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Abstract: Excessive ammonia nitrogen has been demonstrated to cause a serious hazard to water environments. Bacteria performing nitrification and denitrification (SND) simultaneously can be effective biological instruments to remove ammonia nitrogen completely from effluents. For the first time, Pseudomonas oleovorans QS-7 with SND function, isolated from the biogas treatment system of a pig farm, was found to efficiently remove ammonia nitrogen. Through the determination of key enzymes and functional genes related to the nitrogen metabolism of strain QS-7, combined with nitrogen balance measurements of the nitrogen metabolic process, it was speculated that the SND pathway of the novel strain is NH₄⁺→NH₂OH→NO₂^-→NO₃^-→NO₂^-→NO→N₂O→N_2. QS-7 exhibited 98.6% ammonia nitrogen removal and a maximum ammonia degradation rate of 9.2 mg/(L∙h) at 18 h in 100 mg/L ammonia nitrogen solution. This strain also has a certain capacity to remove nitrate and nitrite nitrogen; the maximum removal efficiencies were 54.22% and 73.93%, respectively, in systems with 100 mg/L of nitrate or nitrite nitrogen as the sole nitrogen source. Nitrogen metabolic balance analysis for QS-7, using ammonia (100 mg/L) as the sole nitrogen source, demonstrated that assimilation (56.1%) is the main mode of nitrogen removal, followed by conversion to N₂ (43.6%). Meanwhile, the presence of NO₂^- was not detected eventually and almost no NO_x was produced, which indicates that the nitrogen removal process of QS-7 is an environmentally friendly one. The optimal environmental conditions for QS-7 were found to be sodium citrate as carbon source, C/N=10, pH=7.0, 150 r/min, and 30 °C. The above results indicate that QS-7 may provide a material and conceptual basis for the advancement of SND technology.