Nutrient reconstruction and evaluation of heavy metal environmental behavior in coal gangue–fly ash vegetation substrates

The coordinated utilization of coal-based solid wastes and organic waste-derived amendments offers a potential route for vegetation substrate reconstruction in mining areas. In this study, coal gangue, fly ash, and distillery-sludge-derived nutrient soil were used to prepare reconstructed substrates with coal gangue-to-fly ash ratios of 6:4, 7:3, and 8:2. Their effects on nutrient reconstruction and heavy metal environmental behavior were evaluated. The nutrient soil provided the main source of organic matter and nutrients, whereas coal gangue and fly ash regulated substrate structure and chemical properties. R1, with a higher fly ash proportion, showed stronger mineral nutrient supply, while R3, with a higher coal gangue proportion, favored organic matter and alkali-hydrolyzable nitrogen maintenance. Total-concentration-based assessment identified Cd as the dominant contributor to the solid-phase pollution index. However, DTPA-extractable Cd and Ni increased after planting, whereas available As decreased, demonstrating that total concentrations alone cannot fully characterize metal mobility and availability. Leachate analysis further identified As and Ni as the main migration-sensitive elements, indicating that solid-phase accumulation and liquid-phase release were governed by different processes. Overall, R2 represented an intermediate compromise between nutrient supply and environmental-risk control among the tested ratios. These results support the coordinated use of coal gangue, fly ash, and fermented distillery-sludge-derived nutrient soil for non-food vegetation substrate reconstruction, while emphasizing the need to jointly evaluate nutrient improvement, total heavy metal contents, available fractions, and leaching release before practical application.