.1. Sample Preparation

Soil samples for the study were collected from the surface layer (0–20 cm) in Tomaszkowo (53.7161° N, 20.4167° E), Warmian-Masurian Voivodeship in Poland. The collected material was homogenized by grinding and air drying, after which it was passed through a sieve with 0.5 cm mesh size to remove macroscopic impurities and obtain samples with a homogeneous particle size composition. The decisive criterion for selecting the soil for testing was its representativeness of the climatic and soil conditions in Poland. Poland is located in the Central European subboreal zone, which is characterised by a temperate transitional climate with distinct oceanic influences. Under these conditions, zonal soils, which usually form on clays and clayey sands, dominate, reflecting the typical textural and geological conditions of much of Poland’s land. Therefore, selecting these soils ensures that the results obtained regarding the impact of chromium are highly valuable for national land management and environmental strategies.
The examined soil was classified as Eutric Cambisol according to the World Reference Base for Soil Resources (WRB) [39]. The soil material was collected from the humus horizon developed from sandy loam. The granulometric composition and the basic physicochemical properties of soil were determined before the experiment (Table 1). The physicochemical properties and granulometric composition of soil were determined according to the methods described previously by Wyszkowska et al. [40].

2.2. Experimental Design

The experiment was conducted under controlled greenhouse conditions using plastic pots with a capacity of 3.5 kg. The experimental design involved four treatments with four replicates each to ensure the statistical reliability of the results. Each pot was filled with 3.4 kg of the prepared soil. Soil was contaminated with chromium (Cr) in two oxidation states: Cr(III) as KCr(SO4)2·12H2O and Cr(VI) as K2Cr2O7. Both chemical forms of chromium were applied at 160 mg Cr kg−1 soil. Samples of uncontaminated soil served as control treatments. Chromium was selected as the stressor because it is highly toxic to the environment [41,42] and has been classified as one of the six most dangerous environmental pollutants to human health [43]. The choice of a chromium dose of 160 mg Cr per kg of soil is determined by its position relative to the critical pollution threshold in Poland’s regulatory framework. According to the 2016 Regulation of the Minister of the Environment (for example, for groups B and C soils) [44], chromium concentrations are often regulated at levels exceeding 100 mg kg−1. The selected dose of 160 mg kg−1 intentionally exceeds concentrations typically found in background soils and lower intervention thresholds. This simulates conditions of severe, yet realistic, industrial pollution (for example, in the vicinity of tanneries or steel mills) [6,7]. This dose allows observation of the toxic and selective effects (that is, changes in biodiversity) of both forms of chromium, which is essential for achieving the objectives of our comparative ecotoxicological study.
Soil was fertilized with the following rates of macronutrients (mg kg−1 soil) to ensure the optimal growth and development of oat (Avena sativa L.) plants:
  • nitrogen (N)—140 mg (CO(NH2)2),
  • phosphorus (P)—60 mg (KH2PO4),
  • potassium (K)—120 mg (KH2PO4 + KCl),
  • magnesium (Mg)—20 mg (MgSO4·7H2O).
All components, including chromium compounds and fertilizers, were thoroughly combined with soil before pot filling. Soil moisture was adjusted to 50% of capillary water capacity. The average temperature during the experiment was 16.4 °C, the average air humidity was 77.3%, and the average day length was 14 h and 5 min.
Avena sativa L. cv. Bingo was chosen as the test plant due to its potential use in the remediation of soils contaminated with heavy metals. Avena sativa L. was chosen as the test plant due to its potential for managing contaminated soils. The main criterion was the documented ability of oats to tolerate and accumulate heavy metals [45], making this species suitable for use in phytoremediation of chromium-contaminated soils. This choice enables the assessment of the microbiota’s response to chromium-induced stress. In addition, A. sativa has significant utility beyond standard feed applications. Due to its high caloric content, oat grain is used for energy purposes, particularly in Scandinavian countries such as Sweden [46]. It has also been reported as a raw material for producing biodegradable materials (e.g., plastics) and cat litter [47]. This versatility highlights the importance of this plant in the context of sustainable biomass use within a circular economy. Twelve seeds were sown per pot, and plants were harvested at the BBCH 61 growth stage (beginning of flowering).

2.3. Determination of the Microbial Colony Development Index and Ecophysiological Diversity Index

The colony development (CD) index [48] and the ecophysiological diversity (EP) index [49] were calculated based on the abundance of organotrophic bacteria, actinomycetes, and fungi. Microbiological analysis of soil was performed by the serial dilution method. Soil samples of 10 g each (fresh matter basis) were suspended in 90 cm3 of sterile saline solution (0.85% NaCl) and shaken for 30 min. The resulting suspension was used to prepare a dilution series for organotrophic bacteria and actinomycetes (10−5 and 10−6) and for fungi (10−3 and 10−4). A 1 cm3 aliquot of the suspension from each dilution was spread onto sterile Petri plates and overlaid with the respective culture medium, while gently stirring to ensure even distribution of the inoculum. The cultures were prepared in four replicates. Various culture media were used to isolate the examined microbial groups: Bunt and Rovira medium with soil extract for organotrophic bacteria, Küster and Williams medium supplemented with antibiotics (nystatin and Actidione) for actinomycetes, and Martin’s medium with the addition of Rose Bengal agar and Aureomycin for fungi. The plates were incubated at a temperature of 28 °C, and colonies were counted daily for 10 days to record their increase. The CD and EP indices were calculated from the results and used to evaluate the functional structure of microorganisms in soil exposed to Cr(III) and Cr(VI). Higher values of the CD index indicate a predominance of fast-growing microorganisms, which are most prevalent in the early stages of incubation. In turn, high values of the EP index point to an even rate of colony development and greater diversity of microbial ecological strategies [50,51].

2.4. Genomic DNA Extraction and Sequence Analysis

Genomic DNA was isolated from 1 g of uncontaminated soil, soil contaminated with Cr(III), and soil contaminated with Cr(VI) using the Genomic Mini AX Bacteria+ Kit (A&A Biotechnology, Gdańsk, Poland). Genomic DNA was extracted by mechanical lysis of cells using the FastPrep-24 instrument (MP Biomedicals, Santa Ana, CA, USA), and DNA was purified using the Anti-Inhibitor Kit (A&A Biotechnology, Gdańsk, Poland). The presence of DNA in the analyzed samples was validated by Real-Time PCR using a CFX Connect thermocycler (Bio-Rad, Twinsburg, OH, USA) and SYBR Green dye. Bacterial amplicons were sequenced targeting the V3–V4 hypervariable region of the 16S rRNA gene using primers 341F and 785R. Fungal amplicons were sequenced targeting the ITS1 region using primers ITS1FI2 and 5.8S. Sequencing was performed on the Illumina MiSeq platform (Illumina, Inc., San Diego, CA, USA) in 2 × 300 bp mode, using MiSeq Reporter (MSR) v2.6 software. The quality of the obtained sequences was controlled, and incomplete or chimeric sequences were removed. The taxonomic identification of microorganisms was performed with the Quantitative Insights Into Microbial Ecology (QIIME) package based on the GreenGenes v13_8 reference database for bacteria and the UNITE v8.2 database for fungi. The analytical procedure was described in detail by Borowik et al. [52]. Sequencing data were deposited in the GenBank (NCBI) database under the following accession numbers: (PX387203:PX387520[accn]) for bacteria and (PX363704:PX365542[accn]) for fungi. Metagenomic data were used to calculate the Shannon-Wiener diversity index (H’) and Simpson’s index (D), separately for bacteria and fungi [53,54,55].

2.5. Statistical Analysis

The results were processed statistically using Statistica 13.3 software [56], TBtools-II v2.310 [57], and RSplot [58]. Tukey’s HSD test was used to identify homogeneous groups at a significance level of p ≤ 0.05. Metagenomic data were presented after removing sequences with low abundance, accounting for less than 1% of total operational taxonomic units (OTUs). The effect of Cr(III) and Cr(VI) on the taxonomic composition of bacteria and fungi at the phylum level was determined by the principal component analysis (PCA, a multivariate exploration technique), using the Statistica 13.3 package [56]. Hierarchical cluster analysis (HCA) was performed to explore the similarities and differences in the responses of bacterial and fungal genera to Cr(III) and Cr(VI) ions. A dendrogram was constructed using Ward’s method and Euclidean distances to group microbial taxa based on the similarity in their responses to soil contamination with chromium.

