Reconciling the differences between top-down and bottom-up estimates of nitrous oxide emissions for the US corn belt
2013
Griffis, T. J. | Lee, X. | Baker, J. M. | Russelle, M. P. | Zhang, X. | Venterea, R. | Millet, D. B.
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.
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