Development and evaluation of fly ash–slag composite substrates for greening and their effects on plant growth

​［Objective］Fly ash and Slag are among the major industrial solid wastes in China, and their long-term stockpiling has exerted considerable pressure on the ecological environment. However, owing to their porous structure, high specific surface area, and enrichment in mineral nutrients, these materials exhibit considerable potential for improving soil properties and enhancing substrate performance. Therefore, exhibiting considerable potential for application in soil improvement, enhancement of substrate properties, and vegetation restoration for soil and water conservation.［Methods］A pot experiment using Campsis grandiflora was carried out based on an L16 (4²) orthogonal design, with the fly ash–to–slag mass ratio (4:1–1:1) and soil addition rate (30%–60%) as two experimental factors. All treatments were established under uniform substrate composition and mass, with a commercial sprayed greening substrate without solid waste addition serving as the control (CK). Plant growth indicators, including plant height and biomass, were measured periodically. After the experiment, destructive sampling was performed to determine substrate organic matter and nitrogen and phosphorus contents. Analysis of variance and structural equation modeling (SEM) were used to elucidate the relationships between substrate nutrient characteristics and plant growth responses across treatments.［Results］The results show that: 1) Under the present experimental conditions, substrates formulated with a fly ash–to–slag ratio of 3:1 in combination with 60% soil exhibited the most favorable overall performance,improving substrate organic matter and total nitrogen and phosphorus contents, thereby promoting plant height and biomass. The A1–B1 combination increased ammonium and nitrate nitrogen contents, whereas the fly ash–to–slag ratio of 3:1 with a low soil addition rate (30％) enhanced available phosphorus supply. The fly ash–to–slag ratio of 3:1 and soil addition rate (60％) favored nitrogen and phosphorus accumulation in plant tissues, and moderate soil addition rates (50％ and 40％) showed positive effects on plant growth.A substrate with a relatively dominant proportion of fly ash and a moderate slag content (3:1), combined with a higher soil addition rate (60％), effectively improved the physical structure of the substrate and enhanced the retention of organic matter as well as nitrogen and phosphorus, thereby providing a stable and suitable nutrient environment for the growth of C. grandiflora；Excessive slag proportions or insufficient soil addition were unfavorable for nutrient accumulation and plant growth. 2）Fly ash and slag influenced plant root length through both direct inhibitory and indirect promotive pathways, mediated by changes in substrate phosphorus availability.Structural equation modeling showed that fly ash and slag had dual effects on C. grandiflora growth, with direct inhibition and indirect promotion through improved nitrogen availability and organic matter–nitrate nitrogen transformation.［Conculsions］This study elucidates the mechanisms by which fly ash and slag regulate plant growth by modifying substrate physicochemical properties and nutrient forms, highlighting their synergistic effects on substrate improvement and nutrient retention. The feasibility of applying fly ash and slag in artificial substrate construction and vegetation restoration was confirmed, providing a theoretical basis for their resource utilization in mine ecological restoration and soil and water conservation engineering.