Comparative assessment of organic--mineral fertilizer, biochar, and vermiremediation for petroleum-contaminated alkaline soils

Comparative assessment of organic--mineral fertilizer, biochar, and vermiremediation for petroleum-contaminated alkaline soils | Authorea try { document.documentElement.classList.add('js'); } catch (e) { } var _gaq = _gaq || []; _gaq.push(['_setAccount', 'G-8VDV14Y67G']); _gaq.push(['_trackPageview']); (function() { var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true; ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js'; var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s); })(); Skip to main content Preprints Collections Wiley Open Research IET Open Research Ecological Society of Japan All Collections About About Authorea FAQs Contact Us Quick Search anywhere Search for preprint articles, keywords, etc. Search Search ADVANCED SEARCH SCROLL This is a preprint and has not been peer reviewed. Data may be preliminary. 12 July 2025 V1 Latest version Share on Comparative assessment of organic--mineral fertilizer, biochar, and vermiremediation for petroleum-contaminated alkaline soils Authors : Małgorzata Kacprzak 0000-0002-3897-8659 [email protected] , Sławomir Kaczmarek , and Iwona Kupich Authors Info & Affiliations https://doi.org/10.22541/au.175230022.22803261/v1 Published Land Degradation & Development Version of record Peer review timeline 256 views 144 downloads Contents Abstract Information & Authors Metrics & Citations View Options References Figures Tables Media Share Abstract This study evaluates and compares the effectiveness of three bioremediation strategies—biochar amendment, an organic–mineral fertilizer derived from sewage sludge, and vermiremediation using Eisenia fetida —in remediating petroleum hydrocarbon-contaminated soils with challenging physicochemical characteristics. Two alkaline, clay-rich soils (pH 8.0–8.6) were subjected to six-month laboratory incubations, during which total petroleum hydrocarbons (TPH), polycyclic aromatic hydrocarbons (PAHs), dehydrogenase activity (DHA), nutrient availability, heavy metal immobilization, and microbial responses were monitored. Among the tested treatments, the organic–mineral fertilizer (T1), synthesized from municipal sewage sludge, MgO, and H 2 SO 4 , demonstrated superior remediation performance. It achieved reductions of up to 64% in TPH and 59% in PAHs, enhanced microbial activity (as indicated by DHA), and shifted soil pH toward neutrality, thereby improving nutrient bioavailability (notably P, N, Mg, and S). Biochar (T2) showed limited hydrocarbon degradation but effectively immobilized heavy metals (e.g., Ba, Zn, Pb) and stabilized soil carbon. Vermiremediation (T3) improved DHA and nutrient distribution through bioturbation but was less consistent in hydrocarbon removal. Multivariate analyses, including principal component analysis and hierarchical clustering, highlighted the organic–mineral fertilizer as the most comprehensive and effective treatment. These findings underscore the potential of integrated organo–mineral amendments for restoring hydrocarbon-contaminated soils, particularly in alkaline, fine-textured substrates, while promoting circular economy objectives through the valorization of sewage sludge. 1. Introduction Soil pollution from petroleum products remains a significant global concern, arising from oil extraction, transport, refining, and accidental spills. These hydrophobic and persistent compounds are toxic to soil organisms and often lead to structural degradation, reduced microbial diversity, and decreased fertility (Khodaparast and Khoshgoftar 2024; Majeed et al. 2025). The problem is particularly acute in clay-rich, strongly alkaline soils (pH 8.0–8.6), where hydrocarbons exhibit low bioavailability due to strong adsorption onto mineral and organic matter surfaces, limiting microbial degradation (Masy et al. 2016). In such conditions, natural attenuation is typically slow and ineffective, especially in nutrient-deficient or compacted soils (Saeed et al. 2024). Effective remediation of oil-contaminated soils requires integrated strategies that address both pollutant degradation and soil recovery. Among biological approaches, amendments with organic and mineral materials—including biodegradable waste—have shown promise. Biochar, a porous, carbon-rich material produced via pyrolysis of biomass or sewage sludge, has attracted attention for its multifunctionality. In contaminated soils, biochar can enhance structure, water retention, cation exchange capacity, and immobilize heavy metals through adsorption and precipitation (Penido et al. 2019). It can absorb up to 900% of its weight in oil and stimulate microbial populations by up to tenfold, improving biodegradation (Wang et al. 2023). Its effectiveness depends on pyrolysis conditions and feedstock properties, which influence porosity and functional group availability (Zahed et al. 2021). Vermitechnology, involving earthworms such as Eisenia fetida , is another sustainable option. Earthworms enhance aeration, water infiltration, and microbial activity, facilitating organic matter decomposition and hydrocarbon degradation (Ansari et al. 2023). They also improve nutrient availability—particularly nitrogen, phosphorus, and potassium—critical for microbial function. Earthworm-assisted remediation has achieved TPH reductions of up to 93% (Martinkosky 2015), with combined earthworm–microbial treatments enhancing degradation by 58% (Ameen & Al-Homaidan 2024). Additionally, earthworms contribute to broader soil health improvements, including microbial diversity and physicochemical balance (Gbarakoro & Sikoki 2023). The growing volume of biodegradable waste, including municipal sewage sludge, presents a dual challenge and opportunity. Within the circular economy framework, converting waste into soil amendments helps reduce landfill use, limit emissions, and recycle nutrients (Kacprzak et al. 2022; Miao and Zeller 2025). Stabilized sludge, rich in organic matter, nitrogen, phosphorus, and micronutrients, can be safely applied to soils after sanitization (Buta et al. 2021; Gusiatin et al. 2024; Molaey et al. 2024). A promising waste valorization pathway involves producing organic–mineral fertilizers (OMFs), which combine organic substrates with mineral additives like Ca, MgO, and H₂SO₄ to enhance nutrient delivery and buffer pH. These additions also aid pathogen reduction and pollutant stabilization through