Plant-atmosphere exchange of ammonia

The results of recent controlled environment and micrometeorological measure- ments of NH3 fluxes are presented to highlight the processes controlling NH3 plant-atmosphere exchange. The presence of NH+ in leaf tissues results in the existence of an NH3 'compensation point' concentration for substomatal tissues (Xs), so that both emission and deposition are possible from stomata. In addition, NH3 may deposit efficiently on to leaf cuticles, short-circuiting any stomatal emis- sion, so that a 'canopy compensation point' (Xc) may be defined that is smaller than Xs. Ammonia is generally deposited to nitrogen limited ecosystems, indi- cating a small Xs and small leaf cuticle resistance (Rw). In contrast, fluxes over croplands are typically bidirectional and may reflect a larger Xs as a consequence of greater N supply. The paper discusses the processes defining Rw (humidity, acidic pollutants) and Xs (plant phenology, species, N nutrition) and proposes a new resistance approach, which integrates Xs and Rw into one model. Estimating long term bidirectional NH3 fluxes is still uncertain, though it is now possible to apply a single model concept to a range of ecosystem types and satisfactorily infer NH3 fluxes over diurnal time scales.