3. Results

3.1. Effect of Cr(III) and Cr(VI) on the Structure of Bacterial Communities in Soil

The values of the CD index for organotrophic bacteria and actinomycetes were higher in soil contaminated with Cr(III) and Cr(VI) than in the control sample (Figure 1). Organotrophic bacteria exhibited significantly higher CD values than actinomycetes, indicating that the former proliferated more rapidly. The values of the EP index varied depending on the presence of chromium. The EP index of bacteria increased after soil contamination with Cr(III) and decreased under exposure to Cr(VI). The lowest EP values were noted in organotrophic bacteria in soil contaminated with Cr(VI) and in actinomycetes in soil contaminated with Cr(III). The highest EP values were found in actinomycete populations colonizing soil contaminated with Cr(VI), which suggests that these microorganisms could deploy more diverse metabolic strategies under strong environmental stress.
In all analyzed samples, Actinobacteriota was the dominant bacterial phylum with OTU counts ranging from 80,338 (control soil, C) to 109,257 (soil with Cr(III)) (Figure 2). Proteobacteriota were also abundant (20,947–36,903 OTUs) (Figure 2). The PCA revealed clear differences in the taxonomic composition of bacteria in soil samples treated with Cr(III) and Cr(VI). The first two principal components (PC1 and PC2) explained 97.91% of total variance, with PC1 alone accounting for 92.52%. PC1 was negatively correlated with soil samples C, Cr(III), and Cr(VI), whereas PC2 was negatively correlated with soil samples Cr(III) and Cr(VI). In soil contaminated with Cr(III) and Cr(VI), a decrease in the relative abundance of Acidobacteriota, Proteobacteriota, Chloroflexi, Gemmatimonadota, and Planctomycetota was accompanied by an increase in the abundance of Actinobacteriota relative to the control soil. Cr(III) inhibited the growth of Firmicutes and stimulated the proliferation of Bacteroidota, whereas the opposite effect was noted in soil contaminated with Cr(VI). These findings indicate that Cr(VI) exerted a much stronger effect on the structure of bacterial communities at the phylum level than Cr(III).
All soil samples (Figure 3) were characterized by a predominance of bacteria belonging to the class Actinobacteria, whose relative abundance ranged from 51.11% to 79.21%. Alphaproteobacteria were also relatively abundant (10.85–20.28%). Soil contamination with Cr(III) and Cr(VI) significantly decreased the relative abundance of Acidobacteria-6, Acidimicrobia, Gemmatimonadetes, Alphaproteobacteria, and Gammaproteobacteria compared to the control soil, but Cr(VI) exerted more inhibitory effects than Cr(III). Moreover, Cr(III) decreased the relative abundance of bacteria of the class Bacilli, whereas Cr(VI) reduced the abundance of Thermoleophilia and Betaproteobacteria. Both Cr(III) and Cr(VI) stimulated the proliferation of Actinobacteria in soil, and Cr(VI) additionally promoted the growth of bacteria of the class Bacilli.
Soil contamination with Cr(III) and Cr(VI) induced changes in the structure of bacterial communities at the class level (Figure 4). A comparison of treatments C and Cr(III) and treatments C and Cr(VI) revealed the greatest differences in bacteria of the class Actinobacteria, whose abundance in the control soil was lower by 30,123 OTUs relative to treatment Cr(III) and by 42,148 OTUs relative to treatment Cr(VI). Treatments Cr(III) and Cr(VI) differed most significantly in the abundance of Thermoleophilia, which was higher by 12,286 OTUs in soil contaminated with Cr(III). Considerable variations were also noted in the abundance of the classes Actinobacteria and Betaproteobacteria.
Chromium contamination of soil samples modified the structure of bacterial communities at the genus level, with Cr(VI) inducing more profound changes than Cr(III) (Figure 5). Bacteria of the genus Cellulosimicrobium were most prevalent in all samples. In soil contaminated with Cr(III) and Cr(VI), the abundance of Cellulosimicrobium was 27.36% and 39.55% higher, respectively, relative to the control soil. Cr(III) stimulated the proliferation of bacteria of the genera Phycicoccus, Arthrobacter, Nocardioides, and Burkholderia, whereas Cr(VI) significantly inhibited their growth. The relative abundance of Rhodoplanes and Kaistobacter decreased under exposure to Cr(III) and Cr(VI). In addition, Cr(III) decreased the relative abundance of Mycoplana (4.17-fold relative to uncontaminated soil), whereas Cr(VI) increased their abundance 11.89-fold.
Two main clusters of bacterial taxa were identified in the presented dendrogram (Figure 6). The first cluster consisted of the bacterial genera Phycicoccus, Arthrobacter, and Kaistobacter identified in control and Cr(III)-treated soil, Burkholderia in Cr(III)-treated soil, and Cellulosimicrobium in control, Cr(III)-, and Cr(VI)-treated soil. The second cluster was composed of bacterial genera Mycoplana, Rhodoplanes, Aeromicrobium, and Nocardioides that were identified in all samples (C, Cr(III), Cr(VI)), Phycicoccus and Arthrobacter in Cr(VI)-contaminated soil, Burkholderia in control and Cr(VI)-contaminated soil, and Kaistobacter in Cr(VI)-contaminated soil. The dendrogram also indicates that Cr(III) and Cr(VI) exerted different effects on the identified bacterial genera and divided the studied taxa into clear groups. Some taxa exhibited similar tolerance mechanisms, whereas others responded differently to the analyzed contaminants.
Both Cr(III) and Cr(VI) induced significant changes in bacterial diversity, as evidenced by the values of Shannon-Wiener and Simpson’s indices (Figure 7). Cr(III) decreased the Shannon-Wiener index by 2.92% (at the genus level) to 24.55% (at the order level), whereas Cr(VI) decreased this parameter by 31.14% (at the phylum level) to 42.13% (at the class level). The analyzed contaminants had a somewhat different influence on Simpson’s index. Cr(III) decreased the values of Simpson’s index at the phylum (by 17.08%), order (16.20%), and genus (by 2.82%) level, but increased this parameter at the class (by 1.87%) and family (by 4.87%) level. Cr(VI) decreased Simpson’s index by 25.32% (at the genus level) to 36.81% (at the order level), but no significant changes were found at the class level.

