1328 Figure 2 indicates that pak choi is able to grow in all substrates, with a generally similar trend in plant height over time. The growth is relatively slow in 10 d-20 d, accelerates in 20 d-30 d, and then slows again in 30 d-40 d. The trend aligns with the typical growth cycle of pak choi. This suggests that the microbial fertilizer promotes the conversion of nutrients, ensuring rapid crop growth. However, due to the varying ratios of microbial fertilizer added, differences in growth rate are observed among the substrates. The overall trend in plant height follows R8 > CK > R5 > R2, R9 > R6 > R3, and R7 > R4 > R1, with the plant height of R5 close to that of the CK, and that of R8 slightly larger than that of the CK. This indicates that in the same ratio, a higher ratio of microbial fertilizer results in a better effect. When the ratio of microbial fertilizer is fixed, but the ratio of other substrates varies, the growth performance trend follows R2 > R3 > R1, R5 > R6 > R4, and R8 > R9 > R7. This demonstrates that a 1:1 ratio of fine-grained gangue to tailing coal slime provides the most favorable conditions for crop growth. 4.2 Analysis of Properties of Different Substrates Before and After Crop Planting The pak choi stubbles were turned over and returned to the field after harvest. After one year of planting, the pH value, organic matter, available nitrogen, and rapidly available phosphorus in each substrate were tested, and the changes in nutrient elements in the soil of different substrates before and after crop planting were analyzed. 4.2.1 pH and organic matter content of different substrates before and after crop planting Before pak choi planting, the substrates are weakly alkaline, with a pH of 7.43-8.52, and the CK is neutral; after one year of planting, R1-R9 are neutral to weakly alkaline, with a pH of 7.5-8.6, decreasing by 0.01, 0.03, 0.02, 0.06, 0.10, 0.07, 0.07, 0.15, and 0.10, respectively. Before planting, since microbial fertilizer and biomass powder are weakly acidic, the pH of the substrates gradually decreases with the increase in the ratio of microbial fertilizer added, indicating that microbial fertilizer is conducive to regulating the acidity and alkalinity of the substrates. After planting, the organic matter in the coal gangue generates certain organic acids and humic acids through decomposition, further reducing the pH value of the substrates. Figure 3 - Organic matter content of different substrates before and after pak choi planting Figure 3 shows the organic matter content of different substrates before and after pak choi planting. The organic matter content of substrates before planting is greater than that of the CK. With the increase of mining waste, the organic matter content of each substrate increases accordingly. The organic matter content of the substrates after planting generally increases compared with that before planting. Due to the difference in the formulation of substrates, the increase in organic matter after planting intensifies with the increase in the ratio of microbial fertilizer added. The organic matter content of substrates R1-R9 and CK increases by 1.98 g/kg, 4.06 g/kg, 3.74 g/kg, 4.73 g/kg, 6.25 g/kg, 5.88 g/kg, 7.25 g/kg, 8.96 g/kg, 7.78 g/kg, and 5.17 g/kg, respectively, among which that of R8 increases by the maximum amount. On the one hand, microbial fertilizer and biomass powder supplement a large amount of organic matter to the substrates, provide carbon sources for microorganisms' decomposition to mineral nutrients, and promote microorganisms' humification of coal gangue to form new organic matter. Meanwhile, the exudates and stubbles from the pak choi roots also contribute additional organic matter to the soil. On the other hand, the humification process of organic matter affects the diversity of microbial communities in the substrates and the turnover of organic matter, promoting the formation of stable aggregate structures of soil particles, and accelerating the decomposition rate and forward circulation of organic matter.
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