A modeling study of direct and indirect N2O emissions from a representative catchment in the U. S. Corn Belt

Authors: FU, CONGSHENG - YALE UNIVERSITY; LEE, XUHUI - YALE UNIVERSITY; GRIFFIS, TIMOTHY - UNIVERSITY OF MINNESOTA; BAKER, JOHN; TURNER, PETER - STANFORD UNIVERSITY

Submitted to: Water Resources Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/27/2018
Publication Date: 5/21/2018
Citation: Fu, C., Lee, X., Griffis, T.J., Baker, J.M., Turner, P.A. 2018. A modeling study of direct and indirect N2O emissions from a representative catchment in the U. S. Corn Belt. Water Resources Research. 54(5):3632-3653. doi:https://doi.org/10.1029/2017WR022108.
DOI: https://doi.org/10.1029/2017WR02210810.1029/2017WR022108

Interpretive Summary: Nitrous oxide is a greenhouse gas that is 300 times more potent than carbon dioxide on a molecule per molecule basis, and agricultural systems are the primary source of it. Consequently, finding ways to reduce nitrous oxide emissions may be the most important way that agricultural producers can contribute to climate change mitigation. However, our understanding of nitrous oxide emissions processes is incomplete and fragmentary. One area of major uncertainty is "indirect" emissions , or emissions that occur "beyond the field", caused by microbial denitrification of nitrate that has been transported into streams by runoff.We used data from recent measurements of nitrous oxide emissions from streams to add direct and indirect emission predictions to a widely used hydrologic catchment model, the Soil Water Assessment Tool (SWAT). After calibrating the new components, the model was used to simulate both direct ("in field") and indirect nitrous oxide losses from a number of watersheds in southeast MN. We showed that the smallest streams could contribute 26-46% of total nitrous oxide emissions, suggesting that the IPCC (Intergovernmental Panel on Climate Change) may be underestimating total nitrous oxide emissions by not properly accounting for these indirect emissions. These results should help to resolve why the atmospheric concentration of nitrous oxide has been increasing faster than predicted. They also provide additional incentive to reduce nitrate losses from farm fields.

Technical Abstract: Indirect nitrous oxide (N2O) emissions from drainage ditches and headwater streams are poorly constrained. To date, few studies have monitored stream N2O emissions and to our knowledge, no modeling studies have been conducted to simulate stream N2O emissions. In this study, we developed direct and indirect N2O emission modules and a corresponding calibration module for use in the Soil and Water Assessment Tool (SWAT) model, and implemented the expanded SWAT model (termed SWAT-N2O) to a representative 4th stream-order catchment (size 210 km2) and six first-order stream catchments (size 0.22 – 1.83 km2) in southeastern Minnesota. We simulated the spatial and temporal fluctuations of the indirect emission fluxes from streams, identified the ‘hotspots’ and ‘hot moments’ of the N2O emissions, and diagnosed the correlations between direct and indirect emission fluxes. We showed that zero-order streams and first-order streams could contribute 0.037 – 0.74 and 0.013 nmol m-2 s-1 (flux intensity expressed on the basis of unit catchment area) to the total surface emission fluxes, respectively. Emissions from zero and first order streams equals 26 – 46 % of direct emissions, which may explain the emission gap between calculations using top-down and bottom-up methods. Clear spatial patterns were identified for both direct and indirect emissions and their spatial variations were negatively correlated. Our results suggest that the IPCC N2O emission factor for streams in the Corn Belt should be increased by 4 to 7 times. Increasing precipitation and streamflow in the Corn Belt may potentially increase N2O emissions from both soils and streams.