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

Summary

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 μg m⁻³. The results are used to test hypotheses concerning the canopy cycling of reduced nitrogen (NH_x) 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 NH_x exchange provides a useful basis for developing quantitative resistance models to predict NH₃ fluxes.