Ecological restoration of bauxite residue disposal areas remains challenging due to severe salinity–alkalinity stress and limited nutrient availability. Nitrogen accumulation and transformation are fundamental processes controlling the transition of bauxite residue from industrial waste into a functional soil-like substrate; however, the coupling between nitrogen cycling and microbial community assembly remains poorly understood. In this study, a 9-month field simulation experiment was conducted to investigate the effects of organic manure amendment combined with Penicillium oxalicum inoculation on nitrogen transformation and microbial ecological succession in bauxite residue. By integrating nitrogen pool analysis, salinity–alkalinity characterization, extracellular enzyme stoichiometry, and microbial community profiling, we revealed the temporal mechanisms linking environmental improvement, microbial assembly, and nitrogen cycling. Total nitrogen increased by 79.8–146.0% during remediation, with high manure combined with P. oxalicum treatment showing the strongest nitrogen retention capacity. Nitrate nitrogen continuously accumulated, whereas ammonium nitrogen exhibited a transient peak during the intermediate stage, reflecting dynamic transitions among mineralization, nitrification, and nitrogen fixation processes. Microbial resource limitation shifted from persistent nitrogen limitation toward carbon–nitrogen co-limitation during late remediation. Microbial communities showed strong stochastic assembly characteristics, while nutrient availability and salinity–alkalinity stress regulated early succession. Nitrogen fixation potential increased by 1.48–5.74-fold, whereas denitrification remained the dominant nitrogen transformation pathway. These findings demonstrate that functional microbial inoculation combined with organic amendment can reconstruct nitrogen cycling processes and enhance ecological resilience of mining-induced geo-environmental hazards.
 

Bauxite residue; Nitrogen transformation; Microbial community assembly; Ecological restoration; Penicillium oxalicum