Sustainable Valorization of Coal Gangue into a Planting Substrate: Effects of Multiple Amendments on Ryegrass Growth and Pb Leaching

Coal gangue accumulation causes land occupation and environmental risks in mining areas, but its use as a planting substrate offers a pathway for ecological restoration. This study optimized a coal gangue-based planting substrate by integrating phosphogypsum, spent mushroom substrate, biochar, and a water retention agent. Using a three-factor, four-level orthogonal design with a fixed value of 10% phosphogypsum, the study assessed the effects of the coal gangue-to-spent mushroom substrate ratio, biochar, and water retention agent on substrate properties, nutrient availability, plant growth, and Pb leaching. The results showed that the tested formulations maintained substrate pH within a near-neutral range across treatments. The coal gangue-to-spent mushroom substrate ratio was the dominant factor controlling substrate structure, water-holding capacity, nutrient status, and ryegrass growth. Increasing the spent mushroom substrate proportion reduced bulk density and improved field water capacity, pore structure, and nutrient availability. Biochar most effectively reduced Pb leaching, although excessive application inhibited growth, whereas the water retention agent mainly enhanced water retention and plant growth. The optimal formulation was 80% coal gangue, 20% spent mushroom substrate, 1% biochar, 1.5 g kg−1 water retention agent and 10% phosphogypsum, providing a theoretical and practical optimization basis for coal gangue reutilization and ecological restoration in mining areas.