Optimal Fertilizer Nitrogen Rates and Yield‐Scaled Global Warming Potential in Drill Seeded Rice

Drill seeded rice ( Oryza sativa L.) is the dominant rice cultivation practice in the United States. Although drill seeded systems can lead to significant CH 4 and N 2 O emissions due to anaerobic and aerobic soil conditions, the relationship between high‐yielding management practices, particularly fertilizer N management, and total global warming potential (GWP) remains unclear. We conducted three field experiments in California and Arkansas to test the hypothesis that by optimizing grain yield through N management, the lowest yield‐scaled global warming potential (GWP Y = GWP Mg −1 grain) is achieved. Each growing season, urea was applied at rates ranging from 0 to 224 kg N ha −1 before the permanent flood. Emissions of CH 4 and N 2 O were measured daily to weekly during growing seasons and fallow periods. Annual CH 4 emissions ranged from 9.3 to 193 kg CH 4 –C ha −1 yr −1 across sites, and annual N 2 O emissions averaged 1.3 kg N 2 O–N ha −1 yr −1 . Relative to N 2 O emissions, CH 4 dominated growing season (82%) and annual (68%) GWP. The impacts of fertilizer N rates on GHG fluxes were confined to the growing season, with increasing N rate having little effect on CH 4 emissions but contributing to greater N 2 O emissions during nonflooded periods. The fallow period contributed between 7 and 39% of annual GWP across sites years. This finding illustrates the need to include fallow period measurements in annual emissions estimates. Growing season GWP Y ranged from 130 to 686 kg CO 2 eq Mg −1 season −1 across sites and years. Fertilizer N rate had no significant effect on GWP Y ; therefore, achieving the highest productivity is not at the cost of higher GWP Y .