Estimation of In-Canopy Ammonia Sources and Sinks in a Fertilized Zea mays Field

An analytical model was developed to describe in-canopy vertical distribution of ammonia (NH3) sources and sinks and vertical fluxes in a fertilized agricultural setting using measured in-canopy mean NH3 concentration and wind speed profiles. This model was applied to quantify in-canopy air-surface exchange rates and above-canopy NH3 fluxes in a fertilized corn (Zea mays) field. Modeled air-canopy NH3 fluxes agreed well with independent above-canopy flux estimates. Based on the model results, the urea fertilized soil surface was a consistent source of NH3 one month following the fertilizer application, whereas the vegetation canopy was typically a net NH3 sink with the lower portion of the canopy being a constant sink. The model results suggested that the canopy was a sink for some 70% of the estimated soil NH3 emissions. A logical conclusion is that parametrization of within-canopy processes in air quality models are necessary to explore the impact of agricultural field level management practices on regional air quality. Moreover, there are agronomic and environmental benefits to timing liquid fertilizer applications as close to canopy closure as possible. Finally, given the large within-canopy mean NH3 concentration gradients in such agricultural settings, a discussion about the suitability of the proposed model is also presented.