Managing nitrogen and carbon cycles in landfills is an environmental challenge. In this study, our purpose was to test two types of methane oxidation processes coupled to denitrification inside landfills: microaerobic and hypoxic methane oxidation coupled to denitrification (MAME-D and HYME-D). Leach bed bioreactors were designed and operated for >100？d with NO3--N concentration ranging from 100 to 400？mg？N/L. During six runs of the leach bed bioreactor experiment, leach bed bioreactor 2 (MAME-D) reached 100% denitrification efficiency and the highest average specific denitrification rate of 20.36？mg？N/(L·d) in run 5, while leach bed bioreactor 3 (HYME-D) achieved 75% denitrification efficiency and the highest average specific denitrification rate of 8.09？mg？N/(L·d) in run 6. Subsequently, waste from leach bed bioreactors 1, 2, and 3 was inoculated into anaerobic bottles to run a batch experiment for 13？d. The total consumed methane, oxygen, and nitrate amounts in the microaerobic system with no methane and oxygen supplement were 2.33, 2.38, and 2.04？mmol, respectively, which almost matched the theoretical equation of aerobic methane oxidation coupled to denitrification. In the hypoxic system, the total consumed methane and nitrate amounts were 0.23 and 0.41？mmol, respectively, the ratio of which closely matched the HYME-D. In addition, via the diverse functional community analysis, methane oxidation in the microaerobic system was confirmed to be conducted by methanotrophs (i.e., Methylobacter and Methylomonas) using oxygen as an electron acceptor. Subsequently, the generated organic compounds could support denitrifiers (i.e., Methylophilaceae) to complete denitrification. In the hypoxic system, Methylomonas and Methylobacter utilized nitrate as a direct electron acceptor to oxidize methane. The two landfill processes characterized here will expand our understanding of the environmental role of methanotrophs and methylotrophs in both carbon and nitrogen cycles.
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