Abstract: To explore the effects of gradually and abruptly elevated CO₂ concentration and different nitrogen application levels on CH₄ emissions in paddy field, a field rice (cv. Nanjing 9108) experiment was conducted based on the automatic CO₂ concentration control platform, and CH₄ flux was measured by static chamber-gas chromatography method. The experiment was designed as: the control (CK, ambient atmospheric CO₂ concentration), a gradually elevated atmospheric CO₂ (C₁, an increase of 40 μmol·mol^-1 per year within 2016-2018), and an abruptly elevated atmospheric CO₂ (C₂, an increase of 200 μmol·mol^-1 directly). Meanwhile, a nitrogen fertilizer reduction treatment (N₂, 15 g·m^-2) was set up in addition to the conventional nitrogen application amount (N₁, 25 g·m^-2). The results show that the seasonal variation of CH₄ fluxes was not significantly changed under different CO₂ concentrations and nitrogen treatments, and CH₄ fluxes showed a trend of increasing first and then decreasing. During the whole growth period of rice, C₁ and C₂ treatments had no significant effects on CH₄ emission per unit yield. Under C₂ condition, compared with N₁ treatment, N₂ treatment significantly reduced rice yield by 45.2% (P=0.037), and significantly increased CH₄ emission per unit yield by 63.3% (P=0.008). Overall, under the elevated CO₂ concentration, nitrogen fertilizer reduction treatment (15 g·m^-2) reduced rice yield and simultaneously increased CH₄ emission per unit yield.

Key words: rice, CO₂ concentration, nitrogen fertilizer, CH₄ emission