Yield-Scaled Nitrous Oxide and Methane Emissions in Response to N Rate in Direct-Seeded Rice Production Systems.

Fertilizer management practices that reduce soil-based greenhouse gas (GHG) emissions while maintaining yields may represent a sustainable option for reducing the global warming potential (GWP) of agriculture.  Nitrous oxide (N₂O) and methane (CH₄) emissions are of concern in flooded rice systems.  Although nitrogen (N) inputs have been shown to strongly impact N₂O and to some extent CH₄ emissions, there is limited research on the combined effects of N rate on total GWP (N₂O + CH₄) per unit area and per unit grain yield.  To test the hypothesis that optimal N rates result in maximum agronomic productivity and minimum yield-scaled GWP, on-farm experiments were conducted in drill-seeded and water-seeded rice production systems to quantify N₂O and CH₄ emissions on an annual basis.  Five site-years of data were collected in California and Arkansas from 2010-2012.  Fertilizer N in the form of aqua ammonia or urea was applied at five N rates ranging from 0 to 260 kg N ha^-1 (water-seeded) or 0 to 200 kg N ha^-1 (drill-seeded), respectively.  GHG emissions were determined daily or every other day during specific management events (e.g. N application and field drainage periods), and approximately every 7 d otherwise using the vented closed chamber technique.  Results indicate that across sites, low N₂O emissions occurred regardless of N rate when a permanent flood was maintained but that large N₂O fluxes occurred during field drainage periods, particularly at high N rates.  In contrast, CH₄ emissions were generally similar across N rates, with the highest fluxes occurring midseason and during the drainage period before harvest.  Although results differed at each site, annual GWP often increased with N rate, in part due to greater N₂O emissions but more often as a result of greater CH₄ emissions (particularly in water-seeded systems, where CH₄ had a large impact on total GWP relative to N₂O emissions).  However, when accounting for yield, yield-scaled GWP tended to decrease with increasing N rate across sites, with significant differences occurring between N rates in several years.  This data supports previous work on this topic while also highlighting the fact that rice systems are unique compared to other cereal production systems in which the relationship between N rate, yield, and yield-scaled N₂O emissions is potentially more straightforward with yield-scaled emissions generally increasing at surplus N rates.  These results further suggest that optimal yields can be obtained at relatively constant yield-scaled GWP values when current N management recommendations are followed and N fertilizer inputs are closely matched with crop N demand.