Relative Humidity Controls Ammonia Loss from Urea Applied to Loblolly Pine
2010
Cabrera, M. L. | Kissel, D. E. | Craig, J. R. | Qafoku, N. P. | Vaio, N. | Rema, J. A. | Morris, L. A.
In the United States, approximately 600,000 ha of pine trees are fertilized with urea each year, with NH₃ volatilization losses ranging from <1% to >50% depending on environmental conditions. Previous work showed that the timing of rainfall after urea application plays a significant role in controlling NH₃ loss, but the effect of other environmental variables is not well understood. We conducted 10 29-d studies under different environmental conditions during 2 yr to identify important variables controlling NH₃ loss from urea applied to loblolly pine (Pinus taeda L.) at 200 kg N ha⁻¹ Ammonia loss was measured with dynamic chambers that adjusted the rate of air flow through the system based on wind speed at 1 cm above the soil surface. Regression analysis indicated that a variable related to the initial water content of the forest floor and a variable related to the relative humidity (RH) during the study explained 85 to 94% of the observed variability in NH₃ loss. Relatively high initial water content followed by consistently high RH led to large NH₃ losses. In contrast, low initial water contents resulted in slow rates of NH₃ loss, which increased when elevated RH led to an increase in the water content of the forest floor. These results indicate that RH can play a significant role in NH₃ loss by accelerating urea dissolution and by increasing or decreasing the water content of the forest floor, which in turn can affect the rate of urea hydrolysis.
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