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In this study, the effects of ammonium concentration (117.5−1155.0 mg-N·l-1), nitrite concentration (0-50.0 mg-N·l-1), and temperature (15−35℃) on nitrification performance and its functional genes (amoA-arc, amoA-bac, hao) in an enriched consortium inoculated with humic acid were determined. Notably, the maximum nitrification rate value was observed at 315 mg-N·l-1 of ammonium, but the highest functional gene copy numbers were obtained at 630 mg-N·l-1 of ammonium. No inhibition of the nitrification rate and functional gene copy numbers was observed via the added nitrites. The optimum temperature for maximum nitrification performance was observed to be 30℃. The amoA-bac copy numbers were also greater than those of amoA-arc under all test conditions. Notably, amoA-arc copy numbers and nitrification efficiency showed a positive relationship in network analysis. These results indicate that ammonium-oxidizing archaea and bacteria play important roles in the nitrification process.