N Fertilizer Strategies for Reducing Greenhouse Gas Emissions and Sustaining Grain Yield in Irrigated Flooded Rice Systems.

Practices to improve N fertilizer management can reduce greenhouse gas emissions and at the same time maintain high grain yield in irrigated rice systems. Field studies were conducted to quantify the impacts of different N fertilizer strategies that aimed to increase N use efficiency and reduce CH4 and N2O emissions in flooded rice fields in California and Arkansas, US. Rice yield and GHG emissions were determined from drill seeded and wet seeded fields fertilized with various N fertilizer sources (i.e. aqua ammonia, urea, Agrotain™) and fertilizer placements (i.e. broadcast or subsurface once or twice during the growing season) at recommended N rates ranging from 100-168 kg N ha-1. Emissions of CH4 and N2O were measured throughout the rice crop cycle using vented flux chamber and gas chromatography method. At all sites, grain yields were 3.5 to 6.2 Mg ha-1 without N fertilizer application. The addition of N fertilizer increased yield by approximately 113% with no consistent trends among N sources, number and mode of fertilizer applications. Fertilizer use efficiency ranged from 17 to 64% with the highest in wet seeded fields. In all locations, different types of N fertilizer had no effect on annual CH4 and N2O emissions however, magnitudes of CH4 and N2O emissions were 1.5 and 1.8 times higher with N application compared with unfertilized treatments (81 kg CH4-C and 0.56 kg N2O-N ha-1 yr-1) , respectively. There were no clear patterns whether application of different N fertilizer sources (urea, polymer coated urea, ammonium sulfate, aqua N) effectively decreased global warming potentials (GWP) because annual GWP were variable in all sites. Yield-scaled annual GWP tended to decrease in split N fertilizer treatments but this was not consistent across sites. Our results show that N management practices that maintain grain yield may have potential to reduce GHG emissions but require further assessment.