Sources and sinks of ammonia within an oilseed rape canopy

Within-canopy profiles of ammonia (NH3) and measurements of the canopy turbulence characteristics were used to calculate the vertical source/sink density profile of NH3 and sensible heat in a mature oilseed rape (Brassica napus) canopy. For the analysis, the inverse Lagrangian technique (ILT) based on localized near-field theory was used. Turbulence was measured with a standard ultrasonic anemometer, which, although not ideal for in-canopy work, is estimated to lead to a parameterization of the normalized standard deviation of the vertical wind component (sigma (w)/u*), which is +/-11% accurate for heights >0.16 m during the day. The NH3 profiles in the canopy consistently show largest concentrations at the ground caused by NH3 release from decomposing litter leaves on the ground surface with values of up to 150 ng m(-2) s(-l) predicted by the ILT. The inverse Lagrangian source/sink analysis performs well for both sensible heat and NH3, although it proves to be sensitive to the choice of the source/sink layers and becomes uncertain at the ground. Despite the large estimated ground level emission (26 g NH3-N ha(-1) per day), the analysis indicates that for the runs considered all NH3 is recaptured by the lowest 0.7 m of the 1.38 m tall canopy, and that the bi-directional net exchange with the atmosphere is governed by the top 0.5 m, leading to a net emission from the canopy of 12 g NH3-N ha(-1) per day. Since measurements of apoplastic [NH4+] and pH indicate that no significant stomatal emission from foliage should have occurred, this suggests that the siliques were a further source of NH3. (C) 2000 Elsevier Science B.V. All rights reserved.