3.2. Effect of Cr(III) and Cr(VI) on the Structure of Soil Fungal Communities

An analysis of the fungal CD index revealed that exposure to Cr(III) and Cr(VI) elicited similar effects in the studied populations (Figure 8). In soil samples contaminated with Cr(III) and Cr(VI), the values of the fungal CD index were comparable and significantly higher than in the control soil, suggesting that both Cr(III) and Cr(VI) stimulated the development of fungal colonies. The EP index followed a similar trend. In chromium-contaminated samples, EP values were similar and significantly lower than in the control soil, which indicates that contaminants exert a negative influence on the ecophysiological diversity of fungi.
The structure of fungal communities (Figure 9) was analyzed using PCA to assess changes in microbial composition and diversity in response to soil contamination with Cr(III) and Cr(VI). PC1 explained the largest proportion of variance (72.85%) and was positively correlated with treatments C (control soil), Cr(III), and Cr(VI). PC2 was associated exclusively with the soil sample contaminated with C (control soil) and Cr(III), which suggests that trivalent chromium exerts a specific influence on the structure of fungal communities. Ascomycota was the dominant fungal phylum (51,071–100,114 OTUs). In comparison with the control soil, the relative abundance of Ascomycota was significantly higher in treatment Cr(III) and lower in treatment Cr(VI). Both Cr(III) and Cr(VI) stimulated the growth of Basidiomycota, and Cr(III) additionally enhanced the proliferation of Mucoromycota. Fungi of the phylum Rozellomycota were also identified in Cr(VI)-contaminated soil.
Fungi of the class Sordariomycetes were predominant in the control soil and Cr(VI)-contaminated soil, accounting for 72.64% and 76.53% of fungal OTUs, respectively, whereas Eurotiomycetes were most prevalent in Cr(III)-contaminated soil (66.75%) (Figure 10). Cr(III) significantly decreased the relative abundance of fungi of the classes Dothideomycetes, Sordariomycetes, and Mortierellomycetes, and increased the abundance of Mucoromycetes relative to the control soil. In turn, Cr(VI) decreased the relative abundance of Eurotiomycetes, Mortierellomycetes, and Mucoromycetes, but stimulated the growth of Dothideomycetes and Sordariomycetes.
The greatest differences between the proportions of fungal classes in treatments C and Cr(III) were noted for Sordariomycetes, whose abundance in control soil was higher by 54,390 OTUs (Figure 11). These treatments also differed considerably in the relative abundance of Eurotiomycetes, which was higher by 43,832 OTUs in Cr(III)-contaminated soil than in the control soil. The greatest differences between treatments C and Cr(VI) and between treatments Cr(III) and Cr(VI) were observed in the relative abundance of Eurotiomycetes, which was higher by 35,021 OTUs in the control soil than in treatment Cr(VI) and by 53,323 OTUs in treatment Cr(III) than in treatment Cr(VI).
In the group of the identified fungal genera (Figure 12), Chaetomium (41,297 OTUs) was dominant in the control soil, Penicillium (59,231 OTUs) was most prevalent in Cr(III)-contaminated soil, whereas Trichoderma (24,652 OTUs) was predominant in Cr(VI)-contaminated soil. Soil contamination with Cr(III) and Cr(VI) decreased the relative abundance of the fungal genera Verticillium, Monacrosporium, Mortierella, and Fusarium. Cr(III) increased the number of OTUs associated with the genera Exophiala and Mucor, while decreasing the relative abundance of Trichoderma and Dichotomopilus. In turn, Cr(VI) significantly reduced the relative abundance of the genera Exophiala, Penicillium, and Mucor, but increased the proportions of Trichoderma and Dichotomopilus.
The dendrogram generated by HCA supported the identification of four main groups of soil samples characterized by different responses of fungi to the presence of chromium (Figure 13). The first cluster consisted of fungi of the following genera: Exophiala identified in treatments C, Cr(III), and Cr(VI), Dichotomopilus, Monocillium, and Mortierella in soil contaminated with Cr(III) and Cr(VI), Trichoderma in the control soil and soil contaminated with Cr(III), Fusarium and Verticillium identified in Cr(III)-contaminated soil, and Chaetomium identified in Cr(VI)-contaminated soil. The second cluster was composed of fungi of the following genera: Penicillium identified in treatment Cr(VI), Chaetomium identified in treatment Cr(III), Monocillium and Mortierella identified in the control soil, Mucor identified in treatments C and Cr(III), and Fusarium identified in treatments C and Cr(VI). The third cluster comprised fungi of the genera Penicillium and Verticillium identified in the control soil and Trichoderma in Cr(VI)-contaminated soil. The fourth cluster consisted of the fungal genus Chaetomium in treatment C and Penicillium in treatment Cr(III). This division suggests that Cr(VI) exerts a stronger influence on the structure of fungal communities at the genus level than Cr(III).
Soil contamination with Cr(III) and Cr(VI) had a varied effect on fungal biodiversity (Figure 14). Cr(III) decreased the values of the Shannon-Wiener index at all taxonomic levels, from 2.22% at the phylum level to 31.71% at the genus level. The influence of Cr(VI) was less unequivocal. This contaminant increased the Shannon-Wiener index by 1.68% (at the family level) to 8.89% (at the phylum level), and the only decrease in this parameter relative to the control soil was noted at the order level (by 5.45%). In soil exposed to Cr(III), the value of Simpson’s index decreased at the order (by 22.45%), family (by 24.02%), and genus (by 25.18%) level, but increased at the phylum level (by 5.86%). In turn, Cr(VI) increased Simpson’s index at the phylum level (by 9.77%) relative to the control soil, but decreased its value at the remaining taxonomic levels, from 0.54% (class) to 9.84% (order).

4. Discussion

4.1. Structure of Soil Bacterial Communities Under Exposure to Cr(III) and Cr(VI)

The results of the experiment clearly indicate that soil contamination with chromium, both Cr(III) and Cr(VI), leads to significant changes in the structure and functions of soil bacterial communities. The observed decrease in the values of Shannon-Wiener and Simpson’s diversity indices in chromium-contaminated samples confirms the hypothesis that heavy metals reduce species richness and the evenness of species distribution. These changes were particularly pronounced in Cr(VI)-contaminated soil, which is consistent with previous reports showing that Cr(VI) is more toxic than Cr(III) [17,59,60].
Diverse microbial responses to the presence of chromium were also reflected in the values of the bacterial CD index and EP index. An increase in the CD index of organotrophic bacteria and actinomycetes in chromium-contaminated soil points to a dominance of fast-growing microorganisms that rapidly colonize stressed environments. At the same time, the observed changes in the values of the EP index confirm that Cr(III) and Cr(VI) trigger different microbial adaptation strategies. The high EP values of actinomycetes in Cr(VI)-contaminated soil could suggest that these microorganisms adapt highly effectively to extreme environmental conditions, which supports previous observations of their heavy metal resistance [30,31].
The study demonstrated that the presence of chromium ions in soil significantly affected the structure of bacterial communities, but the observed changes were clearly influenced by the oxidation state of chromium. Cr(VI) proved to be a stronger selective factor than Cr(III), leading to a significant decrease in the diversity and dominance of several taxa that are tolerant to high concentrations of this element. The analyzed soils were characterized by a predominance of bacteria of the phyla Actinobacteriota and Proteobacteriota, but the relative abundance of the latter was slightly lower. Similar trends were reported by Wyszkowska et al. [61] who studied the effect of Cr(VI) applied at 60 mg kg−1 soil DM on the structure of bacterial communities. The addition of Cr(III) and Cr(VI) to soil exerted inhibitory effects on Acidobacteriota, Proteobacteriota, Chloroflexi, Gemmatimonadota, and Planctomycetota. In addition, Cr(III) was toxic to Firmicutes, and Cr(VI)—to Bacteroidota. Wei et al. [62] also observed a decrease in the abundance of dominant bacterial phyla, including Proteobacteriota, Actinobacteriota, Acidobacteriota, and Chloroflexi, in soil contaminated with Cr(VI) at 200 mg kg−1 soil DM.
The mechanism of action of Cr(VI) involves easy penetration into cells and strong oxidizing properties, which cause damage to DNA, proteins, and cell membranes, disrupt metabolic pathways, and alter gene expression [63,64,65,66,67,68]. Despite its potentially toxic effect, Cr(III) is far less mobile, weakly interacts with cells, and can act as a microelement in some cases. These observations explain the less extensive changes in the structure of bacterial communities exposed to Cr(III) [69,70].
Bacterial taxa resistant to chromium were also identified in the analyzed samples. The relative abundance of Actinobacteriota was higher in soil contaminated with Cr(III) and Cr(VI). Firmicutes predominated in soil treated with Cr(III), whereas Bacteroidota were most prevalent in samples containing Cr(VI). The survival of these bacteria under chromium-induced stress may result from their ability to adjust nutrient utilization under extreme environmental conditions, enabling survival through enhanced secondary metabolism [23,24], or from limiting growth by actively regulating energy supply forms and the use of C, N, and P under adverse conditions [24,62]. The observed changes could also be explained by differences in the mechanisms of action of Cr(III) and Cr(VI), as well as the adaptive capabilities of bacteria. Some microorganisms can chelate metals, which enables them to bind chromium ions, thus reducing their toxicity and, in some cases, to transport these ions into cells [71]. The results of this study indicate that Cr(III) and Cr(VI) promoted the growth of bacteria of the genus Cellulosimicrobium in soil, as evidenced by the higher relative abundance of these microorganisms in chromium-contaminated samples relative to the control soil. The above can likely be attributed to the ability of Cellulosimicrobium bacteria to detoxify chromium through extracellular and intracellular reduction mechanisms [71]. Bacteria of the Cellulosimicrobium genus exhibit multifaceted chromium tolerance mechanisms, including biosorption and bioaccumulation of the metal by cell wall components and exopolysaccharides. They also undergo enzymatic reduction of Cr(VI) to the less toxic Cr(III), with the participation of chromate reductases. In addition, they utilize efflux systems, such as the chrA gene. The activation of antioxidant mechanisms, such as superoxide dismutase, also mitigates the oxidative stress caused by Cr(VI). Studies indicate that Cellulosimicrobium isolates can effectively reduce and absorb chromium, making them potential candidates for further research on heavy metal bioremediation [72,73,74]. Other researchers [61,62,66] also found that many bacterial genera, including Arthrobacter, Pseudomonas, Bacillus, Serratia, Agrobacterium, Lysinibacillus, Ochrobactrum, and Geobacter, are highly resistant to Cr(VI) and are able to effectively reduce hexavalent chromium to the less toxic Cr(III). The level of resistance is determined by, among others, the effectiveness of intracellular reduction mechanisms and the presence of Cr(VI) efflux systems in cells [64,75].
According to Zha et al. [76], the adsorption of Cr(VI) by extracellular polysaccharides, enzymes, and other bacterial secretions, followed by its reduction and precipitation in metabolic processes play an important role in the detoxification process. Bacteria also form polysaccharide slime layers that readily present amino, carboxyl, phosphate, and sulfate groups for binding metal ions. In addition, bacterial tolerance to Cr(VI) is associated with the presence of numerous resistance genes, mechanisms for the detoxification of reactive oxygen species (ROS), and DNA repair mechanisms [62]. In the present study, the growth of bacteria of the genera Phycicoccus, Arthrobacter, Nocardioides, and Burkholderia was inhibited by Cr(VI) but stimulated by Cr(III). Both Cr(III) and Cr(VI) inhibited the development of Rhodoplanes and Kaistobacter. In turn, bacteria of the genus Mycoplana were highly resistant to Cr(VI), as evidenced by the significant increase in their relative abundance in soil contaminated with hexavalent chromium.
Changes in the structure of the soil microbiome can have profound environmental consequences. A decline in biodiversity can decrease the resilience and stability of an entire ecosystem, particularly by inhibiting key processes such as the nitrogen cycle and organic matter decomposition. However, the selection and proliferation of bacteria tolerant to high chromium concentrations, including strains capable of reducing Cr(VI) to the less toxic Cr(III), indicate that these microorganisms have potential for the bioremediation of soils contaminated with heavy metals [69,77,78].
Both Cr(III) and Cr(VI) caused significant changes in soil bacterial diversity, as confirmed by the Shannon-Wiener and Simpson indices. A decrease in the Shannon-Wiener index indicates reduced taxon richness and evenness, potentially leading to decreased stability and resilience of the microbiome [79]. For Cr(III), a decrease in the Shannon-Wiener index was observed at the order level, while Cr(VI) had a stronger effect at the phylum and class levels. This confirms that Cr(VI) is significantly more toxic to the microbiome than Cr(III) [14]. The impact of pollutants on the Simpson index varies slightly depending on the taxonomic level. Cr(III) reduced Simpson values at the phylum, order and genus levels but increased them at the class and family levels. This may indicate partial stabilisation of dominant taxa in the face of reduced diversity. Conversely, Cr(VI) decreased Simpson’s index at the genus and order levels, with no significant changes at the class level. This suggests that certain bacterial classes are relatively resistant to the toxic effects of Cr(VI). This can be explained by the direct toxic effect of chromium on bacterial cells, including DNA damage, metabolic disruption, and oxidative stress [80]. The varying sensitivity of bacteria depending on taxonomic level reflects differences in tolerance and metal detoxification mechanisms, such as the activity of Cr(VI)-reducing enzymes or the presence of efflux systems [81,82]. These results emphasize that exposure to chromium, particularly hexavalent chromium (Cr(VI)), causes significant disturbances to the diversity of the bacterial microbiome. This can limit the functioning of soil ecosystems, including nutrient cycling and resistance to environmental stress [83].

