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

Abstract

N 2 O is an important greenhouse gas and the primary stratospheric ozone depleting substance. Its deleterious effects on the environment have prompted appeals to regulate emissions from agriculture, which represents the primary anthropogenic source in the global N 2 O budget. Successful implementation of mitigation strategies requires robust bottom-up inventories that are based on emission factors (EFs), simulation models, or a combination of the two. Top-down emission estimates, based on tall-tower and aircraft observations, indicate that bottom-up inventories severely underestimate regional and continental scale N 2 O emissions, implying that EFs may be biased low. Here, we measured N 2 O emissions from streams within the US Corn Belt using a chamber-based approach and analyzed the data as a function of Strahler stream order (S). N 2 O fluxes from headwater streams often exceeded 29 nmol N 2 O-N m −2 ·s −1 and decreased exponentially as a function of S. This relation was used to scale up riverine emissions and to assess the differences between bottom-up and top-down emission inventories at the local to regional scale. We found that the Intergovernmental Panel on Climate Change (IPCC) indirect EF for rivers (EF 5r ) is underestimated up to ninefold in southern Minnesota, which translates to a total tier 1 agricultural underestimation of N 2 O emissions by 40%. We show that accounting for zero-order streams as potential N 2 O hotspots can more than double the agricultural budget. Applying the same analysis to the US Corn Belt demonstrates that the IPCC EF 5r underestimation explains the large differences observed between top-down and bottom-up emission estimates.aquatic nitrous oxide fluxes | IPCC emission factors | river emission hotspots | regional emission upscaling N 2 O is projected to remain the dominant stratospheric ozonedepleting substance of the 21st century (1) and is a powerful greenhouse gas (GHG) that currently accounts for about 6% of the net radiative forcing associated with long-lived anthropogenic GHGs (2). The detrimental environmental impacts of N 2 O have stimulated appeals to regulate emissions from agricultural lands (1, 3), which account for nearly 80% of the global anthropogenic N 2 O budget (4, 5). The successful regulation and mitigation of N 2 O emissions requires a sound understanding of the direct and indirect emission processes and reduced uncertainty regarding the emission factors (EFs) (6).The Intergovernmental Panel on Climate Change (IPCC) tier 1 approach uses EFs to provide first-order approximations of annual N 2 O emissions based on mechanistic and empirical information that have been constrained by field studies. These EFs are widely used in bottom-up inventories such as the Emission Database for Global Atmospheric Research (EDGAR) (7) and the Global Emissions Initiative (GEIA) (8). These inventories are essential tools for tracking country specific emission trends, assessing thresholds for international treaties, and evaluating the impacts of mitigation policies. Recent i...

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

“…3), one of the most intensively managed agricultural regions of the world. The magnitude of this upward adjustment supports emission underestimates previously found for this region (Kort et al, 2008;Miller et al, 2012;Griffis et al, 2013), which have been attributed to underrepresentation of indirect N 2 O emissions following leaching and runoff from agricultural soils (Turner et al, 2015b;Chen et al, 2016). However, other processes could also contribute, such as freeze-thaw emissions or direct emissions after spring fertilizer application.…”

“…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.…”

Air pollution and risk of hospitalization for epilepsy: the role of farm use of nitrogen fertilizers and emissions of the agricultural air pollutant, nitrous oxide

“…The current finding merges critical concepts of climate change into the growing burden of chronic human neurological illness and, in particular, suggests an important direction to take in the study of epilepsy and related comorbidity. Recent reports are indicating that current Intergovernmental Panel on Climate Change guidelines are underreporting indirect emissions of N 2 O by a magnitude from three-to nine-fold 32,33 , while Intergovernmental Panel on Climate Change methodologies presuming linear increases in direct soil N 2 O emissions as a function of nitrogen inputs may be missing true exponential rises in global emissions, especially under conditions of heavy fertilizer use (i.e., agriculture) 19 . Our data may be particularly revealing in this regard, considering that the greatest protective effect against hospitalization for epilepsy occurs in states that report the greatest amount of nitrogen fertilizers used in agriculture.…”

Agroecology: A Global Paradigm to Challenge Mainstream Industrial Agriculture

“…Furthermore, new data indicate that previous estimates of nitrous oxide emissions from industrial agricultural systems may have been grossly underestimated, and that when "riverine" watershed emissions are considered, the levels of nitrous oxide emissions from areas such as the MidWestern USA may be up to 40% greater than earlier estimated [40]. The overall global environmental impact of these increased emissions on climate change could be significant as other similar regions of the world where intensive industrial farming practices are followed represent, globally, an area of over 230 million ha.…”