Relative Humidity Controls Ammonia Loss from Urea Applied to Loblolly Pine

In the United States, approximately 600,000 ha of pine trees are fertilized with urea each year, with NH 3 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 NH 3 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 NH 3 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 NH 3 loss. Relatively high initial water content followed by consistently high RH led to large NH 3 losses. In contrast, low initial water contents resulted in slow rates of NH 3 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 NH 3 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.