Integrated application of agriculture waste Biochar and Rhizobium for the restoration of acidic mine overburden soil: A biogeochemical approach

Introduction

Coal mining generates large quantities of overburden (OB) material that are typically deposited as dumps, leading to severe land degradation. These soils are often characterized by low pH, poor organic carbon, nutrient deficiency, and elevated concentrations of heavy metals. Acidification mainly occurs due to the oxidation of sulfide minerals such as pyrite, producing sulfuric acid and creating unfavorable conditions for plant growth and microbial activity. The given OB soil (pH 5.1, OC 0.15%, ORP +420 mV) represents a highly oxidized and biologically constrained system. Biochar has emerged as an effective soil amendment for restoring degraded soils due to its alkaline nature, high surface area, and functional groups that enhance nutrient retention and immobilize heavy metals. Cotton-derived biochar contributes stable carbon, while pressmud biochar, a sugar industry by-product, is rich in calcium, phosphorus, and potassium, improving soil fertility and buffering capacity. In addition, microbial inoculants such as Rhizobium play a critical role in nitrogen fixation and stimulation of soil biological activity. The integration of biochar with beneficial microorganisms offers a synergistic approach to soil remediation by combining physicochemical stabilization with biological enhancement. This study aims to evaluate the combined effect of cotton biochar, pressmud biochar, and Rhizobium on improving soil properties, reducing heavy metal availability, and enhancing microbial activity in coal mine OB soil over a 6-week period.

Literature Review

Biochar is widely used for improving degraded soils due to its ability to increase pH, enhance cation exchange capacity, and immobilize heavy metals through adsorption and complexation (Lehmann and Joseph, 2015;Ahmad et al., 2014). Cotton-derived biochar provides stable carbon, improves soil structure, and supports microbial activity, thereby enhancing nutrient cycling (Zhang et al., 2022) [1, 3, 7, 12] .

Pressmud biochar, rich in calcium and nutrients, improves soil fertility and buffering capacity while facilitating the immobilization of heavy metals such as lead and zinc (Srinivasan et al., 2021). Microbial inoculants like Rhizobium contribute to nitrogen fixation and enhance soil biological activity, particularly when supported by biochar as a favorable habitat (Peoples et al., 2009) [5, 6] . Recent studies indicate that the combined use of biochar and microbes produces synergistic effects, significantly improving soil physicochemical properties and accelerating the remediation of contaminated soils (Palansooriya et al., 2019;Gadd, 2010) [2, 4] .

Materials 3.1 Soil Sample:

Coal mine overburden (OB) soil was collected from an abandoned dump site. The sample was air-dried at room temperature, homogenized, and sieved through a less than 2 mm mesh to remove debris and ensure uniformity. The soil was stored in clean polyethylene containers prior to experimentation.

Biochar Materials Cotton Biochar (CB)

Cotton stalk residues were used as feedstock and subjected to slow pyrolysis at 450-500°C under limited oxygen conditions. The resulting biochar was ground and sieved (<2 mm).

Key properties

 Alkaline pH (~8.5)  High fixed carbon (~65-70%)  Moderate surface area (200-300 m²/g)  Functional groups: hydroxyl (-OH), carboxyl (-COOH)

Pressmud Biochar (PB)

Pressmud obtained from a sugar processing unit was pyrolyzed at 400-450°C and processed similarly.

Key properties

 Strongly alkaline (pH ~9-9.5)  Rich in CaCO₃, phosphates, and potassium  High cation exchange capacity  Higher ash content than cotton biochar

Microbial Inoculum

A pure culture of Rhizobium was obtained from a microbiology laboratory and maintained in yeast extract mannitol broth. The inoculum density was adjusted to approximately 10⁸ CFU mL⁻¹ before application.

