Evaluating uncertainties in nitrous oxide emission inventories with multi-scale observations for an agriculture-dominated region

Abstract:

Although agriculture accounts for about 80% of the global anthropogenic nitrous oxide (N₂O) emissions, large uncertainties exist in regional inventories of N₂O emissions from agriculture. The uncertainties mainly include poorly quantified plant flux, large heterogeneity of direct N₂O emissions from cropland, and underestimated N₂O lost through leaching and run off.

To evaluate these uncertainties we conducted observations on three contrasting scales in the Midwest U.S., an agriculture dominated region (Zhang et al., 2014a). Observations at the plant, ecosystem, and regional scales include: 1) N₂O flux measurements from the aboveground section of corn and soybean plants using newly designed plant chamber; 2) N₂O flux-gradient measurements in a soybean-corn rotation field; and 3) N₂O concentration measurements at 3 m and 200 m level on a communication tower (KCMP tower, 44°41'19''N, 93°4'22''W) that were used to estimate regional N₂O fluxes with boundary layer methods (Zhang et al., 2014b).

With these observations we evaluated the uncertainties in two frequently-used N₂O inventories: EDGAR42 (Emission Database for Global Atmospheric Research, release version 4.2); and a national GHG inventory (U.S. EPA, 2014). The results indicate that EDGAR42 and EPA inventory underestimated N₂O emissions for the region around the KCMP tower at least by a factor of three and two respectively. The underestimation is not likely caused by neglecting N₂O flux from crops since N₂O fluxes from unfertilized soybean and fertilized corn plants were about one magnitude lower than N₂O emissions from the soil-plant ecosystem. The direct N₂O emissions from cropland accounted for less than 20% of the regional flux, suggesting a significant influence by other sources and indirect emissions in the regional N₂O budget.

References

U.S. EPA (2014) Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2012, 529 pp., Washington, D.C..

X Zhang, X Lee, TJ Griffis, AE Andrews, JM Baker, MD Erickson, W Xiao, N Hu (2014 a) Quantifying nitrous oxide fluxes on multiple spatial scales in the Upper Midwest, USA, Int J Biometeorol.

X Zhang, X Lee, TJ Griffis, JM Baker, W Xiao (2014 b) Estimating greenhouse gas fluxes from an agriculture-dominated landscape using multiple planetary boundary layer methods, Atmos Chem Phys Discuss.