Impact of long-term organochlorine pesticide contamination on soil microbial communities and enzymatic activityAbstract
Soil microorganisms play a crucial role in maintaining ecosystem stability and soil fertility; however, long-term pesticide contamination can significantly disrupt microbial functioning. The present study provides a comprehensive assessment of the microbiological and biochemical status of soils chronically contaminated with pesticides at a chemical burial site and adjacent areas. Soil samples were collected from several zones within the landfill and surrounding territories, including a reference site with minimal anthropogenic impact. The abundance and structure of major physiological groups of microorganisms were determined using culture-based methods, while soil enzymatic activities (catalase, peroxidase, polyphenol oxidase, and invertase) were assessed as indicators of biological activity. Residual concentrations of organochlorine pesticides were analyzed using gas chromatography with an electron capture detector. The results revealed a pronounced suppression of enzymatic activity and a significant reduction in microbial abundance in contaminated soils compared to the control. The most severe disturbances were observed in the central, western, and eastern zones of the burial site, characterized by elevated concentrations of dieldrin and endrin exceeding permissible levels by tens to hundreds of times. Bacterial communities dominated the microbial structure, whereas fungi and actinomycetes were strongly suppressed, indicating degradation of soil ecological functions. Spatial heterogeneity of biological responses suggested partial adaptation or limited recovery in peripheral zones. These findings demonstrate that even localized pesticide contamination leads to persistent impairment of soil microbial activity and enzymatic functioning. The results emphasize the importance of integrated microbiological and biochemical indicators for environmental monitoring and provide a scientific basis for the development of soil bioremediation strategies in chronically contaminated areas.
Keywords:
soil; microbiota; organochlorine pesticides; enzymatic activity; bioremediation
Resumo
Os microrganismos do solo desempenham um papel crucial na manutenção da estabilidade dos ecossistemas e da fertilidade do solo; no entanto, a contaminação prolongada por pesticidas pode comprometer significativamente o funcionamento microbiano. O presente estudo fornece uma avaliação abrangente do estado microbiológico e bioquímico de solos cronicamente contaminados por pesticidas em um local de enterramento químico e nas áreas adjacentes. Amostras de solo foram coletadas em várias zonas dentro do aterro e nos territórios circundantes, incluindo um local de referência com mínimo impacto antropogênico. A abundância e a estrutura dos principais grupos fisiológicos de microrganismos foram determinadas por métodos baseados em cultivo, enquanto as atividades enzimáticas do solo (catalase, peroxidase, polifenoloxidase e invertase) foram avaliadas como indicadores da atividade biológica. As concentrações residuais de pesticidas organoclorados foram analisadas por cromatografia gasosa com detector de captura de elétrons. Os resultados revelaram uma supressão acentuada da atividade enzimática e uma redução significativa na abundância microbiana nos solos contaminados em comparação com o controle. As perturbações mais severas foram observadas nas zonas central, oeste e leste do local de enterramento, caracterizadas por concentrações elevadas de dieldrina e endrina, excedendo os níveis permitidos em dezenas a centenas de vezes. As comunidades bacterianas dominaram a estrutura microbiana, enquanto fungos e actinomicetos foram fortemente suprimidos, indicando a degradação das funções ecológicas do solo. A heterogeneidade espacial das respostas biológicas sugeriu adaptação parcial ou recuperação limitada nas zonas periféricas. Estes achados demonstram que mesmo a contaminação localizada por pesticidas leva a um comprometimento persistente da atividade microbiana e do funcionamento enzimático do solo. Os resultados enfatizam a importância de indicadores microbiológicos e bioquímicos integrados para o monitoramento ambiental, e fornecem base científica para o desenvolvimento de estratégias de biorremediação do solo em áreas cronicamente contaminadas.
Palavras-chave:
solo; microbiota; pesticidas organoclorados; atividade enzimática; biorremediação
1. Introduction
Soil microorganisms constitute a key component of soil biocenoses and play a fundamental role in the functioning of terrestrial ecosystems by sustaining major biogeochemical cycles, contributing to soil structure formation, and maintaining ecological stability. The soil microbiome, composed of bacteria, archaea, and fungi, is characterized by high taxonomic and functional diversity and governs the transformation of carbon, nitrogen, phosphorus, and other elements, thereby directly influencing soil fertility and environmental quality (Prosser et al., 2007; Putatunda et al., 2024; Sun et al., 2019; Sun et al., 2020; Kadirova et al., 2025).
