The exchange of atmospheric ammonia with vegetated surfaces II: Fertilized vegetation

Measurements of the exchange of gaseous ammonia (NH3) have been made over fertilized plant canopies using micrometeorological techniques. The NH3 exchange over ungrazed grassland and arable crops was examined in a range of environmental conditions: a bidirectional exchange of ammonia was observed, with warm dry conditions favouring emission (NH3 fluxes up to 24 ng m-2s-1) and cool wet conditions favouring deposition (fluxes in the range 1-35 ng m-2s-1). Limited measurements made when the ground was frozen suggest an increased surface resistance may occur in these conditions (Measured Rc up to 80 s m-1), resulting in reduced 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 exchange processes; the increased tendency of fertilized vegetation to emit ammonia is attributed to the larger fixed nitrogen concentrations in these ecosystems. The bidirectional fluxes may be related to the existence of ammonia compensation points. Estimates of the stomatal compensation point for fertilized croplands were in the range 1-7 g NH3m-3, though because of deposition to leaf surfaces, this frequently does not control net fluxes. The Mean surface concentration of ammonia X(z'o), or canopy compensation point, was therefore also estimated and found to vary, with environmental conditions, in the range 0-3 g m-3. A simplified procedure is used to estimate the net annual exchange of ammonia over an example fertilized crop in the British Isles. This predicts an emission of fixed nitrogen of less than 1 kg per hectare, which is the net result of emission and deposition throughout the year.