Nitrous Oxide Emissions from the Application of Fertilizers: Source Partitioning

Nitrous oxide (N₂O) emissions account for 7% of total U.S. greenhouse gas emissions and have been identified as the dominant driver of stratospheric ozone depletion in the 21st century. In the U.S., agriculture accounts for 75% of total N₂O emissions, of which 92% is attributable to soil management practices such as fertilizer N and manure application. N₂O emissions from soil are mainly mediated by nitrifying and denitrifying microorganisms and are controlled by C and N availability, O₂ availability (often approximated by soil moisture content), and soil pH. The objective of the current study is to expose data availability and then quantitatively summarize effects of a suite of agronomic management practices on N₂O emissions in corn cropping systems in the Midwestern U.S. and southeastern Canada through meta-analysis. This approach explores the potential of alternative agronomic management practices to mitigate N₂O emissions from corn cropping systems in major corn producing regions in the U.S. and Canada, by synthesizing available data from peer-reviewed literature. Corn was selected as a focus crop, because it covers the greatest proportion of U.S. crop land and receives more N than any other major U.S. crop. The geographic area of interest confines a large corn-growing region with relatively comparable climate, environmental conditions, and agronomic management.

The use of the urease inhibitor N-(n-butyl) thiophosphoric triamide (NBPT) in combination with the nitrification inhibitor dicyandiamide (DCD) was the only management strategy that consistently reduced N₂O emissions. Manure application caused higher N₂O emissions compared to the use of synthetic fertilizer N. This warrants further investigation in appropriate manure N management, particularly in the Lake States, where often over 30% of corn crop land receives manure. The N₂O response to increasing N rate varied by region, indicating the importance of region-specific approaches for quantifying N₂O emissions and mitigation potential. In general, more data collection on side-by-side comparisons of common and alternative management practices, especially those pertaining to N placement, N timing and N source, in combination with biogeochemical model simulations, will be needed to further develop and improve N₂O mitigation strategies for corn cropping systems in the major corn producing regions in the U.S.