exothermic reactions (Kacprzak et al. 2018). OMFs serve as both nutrient sources and soil conditioners, capable of immobilizing or degrading contaminants (Molaey et al. 2024). OMFs are now being explored for soil remediation, including hydrocarbon-contaminated sites. Formulations like “Kazuglegumus” have demonstrated the ability to reduce oil concentrations by 12–22% while stimulating biological activity and supporting plant growth (Akhanova et al. 2023). Other studies, such as that by Kisić et al. (2022), report improved degradation of petroleum pollutants when mineral–organic fertilizers are combined with sorbents like Spill-Sorb, although the latter was more effective alone. Despite growing evidence for the individual benefits of these methods, their comparative performance in petroleum-contaminated, alkaline, clayey soils remains unclear. Furthermore, long-term effects on soil biochemical functioning, nutrient cycling, and heavy metal dynamics are not well understood. This study addresses these gaps by comparing three bioremediation strategies: (i) biochar application, (ii) vermiremediation using Eisenia fetida , and (iii) amendment with an organic–mineral fertilizer derived from sewage sludge. The key objectives are to (1) evaluate the degradation efficiency of total petroleum hydrocarbons (TPH) and polycyclic aromatic hydrocarbons (PAHs), (2) assess changes in microbial activity through dehydrogenase activity (DHA), (3) track changes in soil pH, nutrient status, and heavy metal mobility, and (4) apply multivariate statistical methods to determine the most effective and sustainable treatment. Additionally, the study investigates the reuse of sewage sludge for soil restoration, supporting circular economy and sustainable land management goals. 2. Materials and methods 2.1. Preparation of Organic-Mineral Fertilizer The organic-mineral fertilizer used in this study was synthesized from municipal sewage sludge, a waste byproduct of wastewater treatment processes, combined with magnesium oxide (MgO) and sulfuric acid (H₂SO₄). The process involved the controlled addition of H₂SO₄ to the sludge to initiate acidification and partial hydrolysis, followed by neutralization with MgO. H 2 SO 4 + H 2 O → H 3 O + + HSO 4 - The reaction between MgO and H₂SO₄ generated magnesium sulfate (MgSO₄·7H₂O), contributing to the stabilization of the fertilizer and the enhancement of its mineral content (Tab.1). MgO + H 2 SO 4 + H 2 O → MgSO 4 ·7H 2 O The exothermic nature of the reaction also aided in the hygienization of the product. The final fertilizer was air-dried, homogenized, and sieved to obtain a uniform particle size suitable for soil application. Table 1. The physicochemical properties of organic-mineral fertilizer used for soil remediation % of d.m. 9.81 31.52 2.21 3.84 4.51 7.96 6.57 7.73 B Zn Mn Cu Fe Cd Pb Hg mg/kg d.m. 6.23±1.31 55.44±10.71 78.15±9.27 4.78±1.31 585.7±49.13 1.41±0.91 6.11±1.11 <0.01 2.2. Biochar The biochar applied in the experiment was commercially procured from Pyropower Europe (pyropowereurope.com), a certified producer specializing in biochar derived from sustainably sourced biomass through pyrolysis technology. The biochar was produced at a pyrolysis temperature of approximately 550°C. The physicochemical properties are consistent with biochars produced at higher pyrolysis temperatures, which tend to exhibit increased surface area, porosity, and pH, beneficial for soil amendment applications (Tab.2). Table 2. The physicochemical properties of biochar used for soil remediation Moisture % fresh matter 8.0 Ash % d.m. 13 Carbon (C) % d.m. 80.0 BET specific surface area m²/g 350 Hydrogen (H) % d.m. 2.0 Total pore volume cm³/g 0.4 Nitrogen (N) % d.m. 0.9 Micropore diameter nm < 2 Oxygen (O) % d.m. 16.0 Micropore surface area contribution m²/g 200.0 C:N ratio 90.0 Micropore proportion (BET) % 60.0 P₂O₅ % d.m. 0.9 Mesopore diameter nm 30.0 K₂O % d.m. 0.5 Mesopore surface area contribution m²/g 30.0 CaO % d.m. 7.0 Mesopore proportion (BET) % 30.0 MgO % d.m. 1.0 Macropore diameter nm > 50 Na₂O % d.m. 0.09 Macropore surface area contribution m²/g < 10 Cadmium (Cd) mg/kg d.m. < 1.0 Macropore proportion (BET) % 10.0 Lead (Pb) mg/kg d.m. < 10.0 Cation exchange capacity (CEC) cmol(+)/kg 100.0 Zinc (Zn) mg/kg d.m. < 50.0 Water holding capacity % d.m. 300.0 Cuprum (Cu) mg/kg d.m. < 30.0 pH (in H₂O) 9.5 Bulk density kg/m³ 400 Electrical conductivity (EC) mS/cm 1.5 2.3. Earthworm inoculation The earthworms (mix of Eisenia fetida and Eisenia andrei ), commonly known as red wigglers, were sourced from Ekagro (https://www.ekagro.pl/), a Polish company specializing in composting worms. Key characteristics of earthworms used for experiment: • Size: typically 5-10cm in length • Coloration: reddish-brown with distinct banding • Habitat preference: thrives in decomposing organic matter • Reproduction: hermaphroditic, capable of rapid population growth under favorable conditions. 2.4. Experimental Design Soil samples were collected from a site located in central Poland (Kuyavian-Pomeranian Voivodeship), affected by a failure of a crude oil transmission pipeline. The accident resulted in a localized petroleum contamination zone, where visible surface and subsurface oil traces were observed. The area had been exposed to the crude oil for an extended period prior to sampling, allowing weathering and partial infiltration into the soil profile. Two representative composite soil samples were collected at different distances (0-10 m and 10-20 m) from the leak site at the depth to 1m. The initial concentrations of total petroleum hydrocarbons (TPH) measured in these samples were: sample 1: 1,313 mg/kg d.m.; sample 2: 2,523 mg/kg d.m. The soil was classified as silty clay (USDA: silty clay loam), with the following texture composition: ~30% clay (0.05 mm). This type of soil is characterized by high water retention, low permeability, and limited aeration, all of which can inhibit natural attenuation of hydrocarbons and slow down microbial degradation processes. The baseline physicochemical characteristics of the contaminated soil (prior to treatment) were as follows: • pH: 8.02 to 8.62 (moderately to strongly alkaline); • organic dry matter content: 4.72% to 5.06%; • hydrolytic acidity (Hh): 0.15 cmol(+)kg⁻¹, indicating very low buffering capacity and poor soil acid-base reactivity, typical for carbonate-rich or alkaline mineral soils • bulk density: ~1.3–1.4 g/cm³ • color and odor: dark brown to black, with a persistent petroleum odor and slightly compacted structure The experiment included the following treatment variants: • Control (C) : Contaminated soil without any amendment (monitored natural attenuation, MNA). • Organic-mineral fertilizer (T1) : Contaminated soil treated with the organic-mineral fertilizer, which was applied at a rate of 50 g/kg soil • Biochar (T2) : Contaminated soil amended with biochar . • Earthworms (T3) : Contaminated soil inoculated with earthwors, a density of 50 individuals per kilogram of soil was introduced to the designated treatment variants. Both amendments (fertilizer and biochar)were thoroughly mixed into the soil to ensure homogeneous distribution before the incubation phase. Each treatment was performed in triplicate in 10-liter containers under controlled laboratory conditions (temperature capacity). The experimental duration was 6 months, with sampling performed at 0, 2, 4, and 6 months. 