Effects of Coal Gangue–Decomposed Straw Composite Substrates on Soil Properties and Plant Growth for Revegetation of Degraded Open-Pit Mine Slopes

Abandoned open-pit coal mine slopes are compacted, infertile and water-limited, hindering vegetation restoration. This study evaluated the combined use of two waste materials—coal gangue (an alkaline, mineral-rich mining residue) and decomposed straw (a nutrient-rich agricultural residue)—as a composite substrate for restoring degraded slope soil. In a 60-day greenhouse pot experiment, slope soil from the Haizhou Open-Pit Coal Mine (Fuxin, China) was amended with coal gangue (5%, 10% or 15% by dry mass), decomposed straw (1%, 3% or 5%), or their nine full-factorial combinations (16 treatments in total, including an unamended control, each with three replicates), and sown with three herbaceous species (ryegrass, alfalfa and tall fescue). Soil physical and chemical properties and plant growth traits were measured. Relative to the unamended control, the composite substrates improved soil structure and fertility: soil water content and field water-holding capacity were highest under the 10% gangue + 5% straw treatment (25.6% and 25.89%, increases of 28.0% and 29.5%), bulk density was lowest, with the correspondingly highest total porosity, under the 15% gangue + 3% straw treatment (1.35 g/cm3, a 14.9% decrease). Organic matter (9.42 g/kg), total nitrogen (0.71 g/kg, +65.1%), available nitrogen (45.3 mg/kg, +33.0%) and available potassium (106.44 mg/kg, +26.2%) were highest under the 5% gangue + 5% straw treatment, which also gave the best growth of ryegrass and alfalfa (alfalfa aboveground and underground dry weight increased by 221% and 188%, respectively), whereas tall fescue grew best under the 5% gangue + 3% straw treatment. These results indicate that a 5% coal gangue + 5% decomposed straw composite substrate is a promising amendment for revegetating degraded open-pit slopes, although field validation is still required.