Indirect nitrous oxide emissions from streams within the US Corn Belt scale with stream order - PubMed (original) (raw)

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

Peter A Turner et al. Proc Natl Acad Sci U S A. 2015.

Abstract

N2O 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 N2O 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 N2O emissions, implying that EFs may be biased low. Here, we measured N2O 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). N2O fluxes from headwater streams often exceeded 29 nmol N2O-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 (EF5r) is underestimated up to ninefold in southern Minnesota, which translates to a total tier 1 agricultural underestimation of N2O emissions by 40%. We show that accounting for zero-order streams as potential N2O hotspots can more than double the agricultural budget. Applying the same analysis to the US Corn Belt demonstrates that the IPCC EF5r underestimation explains the large differences observed between top-down and bottom-up emission estimates.

Keywords: IPCC emission factors; aquatic nitrous oxide fluxes; regional emission upscaling; river emission hotspots.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.

Fig. 1.

The relation between N2O flux and the Strahler stream order in southeastern Minnesota. The black line represents the best fit of an exponential function to the mean flux values measured at each stream order. Red lines represent the 95% CI of the model fit, and error bars indicate 1 SD from the mean.

Fig. 2.

Fig. 2.

Results from upscaling N2O emissions. (A) A comparison of local indirect N2O-N sources from default IPCC EFs and our scaling method. (B) Total US Corn Belt emissions from the three methods. (C) The flux densities from the tall-tower source footprint for each method.

Fig. 3.

Fig. 3.

The first-order default EF5r underestimation from the Corn Belt region. The bias is defined as the difference between IPCC EF5r emissions and the results from stream order scaling.

Fig. S1.

Fig. S1.

Photograph and diagram of the system used in this study to sample N2O flux.

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