1325 Microbial decomposition and humification are inherently slow processes. Therefore, continuous screening and optimization of microbial strains are required to ensure their effective integration, survival, and reproduction within the two types of solid waste. The objective is to convert the mineral elements in waste from a solid state into a free state absorbable by plants and enable the effective release of such beneficial trace elements, thereby realizing thorough activation and rapid decomposition of beneficial mineral elements in mining waste, while passivating heavy metals and balancing pH levels. Wang Hanxia[6] found in her research that mixed microbial strains exhibited superior decomposition performance compared to single-strain systems. Liu Ziwei[7] compared the effects of single-, dual-, and triple-microbial inoculants on the improvement of coal gangue-based substrates, and concluded that dual-microbial inoculants yielded better results for mining area soil than a single-microbial inoculant. Traditional aerobic fermentation takes 7-15 days. In the study, phosphate-solubilizing bacteria, thermophilic bacteria, and nitrogen-fixing bacteria suitable for decomposing coal gangue and capable of proliferating vigorously in forestry and agricultural residues were selected to form a composite microbial inoculant, and efficient microbial strains were cultivated and screened through optimal selection and cultivation. Pruned tree branches from the site of CCTEG Tangshan Research Institute, used as experimental materials, were crushed using specialized equipment and fermented efficiently into microbial fertilizer in the fermentation equipment. By measuring changes in nutrient elements and organic matter in the microbial fertilizer, the optimal inoculant amount ratio, optimal fermentation temperature, and optimal fermentation time were determined for complete fermentation of the microbial inoculant. The experimental results indicate that the complete fermentation of forestry, agricultural, and livestock residues is achieved within 24 hours at a composite inoculant amount ratio of 1:10 and a fermentation temperature of 60 °C-80 °C, and all indicators of the produced organic microbial fertilizer meet the national standards for organic fertilizers in China. The product is rich in organic microbial content and exhibits high activity. When subsequently co-processed with coal gangue, it enables the rapid activation and release of nutrient elements from the coal gangue. 3. COAL GANGUE GRADATION AND RAW MATERIAL BLENDING The coal gangue and tailing coal slime used in the experiment were sourced from Tangshan Mine of Kailuan Group. Through testing, the heavy metal content is below the screening limit for soil contamination risk specified by the Soil Environmental Quality – Risk Control Standard for Soil Contamination of Agricultural Land (GB 15618-2018). There are significant differences in the physicochemical properties of coal gangue with different particle sizes. Previous studies[8-9] have shown that an appropriate gradation of coal gangue can improve the substrate's bulk density and porosity, thereby ensuring aeration and promoting crop growth. This study has shown that coal gangue with different particle sizes plays distinct roles in soil utilization. Tailing coal slime (- 0.5 mm) has a large specific surface area and very good water retention, fertilizer retention, and moisture conservation performance, but it is viscous and has poor permeability. Fine-grained gangue (0.5-6 mm) can increase permeability, but has poor moisture conservation. Large gangue (+6 mm) cannot form a stable soil structure when used directly, and has slow rate of activation and decomposition, which is not conducive to plant growth. Therefore, in the soil utilization of coal gangue, particle sizes below 6 mm are selected for gradation. -6 mm coal gangue is divided into tailing coal slime (-0.5 mm) and fine-grained gangue (0.5-6 mm) according to the particle size. In Table 1, fine-grained gangue, tailing coal slime, soil, plant powder, and microbial fertilizer are mixed in a certain ratio for orthogonal test to form 9 different substrates, designated as R1-R9. Soil collected from the experimental site is used as the control check (CK).
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