4.2. Structure of Soil Fungal Communities Under Exposure to Cr(III) and Cr(VI)

The present study clearly demonstrated that Cr(III) and Cr(VI) affect the structure of soil fungal communities, which confirms the hypothesis that the analyzed forms of chromium exert different effects on biodiversity. The type of contaminant, in particular its specific chemical and biochemical properties, was the main factor responsible for the observed differences between soil samples. The analysis of the taxonomic composition of fungi at the phylum and class level provides key insights about fungal adaptation to contaminated environments. The relative abundance of Ascomycota increased in soil contaminated with Cr(III) and decreased in soil contaminated with Cr(VI). Ascomycota are considered the dominant fungal phylum in heavy metal-contaminated soils due to their ability to produce melanin and other chelating compounds that can bind metals [84].
However, the observed differences at the class and genus level point to the presence of more subtle adaptive mechanisms. Fungi of the class Eurotiomycetes were dominant in soil contaminated with Cr(III), whereas fungi of the class Sordariomycetes were most prevalent in soil contaminated with Cr(VI). This taxonomic shift indicates that Cr(III) and Cr(VI) exert different effects on various groups of fungi. An analysis of fungal genera produced more detailed observations. The genus Trichoderma was predominant in soil contaminated with Cr(VI), whereas Penicillium was the most prevalent fungal genus in soil contaminated with Cr(III). These genera are resistant to heavy metals and are used in soil bioremediation [31,32]. Trichoderma species possess mechanisms for reducing Cr(VI) to the less toxic Cr(III), whereas Penicillium species are effective in the biosorption and bioaccumulation of these metals [85]. The differences in the dominance patterns of these fungal genera suggest that they have various mechanisms of resistance to Cr(III) and Cr(VI), which leads to the selection of different fungal groups [69]. Fungi of the genus Trichoderma are known for their versatile enzymatic properties and high tolerance to heavy metals. They have been shown to be effective in metal solubilisation and mineral biooxidation processes, particularly the species T. harzianum and T. reesei. These fungi produce various organic acids that facilitate the removal of toxic metal ions from cells and reduce Cr(VI) to the less toxic form, Cr(III), with the help of enzymes such as chromate reductases. This mechanism relies on the reducing power generated during carbon metabolism, making it an important strategy for Cr(VI) detoxification. Due to these properties, Trichoderma spp. can survive in metal-contaminated environments and have potential applications in bioremediation [29,86].
The present findings clearly indicate that Cr(VI) and Cr(III) differ in their toxicity to soil microorganisms and trigger various responses in the structure of fungal communities. While Cr(III) appears to exert a general inhibitory effect on biodiversity, Cr(VI) is a strong selection factor that disrupts the taxonomic structure of soil mycobiota. The changes observed at the genus level, particularly the increase in the relative abundance of Trichoderma in Cr(VI)-contaminated soil, have significant implications for the phytoremediation and bioremediation of polluted areas. The above suggests that the ability of this fungal genus to biotransform Cr(VI) could play a key role in remediation strategies.
The observed changes in diversity indices, including Shannon-Wiener and Simpson’s indices, provide further evidence for this differentiation. Exposure to Cr(III) disrupted fungal diversity at all taxonomic levels (in particular at the genus level, where the value of the Shannon-Wiener index decreased by 31.71%), indicating that trivalent chromium had a strong overall inhibitory effect on fungal biodiversity. Although Cr(III) is less mobile than Cr(VI), its high concentrations can be toxic to a wide range of organisms. In contrast, exposure to Cr(VI) increased the value of the Shannon-Wiener index, which may suggest that hexavalent chromium exerts more selective effects. Instead of decreasing overall biodiversity, Cr(VI) eliminates sensitive taxa and stimulates the growth of resistant ones, potentially leading to increased biodiversity in the newly adapted community [17].
In summary, differences in the chemical properties of Cr(III) and Cr(VI) can give rise to distinct toxicity mechanisms that select for unique fungal groups, which has fundamental implications for understanding the ecotoxicology of heavy metals in the soil environment.

5. Conclusions

This study, based on high-resolution NGS sequencing analysis, provides pioneering evidence of the different taxon-specific selection exerted by Cr(III) and Cr(VI) ions on soil bacterial and fungal communities. NGS sequencing revealed that Cr(III) and Cr(VI) selectively interact with different groups of microorganisms, resulting in varied alterations to the composition of soil bacteria and fungi. Cr(VI) was found to be more toxic to soil bacteria than Cr(III), as confirmed by reduced diversity indices—particularly the Shannon–Wiener index—in soil samples contaminated with Cr(VI). In the case of soil fungi, however, the presence of Cr(VI) did not cause significant changes in the values of this index, indicating the selective nature of the impact while maintaining overall diversity. Taxonomic analysis revealed an increase in the relative abundance of Actinobacteriota bacteria and Ascomycota fungi in chromium-contaminated soils, alongside a decrease in sensitive bacterial groups such as Acidobacteriota and Proteobacteriota. Cr(III) inhibited the growth of fungi of the genera Verticillium and Fusarium, but promoted the proliferation of Penicillium and bacteria of the genera Phycicoccus and Arthrobacter. In turn, Cr(VI) stimulated the growth of Trichoderma fungi and bacteria of the genera Mycoplana and Cellulosimicrobium, while suppressing the proliferation of other genera. The observed differences in the resistance mechanisms of microorganisms, such as the selection of Penicillium and Arthrobacter in the presence of Cr(III), and Trichoderma and Cellulosimicrobium in the presence of Cr(VI), are of significant practical importance. The dominance of these taxa suggests that they are highly tolerant of chromium, potentially allowing these microorganisms to be used in biopreparations. The obtained profile of indicator microorganisms provides a solid basis for designing effective, sustainable soil remediation strategies adapted to the dominant form of chromium, which is important from an environmental safety perspective. The increase in the relative abundance and dominance of chromium-resistant taxa, such as bacteria of the genus Cellulosimicrobium and fungi of the genera Trichoderma and Penicillium, indicates their tolerance to this metal and suggests their potential use in bioremediation strategies. The loss of soil microorganism biodiversity as a result of chromium exposure has significant consequences for the functioning of the entire ecosystem. Microorganisms play a key role in the proper functioning of soil ecosystems; therefore, a reduction in their diversity can lead to the simplification of trophic networks and the weakening of ecological functions. In the long term, this can result in reduced soil fertility and stability, as well as a limited ability to regenerate under environmental stress.