2.5. Analytical Methods Samples were analyzed for the following parameters: • Soil moisture content was determined gravimetrically by drying a known mass of fresh sample at 105°C for 24 hours until constant weight (ISO 11465:1993). • Bulk density was measured using the core method. Undisturbed samples were collected in steel cylinders (100 cm³), dried at 105°C, and bulk density was calculated as dry mass per unit volume (g cm⁻³) • Total Petroleum Hydrocarbons (TPH): Determined via gas chromatography with flame ionization detection (GC-FID) after solvent extraction in accordance with ISO 16703:2004. • Policyclic Aromatic Hydrocarbons (PAH) were extracted using ultrasonic-assisted extraction with a mixture of acetone and hexane (1:1, v/v), followed by purification on silica gel columns. Quantification was performed using gas chromatography coupled with mass spectrometry (GC-MS), following EPA Method 8270D. Identification and quantification were based on calibration curves prepared from certified PAH standards. • Total organic matter (OM) was estimated via loss on ignition (LOI) at 550°C for 4 hours in a muffle furnace. • Total carbon (TC) content was measured using an elemental analyzer, based on dry combustion at high temperature in an oxygen atmosphere. Total nitrogen (N) and hydrogen (H) were determined simultaneously with total carbon using a CHN elemental analyzer (dry combustion method). Oxygen (O) was calculated by difference according to the formula: O%=100−(C%+H%+N%+Ash%)where:ash content was determined by combusting air-dried samples in a muffle furnace at 750°C for 4 hours, with weight loss recorded to estimate organic vs. mineral fractions. • Soil pH and Electrical Conductivity (EC): Measured in a 1:2.5 soil-to-water extract using a calibrated multiparameter probe according to the PN EN ISO 10390, 2005 standard. • Cation Exchange Capacity (CEC) was determined using the Cohex extraction method by saturating samples (0.5–5 g) with 25 mL Co[NH 3 ] 6 ]Cl 3 (0.0166 M); The CEC values were calculated from the loss of Co in the solution by ICP (Nel et al. 2023). • Water Holding Capacity (WHC) was measured by saturating 100 g of air-dried, sieved soil in a Buchner funnel under vacuum, allowing free water to drain for 24 hours. The retained water was weighed and expressed as percentage of dry soil mass. • Macro- and Micronutrient Content: Analyzed via ICP-OES following acid digestion (HNO₃/HCl) according to US EPA 200.7 (1994) and US EPA 6010 (2014). • The specific surface area (SSA) was measured using the Brunauer–Emmett–Teller (BET) method with nitrogen adsorption at 77 K, conducted on degassed samples using an automatic surface area analyzer (Micromeritics ASAP 2020). • Microbial Activity: Assessed using dehydrogenase activity (DHA) as an indicator of total microbial respiration, following the method of Casida et al. (1964). 2.4. Statistical Analysis Data were analyzed using one-way ANOVA with Tukey’s HSD post-hoc test (p < 0.05) to assess differences between treatments using Statistica 13. A Pearson correlation matrix was constructed to identify relationships among key variables, including TPH/PAH reduction, dehydrogenase activity (DHA), pH, macronutrients (C, N, P, Ca, Mg, K, S), and heavy metals (Ba, Cu, Zn, Pb). Correlations were considered significant at p < 0.05 and p < 0.01 (Statistica 13) and visualized using Seaborn in Python 3.10. Hierarchical cluster analysis (HCA) using Ward’s method and Euclidean distance was conducted in Statistica 13 to group treatments based on integrated biological, chemical, and environmental parameters. Principal Component Analysis (PCA) was used to explore multivariate relationships among soil quality indicators across both soils (C1, C2), with the first two components (PC1 and PC2) used for interpretation. PCA was performed in Python (scikit-learn package). 2.5. Radar plot development and comparative evaluation A radar chart was developed to visually compare the performance of the three remediation strategies (T1, T2, T3) using six key indicators: 1. Nutrient enrichment 2. pH regulation 3. Metal immobilization 4. Carbon stabilization 5. Dehydrogenase activity (DHA) 6. Overall remediation efficiency Each parameter was normalized on a 0–10 scale based on observed improvements relative to controls. Scoring criteria included macronutrient increases (N, P, K, Mg, S), pH shift toward neutrality (7.0 ± 0.3), heavy metal immobilization (Ba, Zn, Cu, Ni, Pb), changes in total and stabilized carbon, and both peak and sustained DHA. Overall remediation efficiency integrated TPH and PAH removal supported by PCA and HCA. The scores were plotted using a radial axis layout (spider chart) in a polar coordinate system. Each axis represented one parameter, and each treatment formed a polygon connecting its respective scores. The chart was constructed using Python 3.10 with Matplotlib, and legends were placed externally to avoid overlap with chart elements. 