Magnitude and Uncertainty of Nitrous Oxide Emissions From North America Based on Bottom‐Up and Top‐Down Approaches: Informing Future Research and National Inventories

Abstract

We synthesized N 2 O emissions over North America using 17 bottom-up (BU) estimates from 1980-2016 and five top-down (TD) estimates from 1998 to 2016. The BU-based total emission shows a slight increase owing to U.S. agriculture, while no consistent trend is shown in TD estimates. During 2007-2016, North American N 2 O emissions are estimated at 1.7 (1.0-3.0) Tg N yr −1 (BU) and 1.3 (0.9-1.5) Tg N yr −1 (TD). Anthropogenic emissions were twice as large as natural fluxes from soil and water. Direct agricultural and industrial activities accounted for 68% of total anthropogenic emissions, 71% of which was contributed by the U.S. Our estimates of U.S. agricultural emissions are comparable to the EPA greenhouse gas (GHG) inventory, which includes estimates from IPCC tier 1 (emission factor) and tier 3 (process-based modeling) approaches. Conversely, our estimated agricultural emissions for Canada and Mexico are twice as large as the respective national GHG inventories.Plain Language Summary Nitrous oxide (N 2 O) is the third most important greenhouse gase (GHG) after CO 2 and CH 4 causing global warming. Among world regions, North America (defined herein as U.S., Canada, and Mexico) is the second largest source of N 2 O emissions globally, and previous source estimates for this region vary widely. This study aims to provide a comprehensive N 2 O assessment over North America including all available estimates based on a number of approaches. We report total emissions, and emissions from four anthropogenic source sectors, over the past four decades. Agriculture and industry are two major N 2 O sources in North America. Our results show a minor increase in the total N 2 O emission due to agricultural trends in the U.S. Our bottom-up estimate of U.S. agSricultural N 2 O emissions are close to those in the EPA national GHG inventory that includes both empirical and model results. The high consistency suggests the need to take process-based modeling results into account for future national GHG inventories.

Net greenhouse gas balance in U.S. croplands: How can soils be part of the climate solution?

“…In terms of N 2 O emissions, our estimates ranged from 0.27 ± 0.02 Tg N year −1 in the 1960s to 0.45 ± 0.03 Tg N year −1 in the 2010s, which were also comparable to previous studies (Chen et al, 2016;Del Grosso et al, 2006;EPA, 2018;Griffis et al, 2013;Lu et al, 2021;Mummey et al, 1998;Tian et al, 2019). Xu et al, 2021), and our estimate of 0.44 Tg N year −1 fell well within this range. Additionally, our estimated CH 4 emissions (averaged to 0.23 Tg C year −1 over the last two decades) was also close to previous estimates centered on an annual emission rate of ~0.25…”

“…In terms of N 2 O emissions, our estimates ranged from 0.27 ± 0.02 Tg N year −1 in the 1960s to 0.45 ± 0.03 Tg N year −1 in the 2010s, which were also comparable to previous studies (Chen et al., 2016; Del Grosso et al., 2006; EPA, 2018; Griffis et al., 2013; Lu et al., 2021; Mummey et al., 1998; Tian et al., 2019). For example, the direct N 2 O emissions from U.S. agricultural soils estimated by the Global N 2 O Model Inter‐comparison Project (NMIP) ranged from 0.3 Tg N year −1 to 0.62 Tg N year −1 during 2007–2016 (Tian et al., 2019; Xu et al., 2021), and our estimate of 0.44 Tg N year −1 fell well within this range. Additionally, our estimated CH 4 emissions (averaged to 0.23 Tg C year −1 over the last two decades) was also close to previous estimates centered on an annual emission rate of ~0.25 Tg C year −1 (EPA, 2015; Sass et al., 1999; Tian et al., 2015).…”

Standardized Data to Improve Understanding and Modeling of Soil Nitrogen at Continental Scale

“…Additionally, NEON’s standardized, continental‐scale data on soil inorganic N pools and fluxes can be used to evaluate process‐based model estimates of soil N 2 O emissions. Even without the N 2 O flux data, soil pools and net N transformation rates can serve as standardized benchmarks and play important roles in the N 2 O Model Intercomparison Project (NMIP, Tian et al., 2018; Xu et al., 2021).…”

Soil application of high-lignin fermentation byproduct to increase the sustainability of liquid biofuel production from crop residues

“…Nitrous oxide (N 2 O) is a greenhouse gas that has ∼300 times the global warming potential compared to CO 2 (Smith et al 2014b). It is estimated that anthropogenic N 2 O emission are increasing and currently amount to 43% of total N 2 O emission with agriculture accounting for about two thirds of the anthropogenic emissions (Xu et al 2021b). Of GHG emissions associated with N fertilizer production and use, N 2 O emissions from soil are responsible for about half of the total and are substantially larger than emissions from fertilizer production (Brentrup et al 2016).…”