Effects of water regime during rice-growing season on annual direct N2O emission in a paddy rice-winter wheat rotation system in southeast China

Annual paddy rice-winter wheat rotation constitutes one of the typical cropping systems in southeast China, in which various water regimes are currently practiced during the rice-growing season, including continuous flooding (F), flooding-midseason drainage-reflooding (F-D-F), and flooding-midseason drainage-reflooding and moisture but without waterlogging (F-D-F-M). We conducted a field experiment in a rice-winter wheat rotation system to gain an insight into the water regime-specific emission factors and background emissions of nitrous oxide (N₂O) over the whole annual cycle. While flooding led to an unpronounced N₂O emission during the rice-growing season, it incurred substantial N₂O emission during the following non-rice season. During the non-rice season, N₂O fluxes were, on average, 2.61 and 2.48 mg N₂O-N m⁻² day^− 1 for the 250 kg N ha^− 1 applied plots preceded by the F and F-D-F water regimes, which are 56% and 49% higher than those by the F-D-F-M water regime, respectively. For the annual rotation system experienced by continuous flooding during the rice-growing season, the relationship between N₂O emission and nitrogen input predicted the emission factor and background emission of N₂O to be 0.87% and 1.77 kg N₂O-N ha^− 1, respectively. For the plots experienced by the water regimes of F-D-F and F-D-F-M, the emission factors of N₂O averaged 0.97% and 0.85%, with background N₂O emissions of 2.00 kg N₂O-N ha^− 1 and 1.61 kg N₂O-N ha^− 1 for the annual rotation system, respectively. Annual direct N₂O-N emission was estimated to be 98.1 Gg yr^− 1 in Chinese rice-based cropping systems in the 1990s, consisting of 32.3 Gg during the rice-growing season and 65.8 Gg during the non-rice season, which accounts for 25-35% of the annual total emission from croplands in China.