Tropical peat swampy forests, distributed throughout Southeast Asia, precipitate a large mass of carbon as woody peat soil. This peatland ecosystem with a thick peat layer (2-10 m) is relatively stable ecosystem as long as it is forest, but once the forest was reclaimed and drained ground water there, it became a potent N₂O efflux source. We searched for vertical distribution of N₂O emitting bacteria using a soil microbiota-culturing assay. In the 10-year-period of oil palm plantation, shallow soils less than 50 cm showed active N₂O emission. In contrast, deeper soils (100-400 cm) were more active in 2-year-old oil palm plantation, suggesting that the N₂O emitters in the shallow soil newly emerged in the ecosystem after the reclamation. Our microbial investigation revealed that major N₂O emitting bacteria in the reclaimed tropical peat soils are eubacteria in subdivision of β-Proteobacteria, genera of Burkholderia and Janthinobacterium. As their conversion rates of NO₃⁻ into N₂O were extremely high (maximum 50%), it was speculated that the active N₂O emitters are missing nitrous oxide reductase (N₂OR) gene. Both an acetylene inhibition assay for denitrificaion of the N₂O emitters and PCR amplification of N₂OR large subunit (nosZ) gene supported our speculation that nosZ was missing in them. In this presentation, we answer to the question why these denitrifiers emit N₂O actively.