Synergistic optimization of soil layers and AMF inoculation enhances water and salt balance to boost alfalfa growth in mining dumps

Ecological restoration in arid coal mining regions of western China faces challenges from severe water scarcity and high soil salinity. This study investigates the combined effects of soil layer reconstruction and microbial remediation on soil water retention and salinity management to promote plant growth. Using an indoor, simulated soil column test, we employed a model with ecological, culvert, and waterproof layers, introducing interlayers of 0, 4, 7, and 10 cm thickness between the culvert and ecological layers. The ecological layer serves as the root zone for plant growth, the culvert layer stores irrigation water and precipitation, and the aquiclude prevents water loss. We tested three conditions for each interlayer thickness: bare soil (CK), alfalfa (Medicago sativa L.) growth (AL), and alfalfa with arbuscular mycorrhizal fungi (AMF). Results showed that interlayer thickness significantly (p < 0.05) influenced soil water and salt distribution. The soil water content in the ecological layer decreased by 12% and 16% under the 7 and 10 cm treatments but increased by 4% with a 4 cm interlayer. Surface soil salt accumulation was reduced by 40% with interlayer treatments. AMF inoculation enhanced alfalfa root development, increasing underground biomass by 2.1 times and boosting water content by 10.5% and salt content by 15.8% in the absence of an interlayer. The AMF + 4 and 7 cm interlayer treatments reduced surface soil salinity by 47% and 41%, respectively, and increased water use efficiency by 42% and 56%, respectively, with aboveground biomass increasing by 96% and 70%. These findings provide strategies for water conservation and salt control in the ecological restoration of mining dumps in arid regions.