Ammonia volatilization from broadcast urea: measurements using a micrometeorological approach and modeling with the Denitrification-Decomposition (DNDC) model



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Synthetic N fertilizers, such as urea, are one of the main anthropogenic sources of atmospheric ammonia (NH₃). NH₃ volatilization from N fertilizer application can significantly reduce agronomic efficiency (AE), contribute to air pollution, soil acidification and eutrophication of water bodies. Therefore, understanding the processes and factors influencing NH₃ volatilization from broadcast urea is pivotal to improve agricultural sustainability. In Chapter 2 of this thesis, the integrated horizontal flux (IHF) approach was used to measure NH₃ volatilization from eight field experiments under cold weather conditions in Kansas. NH₃ volatilization was measured from circular plots (20-m radius) fertilized with urea and urea amended with urease inhibitor (NBPT) both at rate of 60 kg of N ha⁻¹. The impact of NH₃ volatilization losses on winter wheat was evaluated through experimental plots arranged in a complete randomized block design with treatments consisting of four different rates of application (30, 60, 90 and 120 kg N ha⁻¹) of urea and urea + NBPT and control (0 kg N ha⁻¹) . NH₃ cumulative losses varied from <1% to 29% of applied N. Largest losses occurred when urea was broadcast to soils with high water content followed by a dry period. The use of urease inhibitor NBPT reduced NH₃ volatilization losses in more than 20% at locations where the largest losses (> 25%) occurred. No statistical difference was found in terms of grain yield, N recovery and AE, when comparing urea and urea + NBPT treatments. In Chapter 3, simulations provided by two versions of the Denitrification-Decomposition (DNDC) process-based model (DNDC 9.5 and DNDC v.CAN) were compared with flux data obtained in 29 NH₃ volatilization sampling campaigns. These sampling campaigns were conducted in Kansas and Montana using the IHF method over circular plots. Overall, the DNDC v.CAN simulated NH₃ emissions with smaller average root mean square error ((RMSE) ̅ = 10.9 kg of N ha⁻¹) compared to the DNDC 9.5 ((RMSE) ̅ = 32.8 kg of N ha⁻¹). Our sensitivity analysis showed that soil pH and soil temperature were the main input variables affecting NH₃ volatilization in both models. In addition, our analysis demonstrated several drawbacks that could be improved in future versions of the model to better simulate NH₃ volatilization. These potential areas for improvements the DNDC model versions include: i) limitations in the soil-hydrology water sub-model affected the accuracy of the simulations of the effects of soil water content on urea hydrolysis, which has direct effect on NH₃ volatilization; ii) both models failed to simulate the effects of accumulated precipitation (≥ 20 mm) on NH₃ volatilization during the first 5-15 d post fertilization; iii) future developments of the DNDC should consider adding a more robust routine to simulate the effects of urease inhibitor on NH₃ volatilization and iv) the timing of the NH₃ volatilization peak after fertilization was underestimated by the DNDC v.CAN and largely overestimated by the DNDC 9.5.



Wheat, Ammonia, Volatilization, Broadcast, Urea

Graduation Month



Master of Science


Department of Agronomy

Major Professor

Eduardo Alvarez Santos