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DeNitrification DeComposition (DNDC) model predictions of NH 3 fluxes following chemical fertilizer application were evaluated by comparison to relaxed eddy accumulation (REA) measurements, in Central Illinois, United States, over the 2014 growing season of corn. Practical issues for evaluating closure were addressed by accounting for fluxes outside the measurement site and differences in temporal resolution. DNDC modeled NH 3 fluxes showed no significant differences in magnitude (at p = 0.05) compared to measurements and replicated trends satisfactorily (r a 2 > 0.74), during the initial 33 days after fertilizer application, when measured fluxes were to the atmosphere, compared to later time periods when depositional fluxes were measured (r a 2 < 0.52). Among the model input parameters, NH 3 fluxes were most sensitive to air temperature, precipitation, soil organic carbon, field capacity, pH, and fertilizer application rate, timing, and depth. By constraining these inputs for conditions in Central Illinois, uncertainty in daily NH 3 fluxes was estimated to vary from 0% to 70% on a daily basis, during the corn growing season, with the highest uncertainty values estimated for the period of highest positive NH 3 fluxes. These results can guide future improvements in DNDC, which is a valuable tool to assist (1) in the development of NH 3 emission inventories with high spatial (constrained by the spatial resolution of input parameters) and temporal resolution (daily) and (2) in upscaling emissions from the site (farm) to the regional scale. [ABSTRACT FROM AUTHOR]

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]

Highlights • Modeling the bi-directional NH 3 exchange at the field scale using SURFATM-NH 3. • Integrating an operational parameterization of emission potentials in the model. • Parameterizing the effect of urease inhibitor on NH 3 volatilization. • Satisfactory simulations of NH 3 fluxes by integrating the effect of urease inhibitor. • Good predictions of dynamic and order of magnitude of emission potentials by the model. Abstract Agriculture is the main source of ammonia (NH 3) emissions in the atmosphere. NH 3 is precursor to secondary fine particulate matter, which is of concern for its impacts on health and visibility. There are a limited number of field measurements of NH 3 emissions from fertilizer application in the US, and this limits our understanding of the importance of individual NH 3 source and sink processes in controlling timing and magnitude of NH 3 emissions. In this study, a new parameterization of the effect of urease inhibitor on NH 3 emissions from urea-based fertilizer was developed on the basis of experimental results found in the literature. This parameterization was combined with an existing operational parameterization of soil and stomatal emission potentials (Γ g , Γ s) and was implemented in a surface-atmosphere transfer model for NH 3 (SURFATM-NH 3) in order to evaluate the bi-directional fluxes of NH 3 at the field scale. The model was evaluated with field measurements obtained by the flux-gradient (FG) and relaxed eddy accumulation (REA) methods in a fertilized corn field in central Illinois. By integrating the effect of urease inhibitor, the timing of the highest NH 3 emission peak was successfully predicted and its magnitude was close to that measured (predicted 2106 ng m−2 s−1, measured by FG 2312 ± 582 ng m−2 s−1). Based on the model results, urease inhibitor has a considerable effect on the dynamics and order of magnitude of NH 3 fluxes. Furthermore, the model simulated the inhibiting action of N- (n -butyl) thiophosphoric (nBTPT) and suggests that it can reduce NH 3 volatilization by 32%. The model also successfully predicted environmental parameters, such as soil temperature. Finally, this new version of SURFATM-NH 3 is a valuable tool to estimate the NH 3 bi-directional fluxes at the field scale, which describes dynamic modeling of Γ s and Γ g by taking into account the effect of urease inhibitor which is commonly used in the US to improve the efficiency of urea fertilizers. [ABSTRACT FROM AUTHOR]