Author Contributions

Conceptualization, J.W., E.B.-L., M.B. and J.K.; methodology, J.W., E.B.-L., M.B. and J.K.; software, J.W., E.B.-L., M.B. and J.K.; validation, J.W., E.B.-L., M.B. and J.K.; formal analysis, J.W., E.B.-L., M.B. and J.K.; investigation, J.W.; resources, E.B.-L.; data curation, E.B.-L. and M.B.; writing—original draft preparation, J.W., E.B.-L., M.B. and J.K.; writing—review and editing, J.W., E.B.-L., M.B. and J.K.; visualization, E.B.-L., M.B.; supervision, J.W. and J.K.; project administration, J.W. and E.B.-L.; funding acquisition, J.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the University of Warmia and Mazury in Olsztyn, Faculty of Agriculture and Forestry, Department of Soil Science and Microbiology (grant No. 30.610.006-110) and was financially supported by the Minister of Science under the Regional Initiative of Excellence Program.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this paper are available upon request from the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

References

  1. López-Bucio, J.S.; Ravelo-Ortega, G.; López-Bucio, J. Chromium in plant growth and development: Toxicity, tolerance and hormesis. Environ. Pollut. 2022, 312, 120084. [Google Scholar] [CrossRef]
  2. Tumolo, M.; Ancona, V.; De Paola, D.; Losacco, D.; Campanale, C.; Massarelli, C.; Uricchio, V.F. Chromium pollution in european water, sources, health risk, and remediation strategies: An Overview. Int. J. Environ. Res. Public Health 2020, 17, 5438. [Google Scholar] [CrossRef]
  3. Fu, L.; Feng, A.; Xiao, J.; Wu, Q.; Ye, Q.; Peng, S. Remediation of soil contaminated with high levels of hexavalent chromium by combined chemical-microbial reduction and stabilization. J. Hazard. Mater. 2021, 404, 123847. [Google Scholar] [CrossRef]
  4. Zulfiqar, U.; Jiang, W.; Wang, X.; Hussain, S.; Ahmad, M.; Maqsood, M.F. Insights into the plant-microbe interaction, soil amendments and advanced genetic approaches for cadmium remediation: A review. insights into the plant-microbe interaction, soil amendments and advanced genetic approaches for cadmium remediation: A review. Front. Plant Sci. 2022, 13, 1183. [Google Scholar] [CrossRef]
  5. Wyszkowska, J.; Borowik, A.; Zaborowska, M.; Kucharski, J. Calorific value of Zea mays biomass derived from soil contaminated with chromium (VI). Energies 2023, 16, 3788. [Google Scholar] [CrossRef]
  6. Shi, J.; McGill, W.B.; Chen, N.; Rutherford, P.M.; Whitcombe, T.W.; Zhang, W. Formation and immobilization of Cr(VI) species in long-term tannery waste contaminated soils. Environ. Sci. Technol. 2020, 54, 7226–7235. [Google Scholar] [CrossRef] [PubMed]
  7. Oloruntoba, A.; Omoniyi, A.O.; Shittu, Z.A.; Ajala, O.R.; Kolawole, S.A. Heavy Metal Contamination in Soils, Water, and Food in Nigeria from 2000–2019: A Systematic Review on Methods, Pollution Level and Policy Implications. Water Air Soil Pollut. 2024, 235, 586. [Google Scholar] [CrossRef]
  8. Available online: https://www.statista.com (accessed on 10 September 2025).
  9. Available online: https://www.eea.europa.eu/publications/lrtap-1990-2022 (accessed on 10 September 2025).
  10. Wyszkowska, J.; Kucharski, J.; Jastrzębska, E.; Hłasko, A. The biological properties of the soil as influenced by chromium contamination. Polish J. Environ. Stud. 2001, 10, 37–42. [Google Scholar]
  11. Chen, F.; Ma, J.; Akhtar, S.; Khan, Z.I.; Ahmad, K.; Ashfaq, A.; Nawaz, H.; Nadeem, M. Assessment of chromium toxicity and potential health implications of agriculturally diversely irrigated food crops in the semi-arid regions of South Asia. Agric. Water Manag. 2022, 272, 107833. [Google Scholar] [CrossRef]
  12. Zhang, S.; Hao, X.; Tang, J.; Hu, J.; Deng, Y.; Xu, M.; Zhu, P.; Tao, J.; Liang, Y.; Yin, H.; et al. Assessing chromium contamination in red soil: Monitoring the migration of fractions and the change of related microorganisms. Int. J. Environ. Res. Public Health 2020, 17, 2835. [Google Scholar] [CrossRef]
  13. Singh, V.; Singh, J.; Mishra, V. Sorption kinetics of an eco-friendly and sustainable Cr(VI) ion scavenger in a batch reactor. J. Environ. Chem. Eng. 2021, 9, 105125. [Google Scholar] [CrossRef]
  14. Li, W.; Wang, T.; Chen, W.; Wen, Y.; Zhang, W.; Guo, B.; Yang, Y. Responses of bacterial communities along vertical soil profile to the chromium-contamination stress. Int. Biodeterior. Biodegrad. 2023, 179, 105584. [Google Scholar] [CrossRef]
  15. Wyszkowska, J. Soil contamination with chromium and its enzymatic activity and yielding. Polish J. Environ. Stud. 2002, 11, 79–84. [Google Scholar]
  16. Boros-Lajszner, E.; Wyszkowska, J.; Kucharski, J. Evaluation and assessment of trivalent and hexavalent chromium on Avena sativa and soil enzymes. Molecules 2023, 28, 4693. [Google Scholar] [CrossRef]
  17. Yu, H.; Huang, Q.; Men, J.; Wang, J.; Xiao, J.; Jin, D.; Deng, Y. Chromium contamination affects the fungal community and increases the complexity and stability of the network in long-term contaminated soils. Environ. Res. 2024, 262, 119946. [Google Scholar] [CrossRef] [PubMed]
  18. Ji, C.; Huang, J.; Zhang, X.; Yang, G.; Xing, S.; Fu, W.; Hao, Z.; Chen, B.; Zhang, X. Response of soil fungal community to chromium contamination in agricultural soils with different physicochemical properties. Sci. Total Environ. 2023, 879, 163244. [Google Scholar] [CrossRef] [PubMed]
  19. Manuel, D.-B.; Peter, B.R.; Chanda, T.; David, J.E.; Brajesh, K.S. Multiple elements of soil biodiversity drive ecosystem functions across biomes. Nat. Ecol. Evol. 2020, 4, 210–220. [Google Scholar] [CrossRef]
  20. Liu, Y.-R.; Delgado-Baquerizo, M.; Bi, L.; Zhu, J.; He, J.-Z. Consistent responses of soil microbial taxonomic and functional attributes to mercury pollution across China. Microbiome 2018, 6, 183. [Google Scholar] [CrossRef]
  21. Gang, H.; Xiao, C.; Xiao, Y.; Yan, W.; Bai, R.; Ding, R.; Yang, Z.; Zhao, F. Proteomic analysis of the reduction and resistance mechanisms of Shewanella oneidensis MR-1 under long-term hexavalent chromium stress. Environ. Int. 2019, 127, 94–102. [Google Scholar] [CrossRef]
  22. Rahman, Z.; Singh, V.P. Bioremediation of toxic heavy metals (THMs) contaminated sites: Concepts, applications and challenges. Environ. Sci. Pollut. Res. 2020, 27, 27563–27581. [Google Scholar] [CrossRef]
  23. Lopez, S.; Piutti, S.; Vallance, J.; Morel, J.-L.; Echevarria, G.; Benizri, E. Nickel drives bacterial community diversity in the rhizosphere of the hyperaccumulator Alyssum murale. Soil Biol. Biochem. 2017, 114, 121–130. [Google Scholar] [CrossRef]
  24. Eichorst, S.A.; Trojan, D.; Roux, S.; Herbold, C.; Rattei, T.; Woebken, D. Genomic insights into the Acidobacteria reveal strategies for their success in terrestrial environments. Environ. Microbiol. 2018, 20, 1041–1063. [Google Scholar] [CrossRef]
  25. Zhu, Y.; Yan, J.; Xia, L.; Zhang, X.; Luo, L. Mechanisms of Cr(VI) reduction by Bacillus sp. CRB-1, a novel Cr(VI)-reducing bacterium isolated from tannery activated sludge. Ecotoxicol. Environ. Saf. 2019, 186, 109792. [Google Scholar] [CrossRef]
  26. Khurshid, S.; Shoaib, A.; Javaid, A.; Abid, K. Bioaccumulation of chromium by Fusarium oxysporum. ScienceAsia 2016, 42, 92–98. [Google Scholar] [CrossRef]
