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

Authors: TURNER, PETER - University Of Minnesota; GRIFFIS, TIMOTHY - University Of Minnesota; LEE, XUHUI - Yale University; Baker, John; Venterea, Rodney - Rod; WOOD, JEFFREY - University Of Minnesota

Submitted to: Proceedings of the National Academy of Sciences (PNAS)
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/27/2015
Publication Date: 7/27/2015
Publication URL: https://handle.nal.usda.gov/10113/61167
Citation: Turner, P.A., Griffis, T.J., Lee, X., Baker, J.M., Venterea, R.T., Wood, J.D. 2015. Indirect nitrous oxide emissions from streams within the U.S. Corn Belt scale with stream order. Proceedings of the National Academy of Sciences. 112(32):9839-9843. DOI: 10.1073/pnas.1503598112.

Interpretive Summary: Nitrous oxide (N2O) is an important atmospheric pollutant. It is a potent greenhouse gas, and also contributes to depletion of stratospheric ozone. Consequently, there has been a major global effort to identify the sources of N2O emission in order to develop strategies to reduce them. Estimates of the amounts emitted from various sources have been used to develop emission factors (EF) for each source. For instance, and emission factor of 0.01 might be used for nitrogen fertilizer, indicating that 1% of applied N fertilizer is estimated to escape to the atmosphere as N2O. Emissions from all known sources can then be summed up to provide a "bottom up" estimate of global emissions. Meanwhile, measured changes in mean global concentration provide a "top-down" constraint on overall emissions - if the two numbers don't match, it's indicative of a problem with the emission factors. That is indeed the case; top-down measurements indicate that annual N2O emissions are mcuh larger than bottom-up estimates would suggest. We measured emissions from streams in agricultural regions and found that they were much larger than had been assumed in bottom -up inventories. We also found that on a unit area basis, emissions scaled inversely with stream size, i.e. - emissions from small drainage ditches near fields were largest, and emission rates declined exponentially as stream order (size) increased. Overall, the results suggest that previous estimates for southern MN may have been low by more than 40%.This relationship was used to scale up riverine emissions for the entire corn belt, showing that improper accounting of these emissions may be a primary cause of the large differences between top-down and bottom-up estimates.

Technical Abstract: Nitrous oxide (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 source in the global N2O budget. Successful implementation of mitigation strategies requires robust bottom-up inventories that are based on emission factors (EFs). Recent 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 the EFs may be biased low. Here, we measured N2O emissions from streams within the US Corn Belt using a novel chamber-based approach and analyzed the data as a function of the Strahler stream order (S). N2O fluxes from headwater streams often exceeded 45 nmol N2O m-2 s-1 and decreased exponentially as a function of S. This relation was used to scale up riverine emissions and to help assess the differences between bottom-up and top-down emission inventories at the local to regional scale. These analyses indicate that the Intergovernmental Panel on Climate Change (IPCC) indirect EF for rivers (EF5r) may be underestimated by up to 9-fold 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 that have been observed between top-down and bottom-up emission estimates.