Anthropogenic factors, particularly the long-term application and disposal of pesticides, exert complex and multilevel impacts on soil microbial communities and physicochemical soil properties. Persistent organic pesticides, including organochlorine compounds such as DDT and hexachlorocyclohexanes, are capable of long-term persistence in soils, accumulation in organic matter, and toxic effects on microorganisms. These processes result in alterations of microbial community structure, suppression of sensitive physiological groups, and inhibition of enzymatic activity, accompanied by changes in soil pH, organic carbon content, and other key parameters (Sun et al., 2020; Regar et al., 2019; Bansal, 2019; Ewere et al., 2024). The study area is located in a region of Central Asia characterized by arid climate conditions, increasing desertification processes, and a legacy of intensive agricultural practices during the 20th century, including large-scale pesticide application and disposal. Such conditions have contributed to the formation of long-term contaminated sites, including pesticide burial grounds, which represent significant sources of persistent organic pollutants in the region. Furthermore, studies conducted in Kazakhstan confirm that organochlorine pesticides are characterized by high environmental persistence and the ability to remain in soils and aquatic systems for extended periods, forming stable residual contamination (Mit et al., 2021). Their capacity for bioaccumulation and biomagnification leads to their transfer through food chains and may result in toxic, including genotoxic, effects in living organisms.
Under conditions of chronic pesticide contamination, microbiological and biochemical indicators are considered sensitive tools for assessing soil degradation and disruption of ecological functions. Soil enzyme activities, together with microbial abundance and community structure, reflect the functional state of soils and their capacity for self-recovery (Chia et al., 2024; Swaine et al., 2025). Although modern molecular approaches such as metagenomic analyses provide deeper insight into microbial diversity, classical microbiological and biochemical methods remain highly relevant for long–term environmental monitoring and comparative assessment of contaminated areas.
Despite numerous studies addressing the effects of individual pesticides on soil microbiota, data on integrated changes in microbial communities and biochemical activity under prolonged chronic contamination – particularly at pesticide burial sites–remain limited. Spatial heterogeneity of biological responses and relationships between residual organochlorine pesticide concentrations and soil functional status are still insufficiently understood (Swaine et al., 2025; Köninger et al., 2026)
Therefore, the aim of this study was to perform a comprehensive assessment of the microbiological and biochemical state of soils chronically contaminated with pesticides at a burial site and adjacent areas, and to identify relationships between organochlorine pesticide contamination and indicators of soil biological activity.
2. Materials and Methods
2.1. Soil sampling and preparation of suspensions
Soil samples were collected from multiple sites with documented long-term pesticide use, as well as from control sites with minimal contamination (40°20′41.411″ N, 67°32′56.411″ E). To ensure representativeness, a multi-point sampling grid was applied following standard protocols for depth and spatial distribution (Table 1). Geographic coordinates of all sampling points are listed in Table 1.
In the laboratory, 10 g of air-dried soil from each of the six samples was suspended in 100 mL of sterile distilled water and left at room temperature (28 °C) for 24 h to prepare soil suspensions. Physical soil properties, including texture, homogeneity, solubility, sedimentation rate, and color, were assessed. The selection of the control soil was constrained by the widespread nature of pesticide contamination in the study region. Therefore, a relatively uncontaminated site within the territory of the Institute of Microbiology (Tashkent) was selected as a reference. Although geographically separated from the main study area, this site represents soil with minimal anthropogenic impact and comparable general soil characteristics. This approach is consistent with environmental studies conducted in regions where truly pristine soils are not available.
2.2. Physicochemical characterization
Soil texture, structure, and other physical characteristics were visually and tactually assessed. Moisture content was determined gravimetrically after drying at 105 °C. pH and electrical conductivity were measured in a 1:5 soil-to-water suspension using a calibrated pH/conductivity meter. Organic carbon content was estimated by the Walkley–Black method. All measurements were conducted in triplicate. Soil pH was measured using a standard mercury–chloride reference electrode with automatic temperature compensation. Measurements were performed in an aqueous soil suspension at a soil-to-water ratio of 1:5, in accordance with GOST 27753.3–88 (Boldyreva, 2020). The humus content in soil samples was determined using the Tyurin method, based on the oxidation of organic matter with potassium dichromate, following GOST 26213–91 (Abdrakhmanov et al., 2023). The content of toxic salts was calculated using the method for determining hypothetical salts, expressed in milliequivalents per 100 g of soil (mg-eq/100 g), according to GOST R (State Standard of the Russian Federation) 59540–2021 (GOST, 2021). Exchangeable bases and water-soluble salts (HCO3−, CO32−, Cl−, SO42−; Ca2+, Mg2+, Na+, K+) were determined by titrimetric methods (GOST 26425-85) (Prudnikova et al., 2023).
