Design and ammonia-recovery evaluation of a wind speed-sensitive chamber system

A previously developed system to measure NH3 volatilization in the field consists of a vacuum pump, volatilization chambers, and acid traps to capture NH3. The vacuum pump of the system draws air through each chamber at a constant rate, which may be faster or slower than the wind speed outside the chamber. Because wind speed affects NH3 volatilization, it would be desirable to draw air through the chambers at a wind speed similar to that outside the chambers. The objective of this work was to improve the existing design by adding the capability of adjusting the speed at which air is drawn through each chamber. A hot needle anemometer was placed next to each chamber to measure wind speed at I cm above the soil surface. Wind speed was used to adjust an electronic valve so that the rate at which air was drawn through the chamber would match the external wind speed. Three N recovery studies were conducted with a sand-CaCO3 mixture in the field to determine if the system could be used to obtain quantitative estimates of NH3 volatilized. The NH3 volatilized in each study was estimated from (i) the loss of soil inorganic N and (ii) the N captured in the acid traps. In all three studies, the NH3 volatilized estimated from the acid traps (10-15% of applied N) was not different from the NH3 volatilized estimated from the loss of soil inorganic N. These results indicate that the system can be used to obtain quantitative measurements of NH3 volatilized under the conditions of these studies.