Effects of biochar on nitrous oxide and nitric oxide emissions from paddy field during the wheat growth season

Agricultural activities are an important source of atmospheric nitrous oxide (N2O). Excess nitrogen (N) fertilization in the Taihu Lake region, southeast China, has resulted in a series of environmental issues such as N2O emissions and water body eutrophication. Optimal fertilization can reduce the loss of surplus N with a penalty of no or low yield. The conversion of plant residues to biochar is an attractive strategy for mitigating atmospheric carbon dioxide emissions and for enhancing carbon storage in soil. A field experiment was conducted to investigate the effect of biochar on N2O emissions, crop yield, and global warming potential (GWP). Five treatments were conducted with four replicates: no nitrogen fertilizer (Control), locally conventional N fertilizer (RN), optimal N fertilizer (ON), optimal N fertilizer plus low amount of biochar [3.75 t ha−1; (ONC1)], and optimal N fertilizer plus high amount of biochar [7.50 t ha−1; (ONC2)]. Results showed that both N2O and NO emissions increased exponentially with the N fertilizer application rate during the wheat growth season, and cumulative N2O emissions were significantly (P < 0.05) reduced in the ON treatment. Biochar amendment at 3.75 and 7.50 t ha−1 did not notably reduce N2O emissions. A significant negative correlation was observed between N2O flux and the soil water-filled pore space (WFPS) in biochar amendment, and the NO/N2O ratio was almost lower than 1, except for tillering fertilization when the soil WFPS was lower, indicating that N2O was primarily produced by denitrification. The GWP was significantly mitigated in the ON treatment compared with the RN treatment, which was significantly lower in the biochar treatment (P < 0.05). The greenhouse gas intensity (GHGI) also decreased from 0.039 to 0.031 kg CO2-eq kg−1 yield after biochar addition. GWP and GHGI results indicated that the application of biochar could significantly mitigate GWP compared with the ON treatment. Our results suggest that the application of biochar slightly reduced N2O emission, which might contribute to promote the reduction N2O to N2. Optimal N fertilization, especially, combined with biochar will exercise a greater effect for mitigating global warming.