2.3. Soil enzymatic activity
Catalase, peroxidase (POD), polyphenol oxidase (PPO), and invertase activities were determined. Catalase activity reflects decomposition of hydrogen peroxide into water and oxygen, detoxifying this compound for soil microorganisms. Peroxidase catalyzes oxidation of organic matter via hydrogen peroxide and contributes to humification. Polyphenol oxidase catalyzes oxidation of phenols to quinones, which condense with amino acids and peptides to form primary humic molecules. Catalase activity was measured gasometrically, while invertase, PPO, and POD were assessed colorimetrically (Minnikova et. al., 2024). All enzyme assays were performed in triplicate.
2.4. Microbial abundance and community structure
The abundance and structure of major physiological groups of microorganisms were determined by plate count of soil suspensions on selective media. Actinomycetes were analyzed as a separate group due to their distinct ecological role in soil ecosystems, particularly in the degradation of complex organic compounds such as lignin and xenobiotics, as well as their known sensitivity to environmental stressors, including pesticide contamination. Their separate quantification allows for a more detailed assessment of functional shifts within soil microbial communities.
Ten grams of air-dried soil were suspended in sterile saline and serially diluted tenfold (Tomar et al., 2024; Bogati et al., 2025; Al-Kalbani et al., 2025). Bacteria were enumerated on meat-peptone agar (MPA), fungi on Czapek-Dox agar or wort agar, and actinomycetes on starch–ammonium agar. Inoculated plates were incubated at 25–28 °C for 3–10 days, depending on the microbial group. Colony-forming units (CFU) per gram of dry soil were calculated. Physiological groups were differentiated based on colony morphology and growth characteristics on selective media. All assays were performed in triplicate.
2.5. Pesticide analysis
Quantitative and qualitative determination of pesticide residues in soil samples was performed using certified reference standards (Sigma-Aldrich, Germany) on a gas chromatograph Chromatek-Crystal 5000 (Russia) equipped with an electron-capture detector (ECD), following GOST R 53 21 7–2008 (ISO, 2002) (Figure 1).
Gas chromatographic analysis of pesticides in soil samples collected from different areas of the toxic substances storage site.
2.6. Statistical analysis
All measurements were conducted in triplicate, and results are reported as mean ± standard deviation (SD). Statistical differences between sites were evaluated using one-way ANOVA followed by Tukey’s post hoc test, with significance set at p < 0.05.
3. Results
3.1. Environmental assessment of soil conditions
The physical and chemical characteristics of soils from different sampling locations are presented in Table 2. Soils from the northern part were heavy and fast-settling, with low humus content and a slightly alkaline pH. The western part was characterized by dense soils with poor settling ability and high mineralization, including elevated concentrations of Na+, Cl-, CO32-, Mg2+, K+ and SO42- ions.
Soils collected from the southern and eastern parts were heavy, dense, and turbid. These samples showed high colloidal content, accumulation of organic pollutants or toxicants, acidic to weakly buffered conditions, and reduced buffering capacity.
The central part of the test site exhibited the most unfavorable properties, with heavy, poorly settling, turbid soils and the highest levels of pesticide accumulation, toxicity, and disturbed redox conditions.
At a distance of 2 km from the settlement, soils showed rapid settling and moderate contamination with residual pesticide content. The control soil demonstrated good settling, loose structure, normal humus content, neutral pH, and absence of toxicants.
3.2. Soil enzymatic activity
At the first stage of the study, the activities of key soil enzymes – catalase, peroxidase (POD), polyphenol oxidase (PPO), and invertase – were determined. The obtained results revealed pronounced differences in the biochemical status of soils depending on the sampling location (Figure 2). In all investigated sites, with the exception of the control soil, a substantial decrease in enzymatic activity was observed, indicating suppression of microbiological and biochemical processes under conditions of prolonged technogenic impact.
