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Plant-Atmosphere Exchange of Ammonia [and Discussion]

M. A. Sutton, J. K. Schjorring, G. P. Wyers, J. H. Duyzer, P. Ineson and D. S. Powlson
Philosophical Transactions: Physical Sciences and Engineering
Vol. 351, No. 1696, The Exchange of Trace Gases between Land and Atmosphere (May 16, 1995), pp. 261-278
Published by: Royal Society
Stable URL: http://www.jstor.org/stable/54415
Page Count: 18
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Plant-Atmosphere Exchange of Ammonia [and Discussion]
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Abstract

The results of recent controlled environment and micrometeorological measurements of NH3 fluxes are presented to highlight the processes controlling NH3 plant-atmosphere exchange. The presence of NH4+ in leaf tissues results in the existence of an NH3 `compensation point' concentration for substomatal tissues (χ s), so that both emission and deposition are possible from stomata. In addition, NH3 may deposit efficiently on to leaf cuticles, short-circuiting any stomatal emission, so that a `canopy compensation point' (χ c) may be defined that is smaller than χ s. Ammonia is generally deposited to nitrogen limited ecosystems, indicating a small χ s and small leaf cuticle resistance (Rw). In contrast, fluxes over croplands are typically bidirectional and may reflect a larger χ s as a consequence of greater N supply. The paper discusses the processes defining Rw (humidity, acidic pollutants) and χ s (plant phenology, species, N nutrition) and proposes a new resistance approach, which integrates χ s 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.

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