3.1. Soil pH modification under different remediation strategies The initial pH of the contaminated soils ranged from 8.02 to 8.62, indicating moderately to strongly alkaline conditions. In the control (C) variants, pH remained stable, reflecting minimal natural attenuation (Fig.1)—consistent with findings that biodegradation is limited in high-pH, nutrient-poor soils (Saeed et al., 2024). However, some studies suggest pH 8 can still support microbial activity under oily sludge conditions (Srinivasarao Naik et al., 2011). The most significant pH changes occurred in the organic–mineral fertilizer treatment (T1), where pH dropped to near-neutral levels (7.1–7.4) in both soils. This was likely due to sulfuric acid and magnesium salts (e.g., MgSO₄) in the fertilizer. The pH reduction enhanced microbial activity, as indicated by increased dehydrogenase activity, and likely improved hydrocarbon bioavailability via desorption from soil particles (Fig.1). In contrast, biochar-treated soils (T2) showed elevated or slightly increased pH due to biochar’s inherent alkalinity (pH ≈ 9.5), potentially limiting microbial degradation in some cases while favoring alkali-tolerant species (Fig.1). Slight pH decreases in earthworm treatments (T3), especially in Soil C2, likely resulted from organic acid production and mineralization processes associated with earthworm activity and microbiota. Earthworm-induced bioturbation may also influence local pH by redistributing organic and mineral components (Fig.1). Figure 1. Soil pH values in C1 and C2 soils at the beginning (April) and after 6 months (October) under different treatment variants: control (C), organic-mineral fertilizer (T1), biochar (T2), and earthworms (T3). 3.2. Reduction in Total Petroleum Hydrocarbons (TPH) Total petroleum hydrocarbons (TPHs) are persistent pollutants that impair soil and microbial health. Initial TPH levels were 1,313 mg/kg in Soil C1 and 2,523 mg/kg in C2. After six months, all treatments reduced TPH concentrations, though effectiveness varied by strategy and soil type (Fig. 2). In control soils, TPH reductions were minimal (<10%), confirming limited natural attenuation in alkaline clay soils. ANOVA and Tukey’s HSD test showed significant differences (p < 0.05), with the organic–mineral fertilizer (T1) achieving the highest TPH reductions: 62.5% in C1 (to 492 mg/kg) and 63.5% in C2 (to 921 mg/kg). This is likely due to nutrient enrichment (N, P, S) and pH adjustment toward neutral, supporting microbial degradation (Fig.2). Earthworm treatment (T3) also performed well, reducing TPH by 46.8% in C1 and 64.4% in C2. These effects are attributed to bioturbation, aeration, and microbial stimulation, though performance varied with soil properties affecting worm activity (Fig.2). Biochar (T2) showed the lowest TPH removal: 46.6% in C1 and 31.8% in C2. While its sorptive capacity may reduce hydrocarbon mobility, biochar’s high alkalinity likely inhibited microbial activity in already alkaline soils (Fig.2). Figure 2. Total Petroleum Hydrocarbon (TPH) content in soil_I (C1)and soil_II (C2)at the beginning (April) and after 6 months (October) across different treatment variants. Error bars represent standard deviation. Different letters indicate statistically significant differences between treatments ( p < 0.05). 3.3. Polycyclic Aromatic Hydrocarbons (PAHs) Degradation PAHs are persistent, hydrophobic pollutants with low bioavailability, especially in alkaline, clay-rich soils. In the control (C) variants, PAH levels remained largely unchanged, confirming minimal natural attenuation (Tab. 3). The organic–mineral fertilizer (T1) achieved the most consistent PAH reductions: 50.7% in Soil C1 and 58.8% in C2. This effectiveness is likely due to improved nutrient availability (e.g., N, P), pH adjustment, and enhanced microbial activity, as supported by elevated DHA levels (Tab.3). Biochar (T2) showed variable results—only 29.5% reduction in C1 but 72.9% in C2—possibly due to differences in soil composition and PAH distribution. In C2, biochar’s high surface area may have immobilized PAHs more effectively, but its low nutrient content and alkalinity likely limited microbial degradation in C1 (Tab.3). Earthworm treatment (T3) also reduced PAHs, with stronger effects in C2 (67.4%) than C1 (29.5%). This likely reflects improved microbial stimulation and organic matter redistribution via bioturbation (Tab.3). Table 3. Polycyclic aromatic hydrocarbons (PAHs) content in soil C1 and C2 before and after 6 Months of remediation treatments initial value final value acenaphthene C1 <0.010 <0,010 <0,010 <0,010 <0,010 C2 <0.010 <0,010 <0,010 <0,010 <0,010 acenaphthylene C1 <0.010 <0,010 <0,010 <0,010 <0,010 C2 <0.010 <0,010 <0,010 <0,010 <0,010 anthracene C1 0.023 0.022 0.020 0.040 0.014 C2 0.028 0.023 <0,004 0.012 0.014 benzo(a)anthracene C1 0.126 0.136 0.110 0.210 0.055 C2 0.161 0.100 0.038 0.049 0.070 benzo(a)pyrene C1 0.136 0.169 0.146 0.230 0.111 C2 0.157 0.127 0.070 0.076 0.099 benzo(b)fluoranthene C1 0.247 0.259 0.202 0.330 0.150 C2 0.267 0.199 0.079 0.071 0.128 benzo(ghi)perylene C1 0.170 0.152 0.122 0.138 0.140 C2 0.149 0.121 0.060 0.067 0.128 benzo(k)fluoranthene C1 0.102 0.085 0.070 0.130 0.043 C2 0.101 0.069 0.025 0.030 0.053 chrysene C1 0.118 0.117 0.078 0.170 0.051 C2 0.140 0.078 0.033 0.045 0.052 dibenzo(a.h)anthracene C1 0.030 0.024 0.017 0.031 <0.010 C2 0.025 0.015 <0.010 <0.010 <0.010 fluoranthene C1 0.318 0.269 0.199 0.440 0.136 C2 0.366 0.217 0.068 0.095 0.112 fluorene C1 <0.010 <0.010 <0.010 <0.010 <0.010 C2 <0.010 <0.010 <0.010 <0.010 <0.010 indeno(1.2.3.cd)pyrene C1 0.125 0.140 0.112 0.120 0.067 C2 0.129 0.108 0.044 0.058 0.057 naphthalene C1 <0.010 0.012 <0.010 0.030 <0.010 C2 0.011 0.011 <0.010 0.026 <0.010 phenanthrene C1 0.232 0.135 0.079 0.140 0.044 C2 0.216 0.128 0.035 0.065 0.047 pyrene C1 0.283 0.246 0.192 0.290 0.131 C2 0.341 0.197 0.114 0.087 0.101 Total PAH (16 compounds) C1 1.91 1.766 1.347 2.299 0.942 C2 2.091 1.393 0.566 0.681 0.861 Among individual PAHs, the fertilizer treatment (T1) showed the most consistent reduction across both low-molecular-weight (e.g., anthracene, phenanthrene) and high-molecular-weight compounds (e.g., benzo[a]pyrene, chrysene). This is particularly significant, as high-molecular-weight PAHs are more recalcitrant due to their low solubility and strong adsorption to soil organic matter. Benzo[a]pyrene, one of the most toxic PAHs, was reduced from 0.136 to 0.111 mg/kg in soil C1 and from 0.157 to 0.099 mg/kg in soil C2 under T1 treatment (Tab.3). Fluoranthene, another persistent compound, was reduced by up to 57% in T1, outperforming both biochar and earthworm treatments. The enhanced degradation of these PAHs suggests that the combined action of nutrient supplementation, pH moderation, and microbial stimulation in the T1 treatment fosters favorable enzymatic pathways for PAH transformation, such as those involving dioxygenase enzymes . 