Catalase activity, reflecting the intensity of redox processes and aerobic microbial metabolism in soils, reached 7.6 mL O2 g-1 soil h-1 in the control sample. In soils collected from the burial site and surrounding areas, catalase activity decreased by 1.8–3.6 times and ranged from
2.1 to 4.2 mL O2 g-1 soil h-1. The lowest catalase values were recorded in the western (2.1 mL O2 g-1 soil h-1), eastern (2.5 mL O2 g-1 soil h-1), and central zones of the landfill (2.9 mL O2 g-1 soil h-1), suggesting strong inhibition of aerobic microbial processes caused by the presence of toxic compounds (Table 3).
Comparison of biological indicators between control soil, peripheral zone, and the most contaminated zone.
The activities of polyphenol oxidase and peroxidase, enzymes involved in the transformation of phenolic and aromatic compounds, were 7.1 and 6.8 mg benzoquinone per 20 g soil over 4 h, respectively, in the control soil. In contaminated soils, PPO activity decreased to 2.5–3.6 mg benzoquinone per 20 g soil, while POD activity ranged from 2.4 to 4.0 mg benzoquinone per 20 g soil. The lowest activities of both enzymes were observed in soils from the central part of the landfill (PPO – 2.5; POD – 2.4 mg benzoquinone per 20 g soil), indicating pronounced inhibition of organic matter oxidation processes in the zone of maximum anthropogenic pressure.
Invertase activity, characterizing carbohydrate metabolism and the degree of organic matter mineralization, reached 6.9 mg glucose g-1 soil h-1 in the control soil. In most contaminated samples, invertase activity was reduced to 2.4–3.5 mg glucose g-1 soil h-1 (Table 3). However, relatively high invertase activity was detected in the western part of the landfill (5.1 mg glucose
g-1 soil h-1) and at the site located 2 km from the landfill (4.6 mg glucose g-1 soil h-1). This may indicate partial preservation or recovery of carbohydrate transformation processes in these zones, possibly due to lower contaminant loads or the formation of functionally adapted microbial communities.
Overall, the obtained data indicate significant disruption of the soil enzymatic complex within the pesticide burial site. The most unfavorable biochemical conditions were characteristic of the central, eastern, and western zones of the landfill, whereas more distant sites exhibited a tendency toward partial restoration of individual enzymatic activity parameters.
The comparison between the control soil, peripheral zone, and the most contaminated central zone revealed a clear spatial gradient of biological degradation (Table 3). In the peripheral zone, enzymatic activities were reduced by 33.3–49.3% relative to the control, whereas microbial abundance decreased by 97.7%. In contrast, the central zone exhibited a much stronger suppression, with enzymatic activity reduced by more than 60% and total microbial abundance by nearly two orders of magnitude. These results indicate partial preservation of soil biochemical functioning at the periphery of the landfill, while the central zone represents a hotspot of severe biological degradation.
3.3. Microbial abundance and structure
Quantitative assessment of soil microorganisms after 72 h of incubation revealed pronounced spatial heterogeneity in the microbiological status of soils collected from the landfill and adjacent areas (Table 3). All samples obtained within the landfill exhibited a sharp decrease in total microbial abundance compared to the control soil, indicating substantial suppression of soil microbiota under prolonged technogenic stress.
The lowest total microbial counts were recorded in soils from the central part of the landfill (25 × 105 CFU g-1), as well as in the northern (43 × 105 CFU g-1) and western zones (36 × 105 CFU g-1). These areas were characterized by the predominance of dense, heavy, and poorly sedimenting soil fractions, which likely limit aeration and reduce substrate availability for microbial growth. Relatively higher microbial abundances were observed in the eastern and southern parts of the landfill (114 and 93 × 105 CFU g-1, respectively); however, these values remained an order of magnitude lower than those of the control soil.
The moderate increase in microbial abundance in these zones may be associated with local heterogeneity in contaminant distribution and with physicochemical soil properties that temporarily support microbial viability. In all landfill soils, bacteria constituted the dominant microbial group, whereas fungal and actinomycete populations were extremely low. The absence of fungal microflora in soil sampled 2 km from the settlement of Egizbulok, along with minimal fungal abundance within the landfill, indicates high sensitivity of this group to toxic compounds, including pesticides and chemical residues.
In contrast, the control soil was characterized by a significantly higher total microbial abundance (4905 × 105 CFU g-1) and a balanced proportion of bacteria, fungi, and actinomycetes. The loose structure and favorable physical properties of the control soil created optimal conditions for the formation of stable and functionally active microbial communities.
