Plant—atmosphere exchange of ammonia

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Abstract
The results of recent controlled environment and micrometeorological measurements of NH 3 fluxes are presented to highlight the processes controlling NH 3 plant-atmosphere exchange. The presence of NH +4 in leaf tissues results in the existence of an NH 3 ‘compensation point’ concentration for substomatal tissues (x s ), so that both emission and deposition are possible from stomata. In addition, NH 3 may deposit efficiently on to leaf cuticles, short-circuiting any stomatal emission, so that a ‘canopy compensation point’ (X c ) may be defined that is smaller than X s . Ammonia is generally deposited to nitrogen limited ecosystems, indicating a small X s and small leaf cuticle resistance ( R w ). In contrast, fluxes over croplands are typically bidirectional and may reflect a larger X s as a consequence of greater N supply. The paper discusses the processes defining (humidity, acidic pollutants) and X s (plant phenology, species, N nutrition) and proposes a new resistance approach, which integrates X s and R w into one model. Estimating long term bidirectional NH 3 fluxes is still uncertain, though it is now possible to apply a single model concept to a range of ecosystem types and satisfactorily infer NH 3 fluxes over diurnal time scales.

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