3.4. Nurient dynamic and metal immobilization Oil-contaminated soils often contain toxic metals like Ba, Cu, Pb, and Zn, whose mobility can increase due to changes in pH or microbial activity. Effective remediation must address both nutrient restoration and metal stabilization (Tab. 4). The organic–mineral fertilizer (T1), made from sewage sludge, MgO, and H₂SO₄, significantly increased essential nutrients: P (↑ from 559 to 2,220 mg/kg d.m.), Mg (↑ to 13,900 mg/kg d.m.), K (↑ to 4,610 mg/kg d.m.), and S (↑ to 9,620 mg/kg d.m.). These improvements reflect both the fertilizer composition and enhanced nutrient solubility due to pH reduction. However, its metal immobilization was moderate—e.g., Ba in C2 ↓ by 54.8%, Pb in C1 ↓ by 40.4%, and Cu in C1 ↓ by 46.8%—while changes in Ni, Zn, and Mn were minimal. Additionally, increased sodium levels raise concerns about potential soil sodicity (Tab.4). Biochar (T2), with high surface area and alkalinity (pH 9.5), acted mainly as a metal immobilizer. Compared to control soils, slight increases in Cu (↑10%), Ni, Pb (↑6%), and Fe (↑5%) were observed, suggesting stable retention rather than removal due to its high CEC (100 cmol(+)/kg), porous microstructure, and reactive functional groups. Biochar also increased total carbon in C2 (from 2.51 to 2.68%), indicating potential for long-term carbon sequestration (Tab.4). However, slight decreases in P and S point to reduced nutrient availability due to adsorption, highlighting the need to pair biochar with nutrient-rich amendments. Vermiremediation (T3) modestly improved nutrient levels—N (↑55%), P (↑58%), and K (↑7%)—through organic matter mineralization and bioturbation (Tab.4). Metal immobilization results were variable: Ba showed slight decreases, while Pb, Zn, and Cu had mixed trends. In some cases, earthworm activity may have increased metal mobility, reinforcing the benefit of combining vermiremediation with sorptive materials like biochar. Table 4. Concentrations of selected inorganic elements in soil C1 and C2 after 6 months of remediation treatments. Arrows indicate differences in relation to the final control values - ↑increase and ↓decrease initial value final value C tot. [%] C1 2.51 2.35 2.02↓ 2.59↓ 2.37↑ C2 2.51 1.20 2.57↑ 2.68↑ 2.31↑ N [mg/kg d.m.] C1 1200 1180 1200↑ 577↓ 1790↑ C2 1160 1080 1195↑ 743↓ 1800↑ P [mg/kg d.m.] C1 611 570 1550↑↑ 577↑ 798↑ C2 559 494 2220↑↑ 463↓ 805↑ K [mg/kg d.m.] C1 1840 1560 3870↑↑ 2820↑ 1960↑ C2 1830 1960 4610↑↑ 2540↑ 1970↑ Mg [mg/kg d.m.] C1 2190 1860 9770↑↑ 2320↑ 2440↑ C2 2240 2440 13900↑↑ 2370↓ 2820↑ Ca [mg/kg d.m.] C1 14700 13700 32500↑↑ 15000↑ 17400↑ C2 14500 13400 43900↑↑ 13900↑ 17500↑ S [mg/kg d.m.] C1 726 771 6540↑↑ 663↓ 920↑ C2 791 619 9620↑↑ 788↑ 1100↑ Na [mg/kg d.m.] C1 81 56 314↑↑ 65↑ 181↑↑ C2 114 88 397↑↑ 68↑ 274↑↑ Al [mg/kg d.m.] C1 7000 6080 6020↓ 6750↑ 6770↑ C2 7000 6660 6320↓ 5960↓ 6630↓ As[mg/kg d.m.] C1 3.02 3.04 3.02↓ 2.11↓ 2.86↓ C2 3.73 3.87 3.81↓ 2.18↓ 3.17↓ Ba [mg/kg d.m.] C1 145 160 82.6↓↓ 140↓ 123↓ C2 147 115 66.5↓↓ 103↓ 121↓ Cu [mg/kg d.m.] C1 19.9 13.4 10.6↓ 14.7↑ 20.1↑ C2 11.5 10.5 11.0↑ 11.3↑ 12.3↑ Mn [mg/kg d.m.] C1 208 199 173↓ 183↓ 212↑ C2 196 212 173↓ 156↓ 203↓ Ni [mg/kg d.m.] C1 10.0 7.9 8.7↓ 8.4↑ 10.4↑ C2 10.1 9,0 8,5↓ 9,4↑ 10,8↑ Pb [mg/kg d.m.] C1 23.3 17.3 13.8↓ 18.3↑ 15.9↓ C2 16.3 15.9 13.0↓ 16.1↑ 14.3↓ Fe [mg/kg d.m.] C1 10800 9440 8490↓ 9950↑ 10200↑ C2 10700 10600 8450↓ 8410↓ 11300↑ Si [mg/kg d.m.] C1 236 194 211↑ 231↑ 290↑ C2 210 190 230↑ 300↑ 289↑ Zn [mg/kg d.m.] C1 52 44 35↓ 45↑ 47↑ C2 41 41 32↓ 41↓↑ 42↑ V [mg/kg d.m.] C1 15.1 12.7 13.6↑ 14.2↑ 15.2↑ C2 15.0 13.2 14.0↑ 14.3↑ 15.2↑ 3.5. Dynamics of dehydrogenase activity (DHA) Dehydrogenase activity (DHA) reflects overall microbial oxidative function and is a reliable indicator of bioremediation progress. In control soils, DHA remained low (<5 µg TPF g⁻¹ d.m.) due to alkaline pH, nutrient deficiency, and hydrocarbon toxicity. All treatments significantly increased DHA within the first two months, followed by stabilization or slight decline (Fig. 3). The strongest stimulation occurred in the organic–mineral fertilizer (T1), reaching peaks of 14.2 µg in C1 and 15.0 µg TPF g⁻¹ d.m. in C2. This was driven by nutrient enrichment (N, P, Mg), pH neutralization (from ~8.6 to ~7.2), and improved microbial conditions (Fig.3). Biochar (T2) resulted in a slower, sustained DHA increase, peaking at 10–12 µg TPF g⁻¹ d.m. by month 4. While not nutrient-rich, biochar likely supported microbial activity via habitat creation, moisture retention, and carbon stabilization (Fig.3). Earthworm treatment (T3) also boosted DHA, particularly early on, reaching 12–13 µg TPF g⁻¹ d.m. at 2 months (Fig.3). This was likely due to bioturbation, aeration, and nutrient cycling through worm casts. DHA declined slightly after 4 months, possibly due to reduced availability of labile organics or microbial equilibrium. Fig. 3. Dynamics of dehydrogenase activity (DHA) in soils C1 and C2 over 6 months for different treatments: control (C), organic-mineral fertilizer (T1), biochar (T2), and earthworms (T3). Error bars indicate standard deviation (n = 3). Significant differences between treatments (p < 0.05) are marked with different letters. 3.6. Principal Component Analysis of Soil Treatment Effects To comprehensively evaluate and visualize the complex relationships among biological, chemical, and ecological indicators of soil quality, Principal Component Analysis (PCA) was employed. This multivariate statistical technique reduces data dimensionality while preserving variance and enables the identification of patterns in treatment responses across multiple correlated variables. The results of the PCA are illustrated in Figure 4. The first two principal components (PC1 and PC2) accounted for the majority of the total variance in the dataset—PC1 explaining 60–70% and PC2 contributing an additional 15–25%. This cumulative variance (>85%) confirmed the adequacy of a two-dimensional PCA plot to distinguish treatment effects. PC1 was primarily associated with variables such as total petroleum hydrocarbon (TPH) reduction, PAH degradation, dehydrogenase activity (DHA), and macronutrient enrichment (notably N, P, Mg, and S). In turn, PC2 was influenced by parameters related to soil metal content, carbon stabilization, and final pH. These principal components effectively separated treatment types based on their mechanisms of action and impact intensity across soil types. The control samples (C1 and C2) clustered separately and negatively along PC1, indicating poor performance across all measured indicators. These soils were characterized