A comparative analysis of key biological indicators between the control soil and the most contaminated zone (central part of the landfill) demonstrated a pronounced decline in soil biological activity (Table 3). Enzymatic activities decreased by 61.8–65.2% relative to the control, indicating severe inhibition of oxidative and hydrolytic processes. The most dramatic effect was observed for total microbial abundance, which was reduced by approximately 99.5% in the central zone. This sharp decline reflects a near-collapse of the soil microbial pool under conditions of chronic organochlorine pesticide contamination. The control soil was characterized by a balanced microbial structure including bacteria, fungi, and actinomycetes, indicating favorable conditions for microbial development.
3.4. Organochlorine pesticide residues
Analysis of organochlorine pesticide residues in soil samples collected from different zones of the landfill and adjacent areas revealed the presence of several compounds whose concentrations in some cases exceeded the maximum permissible concentrations (MPCs) established by sanitary regulations for soils (GN 1.2.3111-13, SanPiN, and analogous standards of CIS countries) (Table 4).
Comparison of biological indicators between control soil, peripheral zone, and the most contaminated zone.
The compound 2,4,5,6-tetrachloro-m-xylene was detected exclusively in soil from the western zone at a concentration of 11.194 µg kg-1. Due to the absence of an officially established MPC for this compound, its presence was interpreted as localized contamination, likely associated with leakage or unauthorized disposal of chemical waste (Тable 4).
Alpha-hexachlorocyclohexane (α-BHC) was detected in five of the six analyzed samples, with the highest concentration (0.398 µg kg-1) observed in the western zone. Considering the approximate MPC of 0.1 µg kg-1, exceedances were recorded in the eastern, central, and 2-km zones by factors of 1.5–4. Beta-BHC was detected in the western zone at a concentration of 0.140 µg kg-1, also exceeding the permissible level.
Particularly high ecological concern was associated with the detected levels of dieldrin, found in soils from the eastern, central, and remote (2 km) zones. In the central part of the landfill, dieldrin concentration reached 5.82 µg kg-1, exceeding the established MPC (0.02 µg kg-1) by more than 290 times. In other zones, exceedances ranged from 6 to 19 times. Endrin was detected in the western zone at a concentration of 0.557 µg kg-1, approximately 28 times higher than the permissible limit.
Other organochlorine compounds, including γ-HCH (lindane), aldrin, DDT and its metabolites (DDE, DDD), heptachlor, as well as endosulfan I, II, and endrin derivatives, were either not detected or were below detection limits.
Overall, the results indicate localized but severe contamination of soils with organochlorine pesticides in specific areas of the study site. Elevated concentrations of dieldrin and endrin point to the presence of a persistent technogenic hotspot exerting a strong negative impact on biological soil activity and ecological condition. These findings underscore the need for targeted measures aimed at restoring microbial functions and enhancing the ecological resilience of contaminated soils.
4. Discussion
4.1. Physical and chemical characteristics of soil
The results indicate a clear relationship between soil physical properties and chemical characteristics across the pesticide burial site. Heavy texture, high density, turbidity, and poor settling suggest a high proportion of fine and colloidal particles. Recent analyses show that fine-textured soils with elevated organic carbon and colloids tend to exhibit greater sorption of organic contaminants due to larger specific surface areas and increased interaction with pollutants (Wang et al., 2024).
Elevated salinity and high mineralization in the western part align with recent findings demonstrating that increased salt concentrations and electrical conductivity alter soil structure and can influence contaminant mobility and nutrient cycling (Tao et al., 2024). Soils from the southern and eastern sectors displayed high turbidity, acidic pH, and low buffering capacity. Acidic conditions are widely reported to enhance retention of certain pesticide compounds and modify soil organic matter interactions (Yang et al., 2025). Moreover, soil pH has been shown to significantly influence pesticide sorption behavior, often increasing retention under lower pH conditions due to enhanced electrostatic attraction and altered humic substance solubility. The central part exhibited the most unfavorable combination of properties, including high pesticide accumulation. These findings are consistent with recent environmental studies indicating that persistent contaminants remain adsorbed in fine-textured soils with limited leaching, particularly under conditions where organic matter content and pH favor sorption. In contrast, soils located 2 km from the settlement exhibited moderate contamination and relatively improved physical properties, whereas the control soil displayed normal physicochemical conditions, supporting the conclusion that the observed alterations are primarily anthropogenic.
