This study investigated microbial population dynamics and performance in lab-scale conventional, anaerobic, and aerobic landfill bioreactors specialized for high-organic wastes. Each reactor (2.35 l) was loaded with 1.5 kg of organic solid waste made of sludge cake, dry dog food, and wood chips. The conventional reactor was operated without leachate recirculation and aeration, but the other reactors used leachate recirculation at 200 ml/d and without aeration (anaerobic bioreactor) or with aeration at 2 l/min (aerobic bioreactor). The respective final waste volumes on day 138 of the conventional, anaerobic, and aerobic reactors were approximately 75%, 65%, and 60% of the initial volumes. Leachate recirculation in the anaerobic bioreactor accelerated biochemical reactions and promoted methane production. However, leachate from the anaerobic bioreactor showed TOC and NH4+-N concentrations that were as high as those of the conventional reactor. Aeration lowered leachate production and methane concentration and decreased organic matter in solid waste and leachate. Furthermore, the MPN value of amoA gene reached 10(5) MPN-copies/g-dry in the aerobic bioreactor, where nitrogen was removed from organic solid waste and leachate. During the first 72 d, the aerobic bioreactor's MPN value of fungal 18S rDNA was the highest among reactors, but it decreased gradually. All reactors showed similar MPN values of eubacterial 16S rDNA, nirS, and nirK.