by low microbial activity, limited contaminant degradation, persistent alkaline pH, and minimal nutrient recovery, confirming their severely impaired biological and chemical status without intervention. The organic-mineral fertilizer (T1) treatments (T1-C1 and T1-C2) formed a tight and well-separated cluster along PC1, indicating strong and consistent positive influence on the primary soil quality metrics. Their positioning was driven by high TPH and PAH reduction, elevated microbial activity (DHA), improved nutrient availability, and successful pH neutralization. This indicates that T1 provided broad-spectrum remediation effects, confirming its superior performance across chemical, biological, and toxicological soil quality dimensions. Figure 4. Principal Component Analysis (PCA) of soil treatments based on 15 soil quality indicators: TPH and PAH reduction, dehydrogenase activity, final pH, heavy metal content (Ba, Cu, Zn, Pb), and macronutrients (C, N, P, Ca, Mg, K, S). Variables were standardized, and metal concentrations were inverted. Treatments are labeled by soil type (C1 or C2) and treatment variant (C, T1, T2, T3). Biochar-treated soils (T2-C1, T2-C2) were positioned moderately along PC2 but closer to the control along PC1, reflecting their role in metal immobilization and carbon stabilization, with limited biological or chemical stimulation. Their separation along PC2 underscores biochar’s geochemical influence, particularly on pH and heavy metal behavior (Fig.3). Earthworm treatments (T3-C1, T3-C2) occupied an intermediate space between control and T1 treatments along PC1. This placement reflects moderate improvements in DHA and TPH removal, with biological effects more pronounced in the more porous soil (C2). Their clustering supports the role of vermiremediation in stimulating microbial processes, though less comprehensively than T1 (Fig.4). 3.7. Multivariate statistical analysis of remediation efficiency To confirm the integrated effects of each treatment, multivariate analyses—including hierarchical cluster analysis (HCA) and Pearson correlation matrices—were applied. These analyses supported trends observed in individual parameters and PCA (Fig. 5, Fig. 6). 3.7.1. Hierarchical Cluster Analysis (HCA) HCA using Ward’s method revealed three distinct clusters: • Cluster I (T1-C1, T1-C2): Characterized by strong co-association of high DHA, TPH/PAH degradation, and macronutrient enrichment (P, S, Mg), reflecting T1’s broad-spectrum efficacy. • Cluster II (T3-C1, T3-C2): Grouped by moderate improvements in DHA and hydrocarbon degradation, driven by microbial stimulation from earthworm activity, though effects varied between soils. • Cluster III (T2 and Controls): Biochar-treated soils (T2-C1, T2-C2) clustered with untreated controls, indicating limited biological improvement. Their grouping was influenced mainly by metal immobilization and carbon stabilization, with minimal microbial activation. These clusters highlight differing remediation mechanisms: T1 promoted integrated chemical–biological recovery, T3 focused on biostructuring, and T2 on geochemical stabilization. Figure 5. Hierarchical clustering of soil treatments based on standardized values of TPH and PAH reduction, dehydrogenase activity (DHA), final pH, heavy metal content (Ba, Cu, Zn, Pb), and macronutrient concentrations (C, N, P, Ca, Mg, K, S). Clustering performed using Ward’s method with Euclidean distance. Labels indicate soil type and treatment. 3.7.2. Correlation matrix and intervariable relationships Pearson correlations revealed key relationships: • DHA positively correlated with P, S, and Mg (r > 0.85), confirming nutrient enrichment enhances microbial activity. • DHA and TPH showed a strong negative correlation (r = –0.91), linking microbial activity to hydrocarbon degradation, particularly in T1 and T3. • Metal concentrations (Ba, Zn, Pb) negatively correlated with DHA (r = –0.74 to –0.81), indicating potential microbial inhibition from metal toxicity, especially in control and biochar-only treatments. • Soil pH and DHA were also negatively correlated (r = –0.79), showing that alkaline conditions suppress microbial function, while pH reduction (as in T1) enhances bioremediation. Figure 6. Correlation matrix of soil remediation indicators showing relationships among microbial activity (DHA), contaminant degradation (TPH and PAH reduction), nutrient availability (P, S, Mg), metal concentrations (Ba, Zn, Pb), and soil pH across different treatment strategies. Together, the multivariate analyses confirm T1 as the most effective treatment across microbial, chemical, and ecological parameters. T3 offered moderate biological improvements, while T2 was primarily effective for metal immobilization and carbon retention, with limited impact on microbial activity. 3.8. Radar chart analysis of remediation strategies To compare treatment performance across multiple soil quality indicators, a radar chart was constructed using six key metrics: nutrient enrichment, pH regulation, metal immobilization, carbon stabilization, dehydrogenase activity (DHA), and overall remediation efficiency (Fig. 7). The organic–mineral fertilizer (T1) showed the most balanced and highest overall performance, scoring strongly across all six indicators. Its strengths included enhanced microbial activity, effective hydrocarbon degradation, and improved nutrient availability. Biochar (T2) scored highest in metal immobilization and carbon stabilization but lower in biological and chemical metrics due to limited nutrient contribution and its alkaline nature. Earthworm treatment (T3) performed moderately overall, with notable effects on DHA and nutrient cycling, particularly in the more porous soil (C2), though less impact on metal stabilization or pH correction. Figure 7. Comparative performance of three remediation strategies—organic-mineral fertilizer (T1), biochar (T2), and earthworms (T3)—based on six integrated soil quality indicators: nutrient enrichment, pH regulation, metal immobilization, carbon stabilization, dehydrogenase activity (DHA), and overall remediation efficiency. 