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Highlights • Inter-comparison of relaxed eddy accumulation and flux gradient measurements. • Strong correlation between the two NH 3 flux measurement methods. • Peak NH 3 flux measured with both systems six days after fertilizer application. • Two elevated emission periods influenced by urease inhibitor and environmental conditions. Abstract Studies of NH 3 flux over agricultural ecosystems in the USA are limited by low temporal resolution (typically hours or days) and sparse spatial coverage, with no studies over corn in the Midwest USA. We report on NH 3 flux measurements over a corn canopy in Central Illinois, USA, using the relaxed eddy accumulation (REA) and flux gradient (FG) methods, providing measurements at 4 h and 0.5 h intervals, respectively. The REA and FG systems were operated for the duration of the 2014 corn-growing season. Flux-footprint analysis was used to select data from both systems, resulting in 82 concurrent measurements. Mean NH 3 flux of concurrent measurements was 205 ± 300 ng m−2 s−1 from REA and 110 ± 256 ng m−2 s−1 from FG for all concurrent samples. Results from both methods were not significantly different at a 95% confidence level for all concurrent measurements. The FG system resolved NH 3 emission peaks at 0.5 h averaging time that were otherwise un-observed with 4 h REA averaging. Two early-season peak emission periods were identified (DOY 130-132 and 140-143), where the timing and intensity of such emissions were attributed to a combination of urease inhibitor, applied as a field-management decision, and localized soil temperature and precipitation. Given the dependence of NH 3 fluxes on multiple parameters, this study further highlights the need for increased spatial coverage and high temporal resolution (e.g., <1 h) of measurements to better understand the impact of agricultural NH 3 emissions on air quality and the global nitrogen cycle. Such measurements are also needed for evaluation of models describing surface-atmosphere exchange of NH 3. [ABSTRACT FROM AUTHOR]