Nitrogen removal is of great importance in management of sanitary landfills. To optimize the process of denitrification, a better comprehension of denitrifiers was needed. No molecular studies using function genes with a special focus on diversity and quantity of denitrifying communities in municipal solid waste (MSW) have been reported to date. This research investigated the communities of denitrifying bacteria in MSW samples collected from fresh (3-month-old) and aged (10-year-old) landfill bioreactors. The main methods were denaturing gradient gel electrophoresis and real-time polymerase chain reaction using nirK and nirS as the markers. Results showed that most nirK-containing denitrifiers related to the known Rhizobiales bacteria were from the aged MSW sample, while denitrifiers related to Alcaligenaceae bacteria were all from the fresh one. A clear difference in the diversity of nirK-containing denitrifiers between two MSW samples was observed. About half of the nirS gene sequences obtained was found closely related to the nirS of β-proteobacterium (Burkholderiales and Rhodocyclales) and γ-proteobacterium (Pseudomonadales). The rest of the nirS gene sequences were specific. Real-time polymerase chain reaction results revealed that nirK was more abundant than nirS in the fresh MSW sample, whereas nirS was predominant in the aged one. Aged MSW had more potential denitrifiers to reduce NO3− to NO2− than fresh MSW. This work provided more accurate information about the characteristics of denitrifying communities as a result of using functional gene, rather than using 16S rDNA.
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