  27. Bibi, S.; Hussain, A.; Hamayun, M.; Rahman, H.; Iqbal, A.; Shah, M.; Irshad, M.; Qasim, M.; Islam, B. Bioremediation of hexavalent chromium by endophytic fungi; safe and improved production of Lactuca sativa L. Chemosphere 2018, 211, 653–663. [Google Scholar] [CrossRef]
  28. Canonica, L.; Cecchi, G.; Capra, V.; Di Piazza, S.; Girelli, A.; Zappatore, S.; Zotti, M. Fungal arsenic tolerance and bioaccumulation: Local strains from polluted water vs. allochthonous strains. Environments 2024, 11, 23. [Google Scholar] [CrossRef]
  29. Viti, C.; Marchi, E.; Decorosi, F.; Giovannetti, L. Molecular mechanisms of Cr (VI) resistance in bacteria and fungi. FEMS Microbiol. Rev. 2014, 38, 633–659. [Google Scholar] [CrossRef]
  30. Rahman, Z.; Thomas, L.; Chetri, S.P.K.; Bodhankar, S.; Kumar, V.; Naidu, R. A comprehensive review on chromium (Cr) contamination and Cr(VI)-resistant extremophiles in diverse extreme environments. Environ. Sci. Pollut. Res. 2023, 30, 59163–59193. [Google Scholar] [CrossRef] [PubMed]
  31. Xu, X.J.; Xia, L.; Chen, W.L.; Huang, Q.Y. Detoxification of hexavalent chromate by growing Paecilomyces lilacinus XLA. Environ. Pollut. 2017, 225, 47–54. [Google Scholar] [CrossRef] [PubMed]
  32. Xu, H.; Hao, R.X.; Xu, X.Y.; Ding, Y.; Lu, A.H.; Li, Y.H. Removal of hexavalent chromium by Aspergillus niger through reduction and accumulation. Geomicrobiol. J. 2020, 38, 20–28. [Google Scholar] [CrossRef]
  33. Dubey, P.; Farooqui, A.; Patel, A.; Srivastava, P.K. Microbial innovations in chromium remediation: Mechanistic insights and diverse applications. World J. Microbiol. Biotechnol. 2024, 40, 151. [Google Scholar] [CrossRef] [PubMed]
  34. Yasir, M.W.; Siddique, M.B.A.; Shabbir, Z.; Ullah, H.; Riaz, L.; Nisa, W.U.; Shafeequr, R.; Shah, A.A. Biotreatment potential of co-contaminants hexavalent chromium and polychlorinated biphenyls in industrial wastewater: Individual and simultaneous prospects. Sci. Total Environ. 2021, 779, 146345. [Google Scholar] [CrossRef]
  35. Rohand, T.; Ben El Ayouchia, H.; Achtak, H.; Ghaleb, A.; Derin, Y.; Tutar, A.; Tanemura, K. Design, synthesis, DFT calculations, molecular docking and antimicrobial activities of novel cobalt, chromium metal complexes of heterocyclic moiety-based 1,3,4-oxadiazole derivatives. J. Biomol. Struct. Dyn. 2021, 40, 11837–11850. [Google Scholar] [CrossRef]
  36. Zulfiqar, U.; Haider, F.U.; Ahmad, M.; Hussain, S.; Maqsood, M.F.; Ishfaq, M.; Shahzad, B.; Waqas, M.M.; Ali, B.; Tayyab, M.N.; et al. Chromium toxicity, speciation, and remediation strategies in soil-plant interface: A critical review. Front. Plant Sci. 2023, 13, 1081624. [Google Scholar] [CrossRef]
  37. Sharma, P.; Singh, S.P.; Parakh, S.K.; Tong, Y.W. Health hazards of hexavalent chromium (Cr (VI)) and its microbial reduction. Bioengineered 2022, 13, 4923–4938. [Google Scholar] [CrossRef]
  38. Mushtaq, Z.; Liaquat, M.; Nazir, A.; Liaquat, R.; Iftikhai, H.; Anwar, W.; Itrat, N. Potential of plant growth promoting rhizobacteria to mitigate chromium contamination. Environ. Technol. Innov. 2022, 28, 102826. [Google Scholar] [CrossRef]
  39. Available online: https://zpe.gov.pl/a/soils-in-poland (accessed on 1 September 2025).
  40. Wyszkowska, J.; Boros-Lajszner, E.; Kucharski, J. The impact of soil contamination with lead on the biomass of maize intended for energy purposes, and the biochemical and physicochemical properties of the soil. Energies 2024, 17, 1156. [Google Scholar] [CrossRef]
  41. Fu, Z.; Guo, W.; Dang, Z.; Hu, Q.; Wu, F.; Feng, C.; Zhao, X.; Meng, W.; Xing, B.; Giesy, J.P. Refocusing on nonpriority toxic metals in the aquatic environment in China. Environ. Sci. Technol. 2017, 51, 3117–3118. [Google Scholar] [CrossRef] [PubMed]
  42. Ali, H.; Khan, E.; Ilahi, I. Environmental chemistry and ecotoxicology of hazardous heavy metals: Environmental persistence, toxicity, and tioaccumulation. J. Chem. 2019, 2019, 6730305. [Google Scholar] [CrossRef]
  43. World’s Worst Pollution Problems 2015. The New Top Six Toxic Threats: A Priority List for Remediation. Available online: http://www.worstpolluted.org/docs/WWPP_2015_Final.pdf (accessed on 21 November 2018).
  44. Regulation of the Minister of the Environment of 1 September 2016 Applicable in Poland (Journal of Laws 2016 item 1395). ISAP—Internet System of Legal Acts. Available online: http://https://isap.sejm.gov.pl/isap.nsf/DocDetails.xsp?id=wdu20160001395 (accessed on 11 September 2025).
  45. Abideen, S.N.U.; Abideen, A.A. Protein level and heavy metals (Pb, Cr, and Cd) concentrations in wheat (Triticum aestivum) and in oat (Avena sativa) plants. IJIAS 2013, 3, 284–289. [Google Scholar]
  46. Tobiasz-Salach, R.; Pyrek-Bajcar, E.; Bobrecka-Jamro, D. Assessing the possible use of hulled and naked oat grains as energy source. Econtechmod Int. Q. J. Econ. Technol. Model. Process. 2016, 5, 35–40. [Google Scholar]
  47. Proszak-Miąsik, D.; Jarecki, W.; Nowak, K. Selected parameters of oat straw as an alternative energy raw material. Energies 2022, 15, 331. [Google Scholar] [CrossRef]
  48. Sarathchandra, S.U.; Burch, G.; Cox, N.R. Growth patterns of bacterial communities in the rhizoplane and rhizosphere of with clover (Trifolium repens L.) and perennial ryegrass (Lolium perenne L.) in long-term pasture. Appl. Soil Ecol. 1997, 6, 293–299. [Google Scholar] [CrossRef]
  49. De Leij, F.A.; Whipps, J.M.; Lynch, J.M. The use of colony development for the characterization of bacterial communities in soil and on roots. Microb. Ecol. 1993, 27, 81–97. [Google Scholar] [CrossRef] [PubMed]
  50. Baćmaga, M.; Wyszkowska, J.; Kucharski, J. Influence of prosulfocarb and polymer supplementation on soil bacterial diversity in Triticum aestivum L. cultivation. Int. J. Mol. Sci. 2025, 26, 5452. [Google Scholar] [CrossRef]
  51. Baćmaga, M.; Wyszkowska, J.; Kucharski, J. Response of soil microbiota, enzymes, and plants to the fungicide azoxystrobin. Int. J. Mol. Sci. 2024, 25, 8104. [Google Scholar] [CrossRef]
  52. Borowik, A.; Wyszkowska, J.; Zaborowska, M.; Kucharski, J. Regulation of the microbiome in soil contaminated with diesel oil and gasoline. Int. J. Mol. Sci. 2025, 26, 6491. [Google Scholar] [CrossRef]
  53. Jiang, X.; Qu, Y.; Zeng, H.; Yang, J.; Liu, L.; Deng, D.; Ma, Y.; Chen, D.; Jian, B.; Guan, L.; et al. Long-term ecological restoration increased plant diversity and soil total phosphorus content of the alpine flowing sand land in Northwest Sichuan, China. Heliyon 2024, 10, e24035. [Google Scholar] [CrossRef]
  54. Yang, X.; Turmuhan, R.-G.; Wang, L.; Li, J.; Wan, L. Five years of natural vegetation recovery in three forests of karst graben area and its effects on plant diversity and soil properties. Forests 2025, 16, 91. [Google Scholar] [CrossRef]
  55. Kitikidou, K.; Milios, E.; Stampoulidis, A.; Pipinis, E.; Radoglou, K. Using biodiversity indices effectively: Considerations for forest management. Ecologies 2024, 5, 42–51. [Google Scholar] [CrossRef]
  56. TIBCO Software Inc Statistica (Data Analysis Software System), Version 13. 2017. Available online: https://www.statistica.com (accessed on 23 April 2024).
  57. Chen, C.; Wu, Y.; Li, J.; Wang, X.; Zeng, Z.; Xu, J.; Liu, Y.; Feng, J.; Chen, H.; He, Y.; et al. TBtools-II: A “one for all, all for one” bioinformatics platform for biological big-data mining. Mol. Plant. 2023, 16, 1733–1742. [Google Scholar] [CrossRef] [PubMed]
