Ammonia dry deposition over grassland - micrometeorological flux-gradient measurements and bidirectional flux calculations using an inferential model

For a 17-day period in late summer 1995 ammonia (NH(3)) fluxes over an extensive area of semi-natural ungrazed short grassland were measured and modelled. The measurements were carried out at the Melpitz field research station near Torgau in the state of Saxony in eastern Germany (86 m above sea level, 51 degrees 31'N, 12 degrees 56'E). For the calculation of the fluxes the micrometeorological flux-gradient technique was applied using a sensitive continuous-flow wet annular denuder system for NH(3) at three heights. Both downward fluxes to the canopy and upward fluxes from the grassland to th; atmosphere were observed. On the basis of the direction of the flux in relation to air concentrations, the ammonia canopy compensation point was estimated to vary within the range 0.2 to 3.0 mug m(-3). Because the simple 'canopy/surface resistance' (R(c)) model presupposes a surface concentration of zero for the observed trace gas, which cannot explain bidirectional fluxes, the results of the measurements were compared with a static 'canopy compensation point-cuticular resistance' (chi (c) - R(w)) model, which is able to quantify the stomatal compensation point (chi (s)) and allows bidirectional fluxes. The agreement between the model and the measured fluxes for the whole period was relatively good. Given realistic ratios of ammonium to proton concentrations (Gamma = [NH(4)(+)]/[H(+)]) for the intercellular fluid of between 150 and 1000 the minimum in the difference was found for Gamma approximate to 1000 (correlation coefficient r(2) = 0.23) and even bidirectional fluxes were obtained. Possible reasons for the differences between the measurements and model estimates are discussed.