4.2. Effects of long-term organochlorine contamination on soil enzymatic activity
The prolonged burial of pesticides at the studied site, as described in the sampling site characterization, exerts a strong and persistent inhibitory effect on soil enzymatic activity (Wołejko et al., 2020; Gianfreda et al., 2005; Yasir et al., 2025). The significant reduction in catalase, peroxidase, polyphenol oxidase, and invertase activities observed in contaminated soils reflects suppression of key metabolic pathways related to oxidative balance, organic matter transformation, and carbohydrate metabolism.
Catalase activity, which plays a critical role in protecting microbial cells from oxidative stress, was particularly sensitive to organochlorine pesticide contamination. Similar reductions in catalase activity under pesticide stress have been reported in soils contaminated with DDT, lindane, and dieldrin, where inhibition of antioxidant enzyme systems led to accumulation of reactive oxygen species and impaired microbial metabolism (Wołejko et al., 2020; Ren et al., 2024). The lowest catalase activity observed in the central and western zones of the burial site likely reflects long-term accumulation of highly persistent organochlorine compounds.
The pronounced decrease in peroxidase and polyphenol oxidase activities indicates disruption of phenolic compound oxidation and humification processes. These enzymes are essential for the stabilization of soil organic matter and formation of humic substances (Gianfreda et al., 2005; Minnikova et al., 2025). Their inhibition may result in reduced humus quality and decreased soil resilience, as previously demonstrated for pesticide-contaminated and industrially polluted soils (Ren et al., 2024).
Invertase activity, reflecting carbohydrate turnover and energy supply to microbial communities, was also markedly suppressed in most contaminated soils. However, relatively higher invertase activity in peripheral zones suggests partial functional recovery or adaptation of microbial communities, consistent with earlier observations of localized resilience in chronically contaminated soils (Riah et al., 2014; Srivastava et al., 2025).
4.3. Alterations in microbial community structure under chronic pesticide exposure
The sharp decline in total microbial abundance and strong dominance of bacterial populations observed in contaminated soils indicate severe disruption of microbial community structure (Caraba et al., 2025). Fungi and actinomycetes appeared to be more sensitive to pesticide contamination, which may be associated with their physiological characteristics and susceptibility to toxic compounds. However, the extremely low abundance of fungi observed even in soils located 2 km from the burial site may also reflect specific local soil conditions, including physicochemical properties and environmental stress factors (Wołejko et al., 2020; Khan et al., 2025; Kumar and Singh, 2025).
The absence or minimal presence of fungi even at sites located 2 km from the burial area suggests possible lateral migration of contaminants and highlights the long-term ecological risk associated with pesticide disposal sites. Adaptation of bacterial communities in some zones may be related to the development of pesticide-tolerant or xenobiotic-degrading strains, as documented in other chronically polluted environments (Nauanová et al., 2025; Mahapatra and Adak, 2025).
4.4. Relationship between organochlorine pesticides and soil biological activity
The observed inverse relationship between pesticide concentrations and biological indicators provides strong evidence for the toxic impact of organochlorine compounds on soil functioning. Highly persistent pesticides such as dieldrin and endrin are known to disrupt cellular membranes, inhibit enzymatic systems, and interfere with microbial metabolism even at low concentrations. These effects result in long-term alterations of microbial community structure and functional capacity, ultimately reducing soil resilience and self-recovery potential (Basapuram et al., 2025; Chia et al., 2024; Dhakal et al., 2025). Soil microbial diversity and community structure have also been shown to shift significantly under organochlorine contamination, with dominant taxa exhibiting altered functional profiles compared to uncontaminated soils (Sun et al., 2019; Liu et al., 2024). These disruptions in microbial populations correspond to reduced activities of soil enzymes involved in nutrient cycling, demonstrating that specific toxic compounds contribute more strongly to functional impairment than do total residue concentrations per se (Dhakal et al., 2025).
The absence of a strictly linear relationship between total pesticide concentration and biological activity reinforces the idea that highly persistent and toxic organochlorines disproportionately determine the soil’s functional status, rather than the cumulative pesticide load (Sun et al., 2019).
4.5. Ecological implications and prospects for soil restoration
The observed degradation of microbial communities and enzymatic systems indicates the formation of stable anthropogenic disturbance with impaired soil ecological functions, including diminished nutrient turnover and self-purification capacity (Chia et al., 2024). Reduced microbial activity can lead to alterations in soil carbon and nitrogen cycles, with potential knock-on effects on plant productivity and soil structure.
