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 CH₄ and N₂O 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 CH₄ and N₂O were measured daily to weekly during growing seasons and fallow periods. Annual CH₄ emissions ranged from 9.3 to 193 kg CH₄-C ha^-1 yr^-1 across sites, and annual N₂O emissions averaged 1.3 kg N₂O-N ha^-1 yr^-1. Relative to N₂O emissions, CH₄ 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 CH₄ emissions but contributing to greater N₂O 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 CO₂ 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.