4. Discussion 4.1. Organic–mineral fertilizer (T1) as a broad-spectrum remediation product The organic–mineral fertilizer (T1) synthesized from municipal sewage sludge, MgO, and H₂SO₄ demonstrated the most effective and consistent performance across biological, chemical, and environmental metrics. Its strong remediation capacity—reflected in TPH reductions up to 63.5% and PAH degradation up to 58.8%—can be attributed to the synergistic interplay of pH regulation, nutrient enrichment, and microbial activation. Other research indicates that the combination of organic materials with mineral amendments can enhance the degradation of PAHs. The addition of organic-mineral mixtures, such as zeolite composites combined with organic matter, has shown significant reductions in soil PAH concentrations. Szerement et al. (2023) demonstrated that these mixtures effectively decreased PAH levels in both soil and plant tissues without compromising agricultural yields. Organic amendments like compost and biosolids can stimulate microbial activity, which is crucial for the biodegradation of PAHs. By buffering soil pH toward neutral values (~7.1–7.4), T1 created more favorable conditions for microbial proliferation. Numerous studies have established that petroleum-degrading microorganisms—particularly bacteria such as Bacillus and Pseudomonas —exhibit optimal enzymatic activity under neutral to mildly acidic conditions (Wang et al., 2020; Odili et al., 2021). Furthermore, the pH shift likely enhanced hydrocarbon bioavailability by weakening the sorptive interactions between hydrophobic compounds and clay particles (Akhanova et al., 2023). T1 also provided essential nutrients—particularly phosphorus, magnesium, sulfur, and nitrogen—that are typically deficient in oil-contaminated soils. These elements play critical roles in microbial metabolism and enzyme synthesis required for hydrocarbon degradation (Kisić et al., 2022; Fernández-Luqueño et al., 2017) and are essential for plant growth (Magela et al., 2019, Varalakshmi et al., 2024). The observed increase in dehydrogenase activity (DHA), reaching up to 15 µg TPF g⁻¹ d.m., is a strong indicator of enhanced microbial respiration and soil biological recovery (Lee et al., 2007). The effectiveness of T1 aligns with previous findings on sludge-based fertilizers, such as “Kazuglegumus” and calcium ammonium nitrate-based organo-mineral formulations, which improved biological activity, reduced hydrocarbon concentrations, and restored plant productivity (Akhanova et al., 2023; Kacprzak et al., 2018). Moreover, the inclusion of sulfuric acid likely contributed to the inactivation of pathogens and partial hydrolysis of organic matter, improving nutrient solubility and fertilizer stability (Gusiatin et al., 2024). PCA and hierarchical clustering confirmed the comprehensive performance of T1, grouping it distinctly from other treatments. These multivariate tools effectively captured its simultaneous contributions to contaminant degradation, nutrient enrichment, microbial activation, and pH normalization. 4.2. Biochar (T2) as effective immobilizer with limited biostimulation Although biochar (T2) was less effective than T1 in reducing TPH and PAHs (TPH removal of 32–47%), it demonstrated strong potential for heavy metal immobilization and carbon stabilization. Its high surface area (BET 350 m²/g), cation exchange capacity (100 cmol(+)/kg), and alkaline pH (~9.5) support its use as a sorbent for potentially toxic elements (Penido et al., 2019; Yang and Cao, 2022). This was reflected in the reductions of Ba and Zn concentrations and modest increases in total organic carbon content. However, biochar alone did not significantly improve DHA or support microbial degradation, especially under alkaline conditions where microbial communities are less active (Wang et al., 2023). The high pH of biochar may have compounded the existing alkalinity of the soil, further limiting microbial activity (Itam et al., 2023). Studies have shown that biochar is most effective when co-applied with nutrient-rich materials or microbial inoculants to overcome its biological inertness (Faloye et al., 2024). Studies show that biochar can decrease total PAH concentrations by 15.4% and free PAHs by 55.6% (Li et al., 2023). Interestingly, in Soil C2, biochar achieved relatively high PAH reductions (~72.9%), likely due to strong sorptive interactions with high-molecular-weight compounds such as benzo[a]pyrene. These results are consistent with Ke et al. (2024), who reported up to 99.3% removal of PAHs using humic acids modified biochars. However, this effect is largely physical—bioavailability reduction rather than microbial degradation—and may limit long-term site recovery unless combined with biologically active amendments. 4.3. Earthworm (T3) influence on biological enhancement of soil processes Vermiremediation using Eisenia fetida (T3) achieved intermediate remediation performance, with TPH reductions up to 64.4% and moderate improvements in DHA and nutrient cycling. These outcomes are consistent with the known ecological functions of earthworms in enhancing soil porosity, organic matter turnover, and microbial dispersion (Macci et al., 2012; Ansari et al., 2023). Earthworms have been effective in soils contaminated with varying concentrations of crude oil, achieving up to 71.05% TPH loss in heavily contaminated soils(Nassar & Said, 2021). Earthworms, particularly Eisenia fetida , can effectively remove PAHs from contaminated soils, achieving up to 99% removal of phenanthrene and 91% of anthracene. Their gut microorganisms also contribute significantly to the degradation of these persistent organic pollutants (Contreras-Ramos et al., 2008). The bioturbation by earthworms likely facilitated oxygen diffusion and redistributed labile organic matter and nutrients, thereby stimulating microbial degradation. Studies have shown that earthworms can increase microbial biomass and enzyme activity in hydrocarbon-contaminated soils, leading to degradation rates of up to 93% under favorable conditions (Martinkosky, 2015; Gbarakoro & Sikoki, 2023). The observed increase in nitrogen, phosphorus, and potassium concentrations supports this interpretation. However, the effects of T3 varied between soils and were somewhat less predictable. In compacted, less porous soils (C1), earthworm movement may have been restricted, reducing their ecological impact. Furthermore, their influence on heavy metal dynamics was inconsistent, with some elements (e.g., Pb, Cu) showing increases—possibly due to bioaccumulation and release via castings (Rorat et al., 2017). While T3 alone may not be sufficient for full remediation, its biological benefits are substantial when used in integrated approaches. For example, Ameen and Al-Homaidan (2024) demonstrated synergistic effects when earthworms were co-applied with microbial inoculants, resulting in >58% TPH reduction. 