Chemicals and Instruments

 Analytical grade reagents  Distilled water  Digital pH meter  Electrical conductivity (EC) meter  Spectrophotometer  Flame photometer  Atomic Absorption Spectrophotometer (AAS)  Incubation trays (5 L capacity)

Initial Soil Parameters

The untreated coal mine OB soil exhibited the following characteristics: These values indicate acidic conditions, low fertility, and moderate heavy metal contamination.

Experimental Design:

A completely randomized design (CRD) was followed with six treatments: Interpretation: Initial rapid increase (week 1-2) due to alkaline buffering; gradual stabilization thereafter. Pressmud biochar (T 3 , T 6 ) shows stronger pH correction due to CaCO₃ content. Interpretation: T 1 remained almost constant with very low carbon levels. T 2 and T 3 showed gradual increases, while T 5 and T6 recorded the highest carbon buildup. This reflects the contribution of biochar and microbial activity. Interpretation T 1 showed negligible improvement in nitrogen content. T 4 increased due to microbial action, while T 5 and T 6 showed the highest nitrogen levels. This confirms the role of Rhizobium in nitrogen enhancement. Interpretation T 1 remained nearly unchanged over time. T 2 and T 3 showed moderate increases, while T 5 and T 6 recorded the highest phosphorus levels. Pressmud-based treatments performed better due to inherent phosphate content. Interpretation: Increase due to mineral release from biochar ash; PB shows highest due to K-rich composition.

Graph 4: Potassium Dynamics in Biochar-Amended Mine Soil Interpretation; Reduction due to adsorption and precipitation; higher reduction in combined treatments. Interpretation; Reduction indicates decreased soluble salts and improved soil condition. Interpretation T 1 showed almost no microbial growth. T 2 and T 3 increased moderately, while T 4 , T 5 , and T 6 showed rapid growth, with T 6 highest. This reflects enhanced microbial activity due to biochar and inoculation. Interpretation T 1 showed only slight decrease in ORP. T 2 and T 3 reduced moderately, while T 5 and T 6 showed the greatest decline. Lower ORP indicates improved biological and less oxidizing soil conditions. Tray T 1 showed only a slight reduction in bulk density, indicating limited structural improvement. In contrast, trays T 2 and T 3 exhibited moderate decreases, while T 5 and T6 showed the greatest reduction, reaching optimal levels by week 6. This trend highlights that combined treatments were most effective in improving soil structure and reducing compaction.

Discussion

The results indicate that biochar and Rhizobium significantly improved the quality of acidic coal mine overburden soil within 6 weeks. The increase in pH from 5.1 to near neutral values was mainly due to the alkaline nature of biochar, particularly pressmud biochar rich in calcium compounds. Organic carbon and nutrient levels (N, P, K) increased substantially, reflecting both biochar contribution and enhanced microbial activity. The reduction in heavy metals (Fe, Pb) suggests effective immobilization through adsorption and precipitation mechanisms. Decreases in electrical conductivity and oxidation-reduction potential indicate improved soil conditions favorable for biological processes. The higher microbial population in combined treatments confirms that biochar provides a suitable habitat, while Rhizobium enhances nutrient cycling. Overall, the combined application, especially pressmud biochar with Rhizobium, showed the most effective remediation due to synergistic interactions between chemical stabilization and microbial activity.

Conclusion

This study confirms that the combined application of biochar and Rhizobium is an effective strategy for the remediation of acidic coal mine overburden soil. Significant improvements were observed in soil pH, organic carbon, nutrient availability, and microbial population, along with a substantial reduction in heavy metal concentrations. Pressmud biochar, due to its nutrient-rich composition and high buffering capacity, proved more effective than cotton biochar, particularly when combined with microbial inoculation. The integrated treatment not only improved soil fertility but also created a more favourable environment for biological activity, accelerating the restoration process. Overall, the findings highlight that biochar microbial synergy offers a low-cost, environmentally sustainable, and scalable approach for reclaiming degraded mine lands. This method has strong potential for practical application in mine site rehabilitation and sustainable land management.

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