Maria Arlene Adviento-Borbe, Cameron Pittelkow, Cesar Abrenilla, Johan Six ,  James Hill, Chris van Kessel, and Bruce Linquist, 1210 PES Bldg., Dept of Plant Sciences, One Shields Avenue, University of California-Davis, Davis, CA

Reducing greenhouse gas emissions per ton of grain yield (i.e. yield-scaled Global Warming Potential) can address both global environmental issues and food security. Fertilizer N application in excess of plant demand will fuel high yield-scaled GWP. Flooding and frequent draining of rice fields lead to conditions favorable for production of greenhouse gases (GHG): methane (CH₄) and nitrous oxide (N₂O). A farmer field study was conducted to quantify annual CH₄ and N₂O emissions from rice fields under different N fertilizer rates (i.e. six urea N rates ranging from 0 to 200 kg N ha^-1 ) with frequent dry periods. Gas concentrations were measured using a gas chromatograph within one hr period at 30-min interval using a steady-state vented flux chamber.  Daily gas emissions in all N treatments ranged from 0 to 88 g N₂O-N ha^-1 d^-1 and 0 to 725 g CH₄-C ha^-1 d^-1. Methane emissions increased after 25 d of flooding, remained high until around flowering stage and declined after heading stage. Highest N₂O emission events were associated with drain periods and high N rate treatments. Consequently, N₂O accounted for about 45% of total GWP at N rates >100 kg ha^-1. Yield-scaled global warming potential was two times lower in rice fertilized at grower’s N rate, than the rest of fertility treatments. Estimated annual yield-scaled GWP had no linear response to increasing fertilizer N rates. The lack of clear GWP trend to N inputs is likely due to combination of low grain yield and high GHG emissions. Our results suggest that yield of lowland rice cultivation close to yield potential coupled with low GWP can be achieved if synthetic N and water resources are at optimum.