Vegetation atmosphere exchange of ammonia: canopy cycling and the impacts of elevated nitrogen impacts

Micrometeorological measurements of atmospheric ammonia (NH3) exchange with semi-natural and agricultural plant communities were made using sensitive new instrumentation capable of determining NH3 fluxes at <0.1 g m–3. The results are used to test hypotheses concerning the canopy cycling of reduced nitrogen (NHx) and the existence of potential feedbacks between total N inputs (from agricultural sources or atmospheric deposition) and the net NH3 flux. The measurements over cropland, together with a model calculating the canopy compensation point for NH3 indicate the importance of stomatal NH3 emission and recapture of NH3 by plant cuticles and water-layers. In contrast, measurements at an extremely clean upland moorland suggest that cuticular desorption of NH3 is also possible at low concentrations. Interpretation of dew measurements suggests that leaf uptake of NH44 + may occur as a result of pH gradients between the leaf surface and apoplast. The combined conceptual model of NHx exchange provides a useful basis for developing quantitative resistance models to predict NH3 fluxes.