<p>Landfill leachate treatment systems experience strong physicochemical fluctuations, yet microbial succession across sequential full-scale treatment compartments remains insufficiently resolved. Here, we investigated a full-scale upflow anaerobic sludge blanket (UASB)-two-stage anoxic/oxic (A/O) landfill leachate treatment system using 16S rRNA gene sequencing, physicochemical profiling, ordination analyses, environmental-driver analysis, null and neutral community models, FastST-based source tracking, and co-occurrence network analysis. Weighted UniFrac-based PERMANOVA showed that both treatment compartment and season were significantly associated with microbial community structure, with treatment compartment explaining 34.21% of the variation and season explaining 29.87%. However, significant multivariate dispersion among compartments (PERMDISP, P = 0.032) indicated that treatment-stage effects should be interpreted cautiously. Taxonomic turnover was pronounced along the treatment continuum, with Methanosaeta enriched in UASB-related samples and nitrogen-cycling taxa such as Nitrospira and Thauera enriched in downstream oxic tanks. Environmental analyses identified pH, total nitrogen, salinity, and C/N ratio as major correlates of community variation. Assembly analyses indicated that stochastic processes contributed substantially across the system, whereas the ANin community showed the weakest neutral-model fit (R² = 0.013) and stronger departure from neutral expectations. FastST estimated seasonally variable upstream contributions to AO2, and co-occurrence analysis indicated higher inferred topological robustness in the second-stage oxic tank. Overall, these results support a stage-dependent ecological framework in which microbial succession is shaped by environmental filtering, upstream source contributions, and local community reassembly, while functional and interaction-level interpretations remain constrained by marker-gene-based and correlation-based inference.</p>