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Title: Ammonia flux measurements above a corn canopy using relaxed Eddy accumulation and a flux gradient system

item NELSON, ANDREW - University Of Illinois
item LICHIHEB, NEBILA - National Oceanic & Atmospheric Administration (NOAA)
item KOLOUTSOU-VAKAKIS, SOTIRIA - University Of Illinois
item ROOD, MARK - Oak Ridge National Laboratory
item HEUER, MARK - National Oceanic & Atmospheric Administration (NOAA)
item MYLES, LATOYA - National Oceanic & Atmospheric Administration (NOAA)
item JOO, EVA - University Of Illinois
item MILLER, JESSE - University Of Illinois
item Bernacchi, Carl

Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/4/2018
Publication Date: 1/15/2019
Citation: Nelson, A.J., Lichiheb, N., Koloutsou-Vakakis, S., Rood, M.J., Heuer, M., Myles, L., Joo, E., Miller, J., Bernacchi, C.J. 2019. Ammonia flux measurements above a corn canopy using relaxed Eddy accumulation and a flux gradient system. Agricultural and Forest Meteorology. 264:104-113.

Interpretive Summary: Ammonia is a usable form of nitrogen for plants, although excess concentrations can be very damaging. Very little is known about how much ammonia is exchanged between agricultural fields and the atmosphere. While ammonia is often deposited on agricultural fields from the atmosphere, an excessive amount of ammonia that leaves agricultural fields and enters the atmosphere can have dramatic impacts on air quality and the global nitrogen cycle. Furthermore, ecosystem models that attempt to predict how much variation in ammonia fluxes occur with changing environmental conditions and/or fertilizer application rates are highly uncertain due to a lack of measurements. This research project compares two different techniques to measure how much ammonia is exchanged between the ecosystem and the atmosphere. The benefits of both techniques used here, the ‘relaxed eddy accumulation’ and the ‘gradient flux’ techniques, is that they allow for continuous measurements of ammonia fluxes thereby removing the sparse and episodic measurement that are traditionally performed. However, the principles of the measurements are quite different with the ‘gradient flux’ technique allowing for 0.5h incremental measurements compared with the 4 hour measurements associated with ‘relaxed eddy accumulation’. The results show that both measurements agreed in their measured ammonia fluxes. The results from both techniques were that ammonia fluxes were highly episodic, with peaks corresponding to application of fertilizer and fertilizer amendments, and that the gradient flux analysis provided better temporal resolution that corresponds to the high variability in ammonia fluxes. The conclusions are the ammonia fluxes can be quite large and variable suggesting that more spatial and temporal resolution measurements are necessary to better model both the rates and the consequences associated with variation in ammonia release from agricultural systems.

Technical Abstract: Studies of NH3 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 NH3 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 NH3 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 NH3 emission peaks at 0.5 h averaging time that were otherwise unobserved with 4 h REA averaging. Two early-season emission periods were identified (DOY 130-132 and 140-143), where the timing and intensity of such emissions are attributed to a combination of urease inhibitor, applied as a field-management decision, and localized soil temperature and precipitation. Given the dependence of NH3 fluxes on multiple parameters, this study further highlights need for increased spatial coverage and high temporal resolution (e.g., < 1 h) of measurements to better understand the impact of agricultural NH3 emissions on air quality and the global nitrogen cycle and for evaluation of models describing surface-atmosphere exchange of NH3.