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Title: Reconciling the differences between top-down and bottom-up estimates of nitrous oxide emissions for the US corn belt

item GRIFFIS, TIMOTHY - University Of Minnesota
item LEE, X - Yale University
item Baker, John
item Russelle, Michael
item XHANG, XIN - Yale University
item Venterea, Rodney - Rod
item MILLET, DYLAN - University Of Minnesota

Submitted to: Global Biogeochemical Cycles
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/3/2013
Publication Date: 11/3/2013
Citation: Griffis, T.J., Lee, X., Baker, J.M., Russelle, M.P., Xhang, X., Venterea, R.T., Millet, D.B. 2013. Reconciling the differences between top-down and bottom-up estimates of nitrous oxide emissions for the US corn belt. Global Biogeochemical Cycles. 27:746-754.

Interpretive Summary: Nitrous oxide is a major greenhouse gas, with 300 times the global warming potential of carbon dioxide, and cropped fields are one of the primary sources of it. However, estimates of the amount of nitrous oxide produced by agriculture are highly uncertain, hampering attribution and mitigation efforts. The Inter-governmental Panel on Climate Change (IPCC) estimates that between 0.75% and 2% of the nitrogen that is applied to crop fields is released to the atmosphere as nitrous oxide, yet global scale assessments of the atmospheric nitrous oxide budget suggest that the percentage loss must be much higher - between 3.8 and 5.1%. We have used long-term, high-precision concentration measurements from the top of a radio tower in Minnesota to show that the estimate based on the global atmospheric budget is more appropriate for the U.S. Corn Belt than the lower IPCC value. These results indicate that Corn Belt sources released 420 +/- 50 Gg of N as N2O in 2010, much higher than the IPCC-based estimate of 230 +/- 180 Gg N. This large discrepancy suggests that there are emission hot spots or missing sources of nitrous oxide on the landscape that are not adequately represented in the IPCC accounting methodology. Identification of these hot spots will be a crucial for developing effective mitigation strategies.

Technical Abstract: Cropland is the dominant source of nitrous oxide (N2O), an important greenhouse gas and an ozone-depleting substance. Estimates of this source category continue to suffer from large uncertainties, hampering agricultural mitigation efforts. According to the IPCC (Inter-governmental Panel on Climate Change) studies, between 0.75 and 2% of the nitrogen (N) added to the various components of a cropland ecosystem escapes to the atmosphere in the form of N2O. However, consideration of the global N budget suggests a much higher emission factor (EF) of 3.8 to 5.1% (ref 5, 6). Here we use high-precision, continuous N2O concentration measurements on a tall tower to show that the global “top-down” EF is more appropriate for the United States Corn Belt, a vast region spanning the US Midwest that is dominated by intensive N inputs to support corn cultivation. These measurements are combined with atmospheric boundary layer methods to derive a regional N2O flux. Our results show that agricultural sources in the Corn Belt released 420 ± 50 Gg N (mean ± 1standard deviation; 1Gg = 109 g) in 2010, in closer agreement with the estimate of 350 ± 50 Gg N using the top-down EF, and 80% larger than the bottom-up estimate based on the IPCC EFs (230 ± 180 Gg N). The large difference between the tall-tower measurement and the bottom-up estimate implies the existence of N2O emission hot spots or missing sources within the landscape that are not fully accounted for in the IPCC and other bottom-up emission inventories.