Season-long ammonia flux measurements above fertilized corn in central Illinois, USA, using relaxed eddy accumulation

The objective of this research is to quantify NH₃ flux above an intensively managed cornfield in the Midwestern United States to improve understanding of NH₃ emissions and evaluations of new and existing emission models. A relaxed eddy accumulation (REA) system was deployed above a corn canopy in central Illinois, USA (40°3′46.209″N, 88° 11′46.0212″W) from May through September 2014 (day of year 115–273) to measure NH₃ fluxes due to chemical fertilizer application. NH₃ flux was measured in four-hour periods during mornings and afternoons. Mean atmospheric NH₃ concentration during the complete measurement period was 2.6 ± 2.0 μg m⁻³. Larger upward fluxes of gaseous NH₃ were measured during the first 30 days after fertilization, with variations observed throughout the field campaign. Measured NH₃ fluxes ranged from −246.0 ng m⁻² s⁻¹ during wintertime background measurements to 799.6 ng m⁻² s⁻¹ within two weeks of fertilization (where negative flux indicates deposition). Mean positive flux was 233.3 ± 203.0 ng m⁻² s⁻¹ in the morning and 260.0 ± 253.3 ng m⁻² s⁻¹ in the afternoon while mean negative flux was −45.3 ± 38.6 ng m⁻² s⁻¹ in the morning and −78.35 ± 74.9 ng m⁻² s⁻¹ in the afternoon. NH₃ volatilization during the first 21 days after fertilization accounted for 79% of total nitrogen loss during the growing season. Such measurements are critical to improve understanding of agricultural NH₃ emissions in managed agricultural ecosystems dominated by rotations of highly fertilized corn and moderately to lightly fertilized soybeans, such as the plot studied herein. These measurements are also important to improve understanding of how managed agricultural ecosystems impact air quality, and contribute to the global nitrogen cycle, and to evaluate current NH₃ emission models.