Yield-scaled global warming potential of annual nitrous oxide and methane emissions from continuously flooded rice in response to nitrogen input

Fertilizer nitrogen (N) has been shown to impact both N₂O and CH₄emissions from flooded rice systems, yet there is limited research on the effects of N rate when assessing global warming potential (GWP=N₂O+CH₄) per unit area and per unit grain yield (yield-scaled) on a seasonal and annual basis. A two-year on-farm experiment was conducted from 2010-2012 to test the hypothesis that optimal N rates result in maximum agronomic productivity and minimal yield-scaled GWP in water-seeded rice systems experiencing continuously flooded conditions during the growing season and fallow period. Five fertilizer N rates (0, 80, 140, 200 and 260kgNha^-1yr^-1) were applied as aqua ammonia and annual N₂O and CH₄ emissions were quantified using the vented, closed chamber method. Results indicate that low N₂O emissions occurred regardless of N rate when a permanent flood was maintained, but that large N₂O fluxes occurred during discrete field drainage periods prior to harvest, particularly at high N rates. Hence, cumulative N₂O emissions increased with N rate in a nonlinear manner during the growing season. Over the entire cropping cycle, the highest CH₄ fluxes occurred during the middle of the growing season and following field drainage periods prior to harvest and at the conclusion of the fallow period. Mean seasonal and annual CH₄ emissions tended to increase with N addition compared to the control, but significant differences were not observed between N rates. While CH₄ and N₂O emissions were generally not affected by N rate during the fallow period, the fallow period contributed significantly to annual emissions (e.g. 56% of annual N₂O emissions across N rates). Across years, CH₄represented 94% of total GWP and as a result, mean annual GWP increased with N rate up to 140kgNha^-1. Maximum yields occurred between 140 and 200kgNha^-1, thus by employing the yield-scaled metric to begin to integrate climate change and global food demand concerns, mean annual yield-scaled GWP significantly decreased by 49% at these N rates. These findings suggest that optimal yields can be achieved with simultaneous reductions in yield-scaled GWP through efficient fertilizer N management in water-seeded rice systems experiencing continuously flooded conditions during the growing season and fallow period.