Estimating Agricultural Nitrous Oxide Emissions

Authors: Del Grosso, Stephen - Steve; WIRTH, J - Environmental Protection Agency (EPA); OGLE, S - Colorado State University; PARTON, W - Colorado State University

Submitted to: Trans American Geophysical Union
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/10/2008
Publication Date: 12/16/2008
Citation: Del Grosso, S.J., Wirth, J., Ogle, S.M., Parton, W.J. 2008. Estimating Agricultural Nitrous Oxide Emissions. Trans American Geophysical Union. 89, 529-530.

Interpretive Summary: Nitrous oxide (N2O) is a potent greenhouse gas, the main anthropogenic source is agricultural soils, and recent research suggests that standard methods as commonly applied tend to under-estimate emissions. The global warming potential of N2O is approximately 300 times that of CO2, and N2O is responsible for ~6% of global anthropogenic greenhouse gas emissions. Proper quantification of N2O emissions is particularly important in the context of calculating net greenhouse gas emissions from biofuel cropping systems because these emissions offset the greenhouse gas benefits of displacing fossil fuel and can even lead to biofuel systems being a net greenhouse gas source. N2O emissions are highly variable in space and time and different methodologies have not agreed closely, especially at small scales. However, as scale increases, so does the agreement between estimates based on soil surface measurements (bottom up approach) and estimates derived from changes in atmospheric concentration of N2O (top down approach). At the global scale, estimates of agricultural N2O emissions based on a simple bottom up methodology agree surprisingly well with the top down approach. But simple bottom up and the global top down approaches are not reliable when trying to discern the smaller scale temporal and spatial patterns in emissions. However, more complex bottom up models often show reasonably good agreement with plot and field scale data and these types of models can be used to identify the hot spots and moments where and when mitigation efforts should be targeted.

Technical Abstract: Nitrous oxide emissions are highly variable in space and time and different methodologies have not agreed closely, especially at small scales. However, as scale increases, so does the agreement between estimates based on soil surface measurements (bottom up approach) and estimates derived from changes in atmospheric concentration of N2O (top down approach). At the global scale, estimates of agricultural N2O emissions based on a simple bottom up methodology agree surprisingly well with the top down approach. Convergence of top down and bottom up approaches increases confidence in emission estimates because the methods are based on different assumptions and suggests that we have at least a rudimentary understanding of the factors that control emissions at large spatial and temporal scales. But simple bottom up approaches are not reliable when trying to discern the smaller scale temporal and spatial patterns in emissions; nor are they expected to reliably identify hot spots and hot time periods of emissions where/when mitigation efforts should be targeted. Similarly, the global top down approach is not expected to be applicable at small spatial and temporal scales because all N inputs are treated equally. However, more complex bottom up models often show reasonably good agreement with plot and field scale data and these types of models can be used to identify the hot spots and moments where and when mitigation efforts should be targeted. [GRACENet publication]