Title: Optimal fertilizer N rates and yield-scaled global warming potential in drill seeded rice Authors
|Adviento-Borbe, Maria -|
|Pittelkow, Cameron -|
|Anders, Merle -|
|Van Kessel, Chris -|
|Hill, James -|
|Six, Johan -|
|Linquist, Bruce -|
Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 5, 2013
Publication Date: N/A
Interpretive Summary: Methane and nitrous oxide are two gases that have a major impact on global warming potential. Development of agricultural practices that minimize the production of these gases while still maintaining high yields for food and feed production is a major research concern. Rice is one of the most important cereal grains, serving as a staple food for half of the world’s population. Most rice is produced in flooded paddies which emit relatively high levels of methane due to anaerobic bacteria that live in flooded soils. Increased application of nitrogen fertilizer results in higher field yields, but excessive amounts can result in nitrogen loss through volatilization as nitrous oxide. This study was conducted to ascertain global warming potential in drill seeded rice production fields in response to different rates of fertilizer nitrogen. Methane and nitrous oxide emissions were determined both during the growing season (summer) and during the winter fallow. The research showed that methane emissions during the growing season had the biggest impact on annual global warming potential, whereas, even when above optimum rates of nitrogen fertilizer were applied, nitrous oxide had less impact. This demonstrates that in drill seeded flooded rice paddies practices which reduce methane emissions during the growing season will likely have the biggest impact on reducing global warming potential in rice production systems.
Technical Abstract: Drill seeded rice (Oryza sativa L.) is the dominant rice cultivation practice in the USA. Although drill seeded systems can lead to significant methane and nitrous oxide emissions due to the presence of both anaerobic and aerobic soil conditions, the relationship between high-yielding management practices, particularly fertilizer nitrogen (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 (GWPY = GWP Mg-1 grain) is achieved. Urea was applied at rates from 0 to 224 kg N ha-1 prior to permanent flood each growing season. Emissions of methane and nitrous oxide were measured daily to weekly during the growing season and during fallow periods. Annual methane emissions ranged from 9.3 to 193 kg CH4-C ha-1 yr-1 across sites while annual nitrous oxide emissions averaged 1.3 kg N2O-N ha-1 yr-1. Relative to nitrous oxide emissions, methane dominated both growing season (82%) and annual (68%) GWP. Impacts of fertilizer N rates on greenhouse gas fluxes were confined to the growing season, with increasing N rate having little effect on methane emissions but contributing to greater nitrous oxide emissions during non-flooded 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 GWPY ranged from 130 to 686 kg CO2 eq Mg-1 season-1 across sites and years. Fertilizer N rate had no significant effect on GWPY; therefore achieving the highest productivity is not at the cost of higher GWPY.