Reconciling the differences between top‐down and bottom‐up estimates of nitrous oxide emissions for the U.S. Corn Belt

Abstract

[1] Nitrous oxide (N 2 O) is a greenhouse gas with a large global warming potential and is a major cause of stratospheric ozone depletion. Croplands are the dominant source of N 2 O, but mitigation strategies have been limited by the large uncertainties in both direct and indirect emission factors (EFs) implemented in "bottom-up" emission inventories. The Intergovernmental Panel on Climate Change (IPCC) recommends EFs ranging from 0.75% to 2% of the anthropogenic nitrogen (N) input for the various N 2 O pathways in croplands. Consideration of the global N budget yields a much higher EF ranging between 3.8% and 5.1% of the anthropogenic N input. Here we use 2 years of hourly high-precision N 2 O concentration measurements on a very tall tower to evaluate the IPCC bottom-up and global "top-down" EFs for a large representative subsection of the United States Corn Belt, a vast region spanning the U.S. Midwest that is dominated by intensive N inputs to support corn cultivation. Scaling up these results indicates that agricultural sources in the Corn Belt released 420˙50 Gg N (mean˙1 standard deviation; 1 Gg = 10 9 g) in 2010, in close agreement with the top-down estimate of 350˙50 Gg N 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 N 2 O emission hot spots or missing sources within the landscape that are not fully accounted for in the IPCC and other bottom-up emission inventories. Reconciling these differences is an important step toward developing a practical mitigation strategy for N 2 O.

Indirect nitrous oxide emissions from streams within the US Corn Belt scale with stream order

“…2C). This estimate is in excellent agreement with the top-down tall-tower measured ensemble flux of 0.35 (0.3-0.4) nmol N 2 O-N·m −2 ·s −1 (9), indicating that top-down and bottom-up budgets can be reconciled by applying our stream order scaling function. Our study suggests that an appropriate EF 5r for the tall-tower source footprint should be closer to 2%, assuming the average application rate of nitrogen fertilizer (88.8 kg N·ha −1 ) (47), which is in agreement with a recent independent investigation (18).…”

“…1 to up-scale emissions within the observed concentration footprint (50-km radius) of our tall-tower N 2 O flux station (9). Land use in this study area consists of 70% crops and pasture, 14% mixed vegetation, 11% forest, 3% developed, and 2% open water and is representative of the US Corn Belt.…”

“…Several studies using these approaches demonstrate that bottom-up inventories underestimate N 2 O emissions by up to ninefold in the Midwest US Corn Belt (9)(10)(11)(12), implying that some EFs are too small. An important problem, therefore, is determining which EFs are biased low and how to reduce their uncertainty.…”

Top-down constraints on global N<sub>2</sub>O emissions at optimal resolution: application of a new dimension reduction technique

“…We assume a measurement uncertainty of 0.4 ppb at all flask sampling sites based on recommendations from the data providers. In addition to the flask-based air samples, we use high-frequency N 2 O measurements (discrete hourly or hourly averaged) from the NOAA Chromatograph for Atmospheric Trace Species (CATS) network (Hall et al, 2007), the Advanced Global Atmospheric Gases Experiment (AGAGE) network (Prinn et al, 2000), and the University of Minnesota tall tower (KCMP tall tower; Griffis et al, 2013;Chen et al, 2016). The hourly measurement uncertainty at these sites is approximately 0.3, 0.6, and 1 ppb, respectively.…”

“…For example, by assuming a linear flux response to fertilizer application, one can either under-or overestimate emissions depending on the application rate (Shcherbak et al, 2014;Gerber et al, 2016). Recent work also suggests that the indirect N 2 O flux could be 2.6-9 times larger than is presently accounted for in bottom-up estimates (Griffis et al, 2013;Turner et al, 2015b), which would imply an underestimate of the agricultural contribution to the overall N 2 O budget. Nonagricultural soils and oceans are thought to contribute an additional 7.4-11 Tg N yr −1 (Saikawa et al, 2013) and 1.2-6.8 Tg N yr −1 (Nevison et al, 1995;Jin and Gruber, 2003;Manizza et al, 2012), respectively, to the global N 2 O source.…”