4.4. Microbial activity as an indicator of remediation Dehydrogenase activity (DHA) proved to be a sensitive and integrative indicator of remediation progress. Strong positive correlations were found between DHA and nutrient availability (notably P, S, and Mg), and negative correlations with TPH (r = –0.91), pH (r = –0.79), and heavy metals (r = –0.74 to –0.81), suggesting that microbial activity is driven by both chemical and physical improvements in soil conditions. These findings are supported by Shi et al. (2023), who found that biochar-enhanced microbial habitats increased DHA and hydrocarbon degradation. Similarly, Fernández-Luqueño et al. (2017) emphasized that the combination of organic and mineral amendments improves microbial community resilience and enzymatic function. The results validate DHA as a practical biomarker for monitoring bioremediation in hydrocarbon-contaminated environments and reinforce the importance of designing amendments that promote microbial function. 4.5. Multivariate insights for integrated approaches Principal Component Analysis (PCA) and Hierarchical Cluster Analysis (HCA) provided robust multivariate validation of treatment performance. The principal component analysis (PCA) is widely used for identify key factors influencing soil quality and pollutant degradation, such general distribution patterns or similarities of individual PAH components (Golobočanin et al. 2004) or to test the effects of individual and co-application of biochar and inorganic fertilizer on soil quality (Faloye et al. 2024) T1 treatments clustered separately due to their high remediation efficiency, biological stimulation, and nutrient enrichment. In contrast, T2 and control soils formed a cluster characterized by limited biological improvement and stronger geochemical influence. T3 treatments formed an intermediate group with moderate performance, primarily biological. This clustering reflects the mechanistic divergence of treatments—chemical-biological synergy in T1, physical sorption in T2, and bioturbation in T3. Radar plot visualization further highlighted T1’s balanced performance across six critical soil health indicators, supporting its suitability for multifunctional soil restoration. These findings underscore the value of combining amendments with complementary mechanisms. As noted in studies by Aziz et al. (2020) and Faloye et al. (2024), the co-application of biochar with organic or mineral inputs can enhance both chemical stabilization and microbial degradation. In highly degraded, multi-contaminated soils, such integrated approaches may be necessary to restore both ecological function and land productivity. Finally, the reuse of sewage sludge to create high-value fertilizers such as T1 contributes to sustainable waste management and circular economy goals (Kacprzak et al., 2022; Gusiatin et al., 2024; Buta et al., 2021), offering an environmentally responsible pathway to rehabilitate contaminated soils. 5. Conclusion This study compared three remediation strategies—organic–mineral fertilizer (T1), biochar (T2), and vermiremediation with Eisenia fetida (T3)—in petroleum-contaminated, alkaline clay soils. All treatments improved soil quality to varying degrees. T1 was the most effective, achieving the highest reductions in TPH (63.5%) and PAHs (58.8%), improving nutrient availability (P, Mg, S), enhancing microbial activity (DHA), and buffering pH toward neutrality. Biochar (T2) excelled in heavy metal immobilization and carbon stabilization but showed limited biological stimulation. T3 moderately enhanced DHA and nutrient cycling, with greater impact in more porous soils. Multivariate analyses (PCA, correlation, clustering) confirmed T1’s superior, well-balanced remediation performance. Results highlight the benefits of combining microbial support, nutrient delivery, and chemical stabilization. Integrated strategies—such as combining T1’s bioactivity with T2’s metal sorption or T3’s biological structuring—may offer more effective and sustainable soil restoration solutions. A key future challenge lies in translating laboratory successes into field-scale, long-term solutions. Soil variability, environmental fluctuations, and the persistence of complex contaminants may limit the effectiveness of single-component treatments. Multi-component approaches—such as combining nutrient-rich organic–mineral fertilizers with biochar—offer promising synergies by uniting biological stimulation, nutrient delivery, pH correction, and metal immobilization. However, optimizing the composition, application rates, and interactions of such hybrid products under diverse soil conditions remains an open research frontier. Further studies are needed to evaluate their agronomic safety, environmental stability, and cost-effectiveness for large-scale remediation and land restoration. 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Hejabi. 2021. “Biochar as a sustainable product for remediation of contaminated soil.” Current Research in Green and Sustainable Chemistry 4: 100055 https://doi.org/10.1016/j.crgsc.2021.100055 Information & Authors Information Version history V1 Version 1 12 July 2025 Peer review timeline Published Land Degradation & Development Version of Record 28 Jan 2026 Published Copyright This work is licensed under a Non Exclusive No Reuse License. Keywords eisenia fetida biochar bioremediation organic–mineral fertilizer petroleum hydrocarbons Authors Affiliations Małgorzata Kacprzak 0000-0002-3897-8659 [email protected] Politechnika Warszawska Filia w Plocku Wydzial Budownictwa Mechaniki i Petrochemii View all articles by this author Sławomir Kaczmarek Uniwersytet im Adama Mickiewicza w Poznaniu Wydzial Chemii View all articles by this author Iwona Kupich Politechnika Czestochowska View all articles by this author Metrics & Citations Metrics Article Usage 256 views 144 downloads .FvxKWukQNSOunydq8rnd { width: 100px; } Citations Download citation Małgorzata Kacprzak, Sławomir Kaczmarek, Iwona Kupich. Comparative assessment of organic--mineral fertilizer, biochar, and vermiremediation for petroleum-contaminated alkaline soils. Authorea . 12 July 2025. DOI: https://doi.org/10.22541/au.175230022.22803261/v1 If you have the appropriate software installed, you can download article citation data to the citation manager of your choice. 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