Similar patterns of long-term organochlorine pesticide contamination and its effects on soil biological activity have been reported in various regions worldwide. For instance, studies conducted in China have demonstrated that residues of legacy organochlorine pesticides such as DDTs and HCHs persist in agricultural soils for decades and are strongly associated with reduced microbial biomass, altered community composition, and decreased enzymatic activity (Zhang et al., 2021, 2024).
In India, investigations of historically contaminated agricultural soils have revealed significant inhibition of soil enzymes, including dehydrogenase and phosphatase, as well as shifts toward bacterial dominance under pesticide stress, which is consistent with the patterns observed in the present study (Sharma et al., 2022; Singh and Singh, 2020).
Legacy organochlorine pollutants (OCPs) continue to pose a significant environmental concern due to their high persistence and bioaccumulative properties. A study conducted in Tajikistan demonstrated that residual levels of OCPs were detected across all investigated environmental compartments, with the highest concentrations observed in soils, in some cases exceeding 10 ppm, whereas contamination levels in food products remained considerably lower (Barron et al., 2017). Among the identified compounds, DDT (dichlorodiphenyltrichloroethane) was the most frequently detected and dominant contaminant, indicating its long-term persistence in the environment even decades after its ban.
Similar findings have been reported for sediment samples from the Yamuna River (India), where total concentrations of organochlorine pesticides ranged from 157 to 844 ng/g depending on the season (Pandey et al., 2011). The major contributors to contamination included compounds such as endrin and endosulfan, suggesting either ongoing inputs or remobilization of these substances within aquatic ecosystems.
Similarly, studies from European regions affected by legacy pesticide pollution have reported long-term ecological impacts, including suppression of microbial diversity and persistence of highly toxic compounds such as dieldrin and endrin in soils, even decades after their ban (Riedo et al., 2021).
These findings collectively indicate that the ecological consequences of organochlorine pesticide contamination are not region-specific but represent a global environmental challenge, particularly in areas with historical pesticide burial or intensive agricultural use.
Nevertheless, zones with partial biological recovery and evidence from bioremediation studies suggest that targeted strategies may restore soil functionality. Enzyme-driven bioremediation pathways, leveraging microbial dehalogenases and oxidative enzymes, represent promising methods to accelerate degradation of recalcitrant organochlorines (Chia et al., 2024). Emerging approaches such as the use of microbial consortia with enhanced catabolic capacities, coupled with omics-informed ecological management, have shown potential for rehabilitating chronically contaminated soils (Dhakal et al., 2025; Mahalle et al., 2025). Thus, despite the restriction or complete ban of many organochlorine pesticides, their residual presence is still detected in various environmental matrices, highlighting the need for continued monitoring and risk assessment.
5. Conclusion
Soil microorganisms play a pivotal role in maintaining ecosystem stability and soil fertility; however, long-term pesticide contamination can severely impair microbial functioning. This study provides a comprehensive assessment of the microbiological and biochemical status of soils chronically contaminated with organochlorine pesticides at a burial site and in adjacent areas.
Soil samples were collected from multiple zones within the disposal site and surrounding territories, including a reference site with minimal anthropogenic influence. Microbial abundance and community structure were evaluated using culture-based methods, while enzymatic activities (catalase, peroxidase, polyphenol oxidase, and invertase) were determined as indicators of soil biological functioning. Residual pesticide concentrations were quantified by gas chromatography with an electron capture detector.
The results demonstrated a pronounced decline in enzymatic activity and a significant reduction in microbial abundance in contaminated soils compared to the control. The most severe effects were observed in the central, western, and eastern zones, where dieldrin and endrin concentrations exceeded permissible limits by several orders of magnitude. Microbial communities were strongly shifted toward bacterial dominance, while fungi and actinomycetes were markedly suppressed, indicating substantial ecological degradation.
These findings highlight the long-term ecological consequences of organochlorine pesticide contamination and underscore the importance of integrating microbiological and biochemical indicators for environmental monitoring and soil restoration strategies
Acknowledgements
We acknowledge the Institute Microbiology of Academy Sciences of the Republic of Uzbekistan, which carried out a basic topic, for creating sufficient conditions to the experiments in the laboratory.
Data Availability Statement
The entire data set that supports the results of this study was published in the article itself.
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Editor:
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