  58. Tang, D.; Chen, M.; Huang, X.; Zhang, G.; Zeng, L.; Zhang, G.; Wu, S.; Wang, Y. SRplot: A free online platform for data visualization and graphing. PLoS ONE 2023, 18, e0294236. [Google Scholar] [CrossRef]
  59. Wang, B.; Zhu, S.; Li, W.; Tang, Q.; Luo, H. Effects of chromium stress on the rhizosphere microbial community composition of Cyperus alternifolius. Ecotoxicol. Environ. Saf. 2021, 218, 112253. [Google Scholar] [CrossRef]
  60. Tian, G.; Li, X.; He, F.; Guo, S.; Wang, Y.; Li, Y.; Zong, W.; Liu, R. Molecular toxicity of Cr (III) and Cr (VI) to defensive protein lysozyme and their differential mechanisms. J. Mol. Liq. 2023, 390, 123151. [Google Scholar] [CrossRef]
  61. Wyszkowska, J.; Borowik, A.; Zaborowska, M.; Kucharski, J. Sensitivity of Zea mays and soil microorganisms to the toxic effect of chromium (VI). Int. J. Mol. Sci. 2023, 24, 178. [Google Scholar] [CrossRef]
  62. Wei, Z.; Sixi, Z.; Baojing, G.; Xiuqing, Y.; Guodong, X.; Baichun, W. Effects of Cr stress on bacterial community structure and composition in rhizosphere soil of iris tectorum under different cultivation modes. Microbiol. Res. 2023, 14, 243–261. [Google Scholar] [CrossRef]
  63. Chen, J.; Tian, Y. Hexavalent chromium reducing bacteria: Mechanism of reduction and characteristics. Environ. Sci. Pollut. Res. 2021, 28, 20981–20997. [Google Scholar] [CrossRef]
  64. Rahman, Z.; Thomas, L. Chemical-assisted microbially mediated chromium (Cr) (vi) reduction under the influence of various electron donors, redox mediators, and other additives: An outlook on enhanced Cr(vi) removal. Front. Microbiol. 2021, 11, 619766. [Google Scholar] [CrossRef] [PubMed]
  65. Sawicka, E.; Jurkowska, K.; Piwowar, A. Chromium (III) and chromium (VI) as important players in the induction of genotoxicity-current view. Ann. Agric. Environ. Med. 2021, 28, 1–10. [Google Scholar] [CrossRef] [PubMed]
  66. Meng, Y.; Ma, X.; Luan, F.; Zhao, Z.; Li, Y.; Xiao, X.; Thandar, S.M. Sustainable enhancement of Cr (VI) bioreduction by the isolated Cr (VI)-resistant bacteria. Sci. Total Environ. 2022, 812, 152433. [Google Scholar] [CrossRef] [PubMed]
  67. Bai, X.; Li, Y.; Jing, X.; Zhao, X.; Zhao, P. Response mechanisms of bacterial communities and nitrogen cycle functional genes in millet rhizosphere soil to chromium stress. Front. Microbiol. 2023, 14, 1116535. [Google Scholar] [CrossRef] [PubMed]
  68. Ren, J.; Huang, H.; Zhang, Z.; Xu, X.; Zhao, L.; Qiu, H.; Cao, X. Enhanced microbial reduction of Cr(VI) in soil with biochar acting as an electron shuttle: Crucial role of redox-active moieties. Chemosphere 2023, 328, 138601. [Google Scholar] [CrossRef]
  69. Jobby, R.; Jha, P.; Yadav, A.K.; Desai, N. Biosorption and biotransformation of hexavalent chromium [Cr(VI)]: A comprehensive review. Chemosphere 2018, 207, 255–266. [Google Scholar] [CrossRef] [PubMed]
  70. Yan, G.; Gao, Y.; Xue, K.; Qi, Y.; Fan, Y.; Tian, X.; Wang, J.; Zhao, R.; Zhang, P.; Liu, Y.; et al. Toxicity mechanisms and remediation strategies for chromium exposure in the environment. Front. Environ. Sci. 2023, 11, 1131204. [Google Scholar] [CrossRef]
  71. He, C.W.; Gu, L.P.; Xu, Z.X.; He, H.; Fu, G.; Han, F.X.; Huang, B.; Pan, X.J. Cleaning chromium pollution in aquatic environments by bioremediation, photocatalytic remediation, electrochemical remediation and coupled remediation systems. Environ. Chem. Lett. 2020, 18, 561–576. [Google Scholar] [CrossRef]
  72. Rizvi, F.Z.; Kanwal, W.; Faisal, M. Chromate-reducing profile of bacterial strains isolated from industrial effluents. Pol. J. Environ. Stud. 2016, 25, 2121–2128. [Google Scholar] [CrossRef]
  73. Karthik, C.; Barathi, S.; Pugazhendhi, A.; Ramkumar, V.S.; Thi, N.B.D.; Arulselvi, P.I. Evaluation of Cr (VI) reduction mechanism and removal by Cellulosimicrobium funkei strain AR8, a novel haloalkaliphilic bacterium. J. Hazard. Mater. 2017, 333, 42–53. [Google Scholar] [CrossRef]
  74. He, Y.; Dong, L.; Zhou, S.; Jia, Y.; Gu, R.; Bai, Q.; Xiao, H. Chromium resistance characteristics of Cr (VI) resistance genes ChrA and ChrB in Serratia sp. S2. Ecotoxicol. Environ. Saf. 2018, 157, 417–423. [Google Scholar] [CrossRef]
  75. Ma, L.; Chen, N.; Feng, C.; Li, M.; Gao, Y.; Hu, Y. Coupling enhancement of chromium (VI) bioreduction in groundwater by phosphorus minerals. Chemosphere 2020, 240, 124896. [Google Scholar] [CrossRef]
  76. Zha, S.; Yu, A.; Wang, Z.; Shi, Q.; Cheng, X.; Liu, C.; Zhou, L. Microbial strategies for effective hexavalent chromium removal: A comprehensive review. Chem. Eng. J. 2024, 489, 151457. [Google Scholar] [CrossRef]
  77. Ji, C.; Huang, J.; Li, J.; Zhang, X.; Yang, G.; Ma, Y.; Hao, Z.; Zhang, X.; Chen, B. Deciphering the impacts of chromium contamination on soil bacterial communities: A comparative analysis across various soil types. J. Environ. Manag. 2023, 348, 119335. [Google Scholar] [CrossRef]
  78. Ye, Y.; Hao, R.; Shan, B.; Zhang, J.; Li, J.; Lu, A. Mechanism of Cr(VI) removal by efficient Cr(VI)-resistant Bacillus mobilis CR3. World J. Microbiol. Biotechnol. 2024, 40, 21. [Google Scholar] [CrossRef]
  79. Zhu, Y.; Song, K.; Cheng, G.; Xu, H.; Wang, X.; Qi, C.; Zhang, P.; Liu, Y.; Liu, J. Changes in the bacterial communities in chromium contaminated soils. Front. Vet. Sci. 2023, 9, 1066048. [Google Scholar] [CrossRef] [PubMed]
  80. El-Sharkawy, G.; Alotaibi, M.O.; Zuhair, R.; Mahmoud, E.; El Baroudy, A.; Omara, A.E.-D.; El-Sharkawy, M. Ecological assessment of polluted soils: Linking ecological risks, soil quality, and biota diversity in contaminated soils. Sustainability 2025, 17, 1524. [Google Scholar] [CrossRef]
  81. Bazzi, W.; Abou Fayad, A.G.; Nasser, A.; Haraoui, L.-P.; Dewachi, O.; Abou-Sitta, G.; Nguyen, V.-K.; Abara, A.; Karah, N.; Landecker, H.; et al. Heavy metal toxicity in armed conflicts potentiates AMR in A. baumannii by selecting for antibiotic and heavy metal co-resistance mechanisms. Front. Microbiol. 2020, 11, 68. [Google Scholar] [CrossRef] [PubMed]
  82. Peng, D.; Zhang, R.; Chen, Y.; Jiang, L.; Lei, L.; Xu, H.; Feng, S. Effects of secondary release of chromium and vanadium on soil properties, nutrient cycling and bacterial communities in contaminated acidic paddy soil. J. Environ. Manag. 2023, 326, 116725. [Google Scholar] [CrossRef]
  83. Yan, X.; Gao, B.; Wang, J.; Zhu, X.; Zhang, M. Insights into remediation effects and bacterial diversity of different remediation measures in rare earth mine soil with SO42− and heavy metals. Front. Microbiol. 2023, 14, 1050635. [Google Scholar] [CrossRef]
  84. Lin, Y.; Ye, Y.; Hu, Y.; Shi, H. The variation in microbial community structure under different heavy metal contamination levels in paddy soils. Ecotoxicol. Environ. Saf. 2019, 180, 557–564. [Google Scholar] [CrossRef] [PubMed]
  85. Kumar, V.; Dwivedi, S.K. Hexavalent chromium reduction ability and bioremediation potential of Aspergillus flavus CR500 isolated from electroplating wastewater. Chemosphere 2019, 237, 124567. [Google Scholar] [CrossRef]
  86. Nkuna, R.; Matambo, T.S. Insights into metal tolerance and resistance mechanisms in Trichoderma asperellum unveiled by de novo transcriptome analysis during bioleaching. J. Environ. Manag. 2024, 356, 120734. [Google Scholar] [CrossRef]
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Figure 1. Colony development (CD) index (a) and ecophysiological diversity (EP) index (b) of bacteria in soil. Org—organotrophic bacteria; Act—actinomycetes; C—uncontaminated soil; Cr(III)—soil contaminated with trivalent chromium; Cr(VI)—soil contaminated with hexavalent chromium; Homogeneous groups (a–c) were created separately for organotrophic and actinomycetes.
