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Nitrous oxide (N2O) generated during biological nitrogen removal in wastewater treatment processes has contributed an important proportion to the global warming effect. To evaluate the possibility of N2O emission mitigating by changing carbon source supply strategies, nitrogen transformation characteristics and N2O emissions with methanol one-time dosing and step dosing were investigated. Two sets of laboratory-scale sequencing batch biofilm reactors (SBBRs) were conducted to treat real domestic wastewater with low carbon source. The results revealed that reactors with methanol step dosing showed a lower N2O emission of 0.0402 +/- 0.0016mg/(Lh), together with a higher total nitrogen and ammonia nitrogen removal efficiencies of 83.30%+/- 1.21 and 93.45%+/- 1.20, respectively. While N2O emission from conventional one-time dosing reactors was 0.0741 +/- 0.0025mg/(Lh), total nitrogen and ammonia nitrogen removal efficiencies were 75.71%+/- 0.54 and 88.45%+/- 0.59, respectively. The N2O emission factor of SBBR was reduced from 6.26%+/- 0.21 to 3.40%+/- 0.14 with methanol step dosing. Moreover, nitrification rates in aerobic phases were reduced, while denitrification rates in anoxic phases were elevated. Hence, carbon source step dosing enhanced nitrogen removal and reduced N2O emission compared with one-time dosing, which is a simply achievable strategy for N2O emission reduction in highly automated systems like wastewater treatment plants.