Microbial community divergence and environmental responses across multi-phase landfill environments.

Municipal solid waste landfills are highly heterogeneous ecosystems comprising solid waste, leachate, and subsoils wherein complex microbial consortia regulate organic matter degradation and contaminant transformation. However, comprehensive insights into their microbiome structure across multi-phase and depth profiles and responses to environmental gradients remain scarce. This study presents a rare, multidimensional characterization of landfill microbiomes that integrates deep drilling (to 50 m), 16S rRNA gene sequencing, and biogeochemical pathway analysis. To quantify the extent of waste stabilization, a solid-phase stabilization index (β) is proposed to link degradation stages with microbial succession. This index indicates a clear transition from a Firmicutes-dominated rapid degradation phase (β < 0.58) to a Proteobacteria-dominated stabilization phase (β > 0.83). Spatially, the microbiomes exhibit distinct solid-liquid niche partitioning, as evidenced by the prevalence of biofilm formers (Advenella and Brevundimonas) on solid waste surfaces and specific enrichment of the thermophilic planktonic Defluviitoga in the surrounding leachate. At the critical waste-soil interface, leachate infiltration exerts strong environmental filtering that drives a pronounced enrichment of the dual-tolerant Ralstonia, which constitutes up to 46.49% of the community. Cu, Be, and Cd emerge as the key heavy metals driving the evolution of subsoil microorganisms. These findings collectively provide an integrated framework that advances the mechanistic understanding of waste stabilization and leachate-soil interactions, offering new insights for assessing landfill maturity and understanding pollution fronts.