The impact of nitrogen application rate on rice yield and greenhouse gas emissions of ratoon rice in the Taihu Lake region

The effects of nitrogen (N) application rate on the yield and greenhouse gas emissions (CH₄ and N₂O) of ratoon rice were investigated to provide a reference for the optimal N application rate for planting ratoon rice in the Taihu Lake region. From April to October 2022, a static chamber-gas chromatography method was used to observe the CH₄ and N₂O emission fluxes of ratoon rice under three N fertilization rates FL1: 75 kg(N)·hm⁻²; FL2: 125 kg(N)·hm⁻²; FL3: 175 kg(N)·hm⁻² in Yixing, Jiangsu Province, while setting no fertilization treatment (FL0) as a control. The yield and its constituent factors were observed during the rice harvest. The results showed that the peak N₂O emission caused by bud-promoting fertilizer was the highest, and it showed a linear increasing trend with the increase of N fertilization rate; the total amount it caused accounted for 10.0%−50.7% of N₂O emissions during the entire growth period. Overall, the total (main reason+ratoon reason) N₂O emissions ranged from 1.30 to 8.69 kg(N)·hm⁻², with 6.94%−22.7% derived from ratoon rice; the N₂O emission factor during the ratoon season ranged from 0.38% to 1.71%. Seasonal cumulative CH₄ emissions ranged from 58.1 kg·hm⁻² to 78.7 kg·hm⁻², with 13.3%−23.8% in ratoon rice. Total CH₄ emissions were not significantly affected by an increase in N application rate. The total rice yield was 8.33–11.6 t·hm⁻², and the ratoon rice yield contributed 34.0%−46.1%. Compared to FL1, the ratoon season yields of FL2 and FL3 increased by 32.3% and 17.4%, respectively, which was related to an increase in effective panicle number by 27.2% and 13.5%, respectively. The greenhouse gas emission intensity during the entire growth period was 0.26−0.54 t(CO₂-eq)·t⁻¹, and compared with FL1, FL3 increased by 38.5%, and FL2 increased by 10.3%. Overall, both high yield and relatively low greenhouse gas emission intensity can be obtained by applying 125 kg(N)·hm⁻² in ratoon season. This N application rate is recommended for the ratoon rice in Taihu Lake.