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

Griffis, T. J., Lee, X., Baker, J. M., Russelle, M. P., Zhang, X., Venterea, R., Millet, D. B.
Global biogeochemical cycles 2013 v.27 no.3 pp. 746
Zea mays, climate change, crop production, cropland, ecosystems, fertilizer application, greenhouse gas emissions, greenhouse gases, landscapes, nitrogen, nitrogen fertilizers, nitrous oxide, Corn Belt region, Midwestern United States
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.