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

Abstract

Fertilizer nitrogen (N) has been shown to impact both N2O and CH4 emissions 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, CH4 represented 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.