Figure 1. Colony development (CD) index (a) and ecophysiological diversity (EP) index (b) of bacteria in soil. Org—organotrophic bacteria; Act—actinomycetes; C—uncontaminated soil; Cr(III)—soil contaminated with trivalent chromium; Cr(VI)—soil contaminated with hexavalent chromium; Homogeneous groups (a–c) were created separately for organotrophic and actinomycetes.
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Figure 2. Dominant bacterial phyla in soil (OTU ≥ 1%): (a) bacterial response to Cr(III) and Cr(VI) contamination in soil, and (b) number of bacterial OTUs at the phylum level. The abbreviations are explained below Figure 1.
Figure 2. Dominant bacterial phyla in soil (OTU ≥ 1%): (a) bacterial response to Cr(III) and Cr(VI) contamination in soil, and (b) number of bacterial OTUs at the phylum level. The abbreviations are explained below Figure 1.
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Figure 3. Relative abundance of the most abundant bacterial classes in soil (OTU ≥ 1%). The abbreviations are explained below Figure 2.
Figure 3. Relative abundance of the most abundant bacterial classes in soil (OTU ≥ 1%). The abbreviations are explained below Figure 2.
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Figure 4. Differences in the structure of bacterial communities at the class level (OTU ≥ 1%). The abbreviations are explained below Figure 2.
Figure 4. Differences in the structure of bacterial communities at the class level (OTU ≥ 1%). The abbreviations are explained below Figure 2.
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Figure 5. Dominant bacterial genera in soil (OTU ≥ 1%). The abbreviations are explained below Figure 2.
Figure 5. Dominant bacterial genera in soil (OTU ≥ 1%). The abbreviations are explained below Figure 2.
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Figure 6. Changes in soil bacterial composition at the genus level (OTU ≥ 1%) under the influence of Cr(III) and Cr(VI). The abbreviations are explained below Figure 2.
Figure 6. Changes in soil bacterial composition at the genus level (OTU ≥ 1%) under the influence of Cr(III) and Cr(VI). The abbreviations are explained below Figure 2.
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Figure 7. Soil bacterial diversity indices: (a) Shannon-Wiener index (H’) and (b) Simpson’s index (D). The abbreviations are explained below Figure 2.
Figure 7. Soil bacterial diversity indices: (a) Shannon-Wiener index (H’) and (b) Simpson’s index (D). The abbreviations are explained below Figure 2.
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Figure 8. Colony development (CD) index (a) and ecophysiological diversity (EP) index (b) of soil fungi. Fun—fungi; C—uncontaminated soil; Cr(III)—soil contaminated with trivalent chromium; Cr(VI)—soil contaminated with hexavalent chromium; Homogeneous groups (a–b) were created separately for fungi.
Figure 8. Colony development (CD) index (a) and ecophysiological diversity (EP) index (b) of soil fungi. Fun—fungi; C—uncontaminated soil; Cr(III)—soil contaminated with trivalent chromium; Cr(VI)—soil contaminated with hexavalent chromium; Homogeneous groups (a–b) were created separately for fungi.
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Figure 9. Dominant fungal phyla in soil (OTU ≥ 1%): (a) fungal response to Cr(III) and Cr(VI) contamination in soil, and (b) number of fungal OTUs at the phylum level. The abbreviations are explained below Figure 2.
Figure 9. Dominant fungal phyla in soil (OTU ≥ 1%): (a) fungal response to Cr(III) and Cr(VI) contamination in soil, and (b) number of fungal OTUs at the phylum level. The abbreviations are explained below Figure 2.
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Figure 10. Relative abundance of the most prevalent fungal classes in soil (OTU ≥ 1%). The abbreviations are explained below Figure 2.
Figure 10. Relative abundance of the most prevalent fungal classes in soil (OTU ≥ 1%). The abbreviations are explained below Figure 2.
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Figure 11. Differences in the structure of fungal communities at the class level (OTU ≥ 1%). The abbreviations are explained below Figure 2.
Figure 11. Differences in the structure of fungal communities at the class level (OTU ≥ 1%). The abbreviations are explained below Figure 2.
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Figure 12. Dominant fungal genera in soil (OTU ≥ 1%). The abbreviations are explained below Figure 2.
Figure 12. Dominant fungal genera in soil (OTU ≥ 1%). The abbreviations are explained below Figure 2.
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Figure 13. Changes in soil fungal composition at the genus level (OTU ≥ 1%) under the influence of Cr(III) and Cr(VI). The abbreviations are explained below Figure 2.
Figure 13. Changes in soil fungal composition at the genus level (OTU ≥ 1%) under the influence of Cr(III) and Cr(VI). The abbreviations are explained below Figure 2.
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Figure 14. Diversity indices of soil fungi: (a) Shannon-Wiener index (H’) and (b) Simpson’s index (D). The abbreviations are explained below Figure 2.
Figure 14. Diversity indices of soil fungi: (a) Shannon-Wiener index (H’) and (b) Simpson’s index (D). The abbreviations are explained below Figure 2.
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Table 1. The granulometric composition and chemical and physicochemical properties of the soil used in the experiment.
Table 1. The granulometric composition and chemical and physicochemical properties of the soil used in the experiment.
Type of SoilGranulometric
Composition
(%)		pHKClCorgNtotalC:NHACEBCCECBS
%
SandSiltClayg kg−1 d.m. soilmmol (+) kg−1 d.m. soil
Sandy loam69.4127.712.885.807.801.206.5013.5031.0044.5069.67
HAC—hydrolytic acidity; EBC—exchangeable base cations; CEC—cation exchange capacity; BS—base saturation; Corg—total organic carbon; Ntotal—total nitrogen; C:N—ratio of organic carbon content to total nitrogen content.
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Boros-Lajszner, E.; Wyszkowska, J.; Baćmaga, M.; Kucharski, J. Analysis of Changes in the Microbial Biodiversity of Soil Contaminated with Cr(III) and Cr(VI). Appl. Sci. 2025, 15, 10951. https://doi.org/10.3390/app152010951

AMA Style

Boros-Lajszner E, Wyszkowska J, Baćmaga M, Kucharski J. Analysis of Changes in the Microbial Biodiversity of Soil Contaminated with Cr(III) and Cr(VI). Applied Sciences. 2025; 15(20):10951. https://doi.org/10.3390/app152010951

Chicago/Turabian Style

Boros-Lajszner, Edyta, Jadwiga Wyszkowska, Małgorzata Baćmaga, and Jan Kucharski. 2025. "Analysis of Changes in the Microbial Biodiversity of Soil Contaminated with Cr(III) and Cr(VI)" Applied Sciences 15, no. 20: 10951. https://doi.org/10.3390/app152010951

APA Style

Boros-Lajszner, E., Wyszkowska, J., Baćmaga, M., & Kucharski, J. (2025). Analysis of Changes in the Microbial Biodiversity of Soil Contaminated with Cr(III) and Cr(VI). Applied Sciences, 15(20), 10951. https://doi.org/10.3390/app152010951

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