THE EXCHANGE OF ATMOSPHERIC AMMONIA WITH VEGETATED SURFACES .2. FERTILIZED VEGETATION

Measurements of the exchange of gaseous ammonia (NH3) have been made o ver fertilized plant canopies using micrometeorological techniques. Th e NH3 exchange over ungrazed grassland and arable crops was examined i n a range of environmental conditions: a bidirectional exchange of amm onia was observed, with warm dry conditions favouring emission (NH3 fl uxes up to 24 ng m-2 s-1) and cool wet conditions favouring deposition (fluxes in the range 1-35 ng m-2 s-1). Limited measurements made when the ground was frozen suggest an increased surface resistance may occ ur in these conditions (measured R(c) up to 80 s m-1), resulting in re duced rates of deposition. The fluxes are contrasted with measurements over unfertilized vegetation, and it is suggested that the net canopy exchange is the combined result of leaf surface, stomatal and soil ex change processes; the increased tendency of fertilized vegetation to e mit ammonia is attributed to the larger fixed nitrogen concentrations in these ecosystems. The bidirectional fluxes may be related to the ex istence of ammonia compensation points. Estimates of the stomatal comp ensation point for fertilized croplands were in the range 1-7 mug NH3 m-3 though because of deposition to leaf surfaces, this frequently doe s not control net fluxes. The mean surface concentration of ammonia ch i(z(o)'), or canopy compensation point, was therefore also estimated a nd found to vary, with environmental conditions, in the range 0-3 mug m-3. A simplified procedure is used to estimate the net annual exchang e of ammonia over an example fertilized crop in the British Isles. Thi s predicts an emission of fixed nitrogen of less than 1 kg per hectare , which is the net result of emission and deposition throughout the ye ar.