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

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.