Impact of the oasis effect on wind tunnel measurements of ammonia volatilization from urea

Abstract

The validity of emission factors derived from small-scale measurements of ammonia (NH3) volatilization has been questioned in the literature because gaseous NH3 concentration gradients differ at the edge of the measurement plot and may result in higher emissions than at field scale. We studied this "oasis effect" using two very long (22 m) wind tunnels constructed indoors over soil plots fertilized with surface-applied urea (20 g N m-2). We hypothesized that NH3 flux would be highest at the start of the tunnel and decrease with distance. Air NH3 concentration was measured every 2 m along each tunnel for 2 wk after urea application; NH3 flux did not decrease along the length of the tunnels. Of the 60 measurement periods, when there was significant NH3 volatilization, only two had a significant nonlinear relationship (P ≤ 0.05) between NH3 concentration and distance. For the other periods, the NH3 concentration increased linearly with distance (P ≤ 0.05). The background NH3 concentration difference between halves of the tunnels was not significantly related to NH3 flux difference (P > 0.1). Our results indicate that wind tunnel measurements of NH3 volatilization fertilized using urea are not impacted by a measurable oasis effect.