Importance of annual monitoring for evaluating the direct nitrous oxide emission factor in temperate mono-rice paddy fields

In temperate mono-rice paddy fields, rice is cultivated for 100–140 days under submerged conditions during the summer period, and thereafter, the field remains under dry conditions during the winter and spring seasons. However, the early developed nitrous oxide (N₂O) emission factor (EF) was only based on seasonal (rice cropping period) N₂O fluxes, which resulted in lower N₂O EF than the default value (0.3%) used by the Intergovernmental Panel on Climate Change (IPCC). Furthermore, the long fallow season may be favorable for nitrification and substantially result in increased N₂O emissions. A two-year field experiment was conducted to evaluate the effect of N₂O emissions during the dry fallow season on the annual N₂O EF. The N₂O emission rates were sequentially characterized during the rice cropping and the fallow season under four different levels of nitrogen (N) fertilizer for rice cultivation. The urea was applied at four different (0, 45, 90 and 180 kg N ha⁻¹) levels, and rice was cultivated under submerged conditions during late May to early October. The seasonal N₂O fluxes during the rice cropping and fallow seasons clearly increased with increasing N application rates. In the N fertilized plots, the mean N₂O emission rates were higher during the fertilized cropping season than the fallow season, but the seasonal fluxes were much higher during the unfertilized fallow season, due to the long dry period. The seasonal N₂O EF, which was estimated by the increased N₂O flux with N fertilizer, was only 0.0015–0.0017 kg N₂O-N kg⁻¹ N during rice cropping. However, the annual N₂O EF combining the two seasonal N₂O fluxes markedly increased to 0.0028–0.0031 kg N₂O-N kg⁻¹ N, which was very close to the N₂O EF of the IPCC. Conclusively, the N₂O EF in mono-rice paddy fields should be developed using the annual N₂O fluxes and not only the cropping seasonal N₂O fluxes.