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This article in AJ

  1. Vol. 72 No. 6, p. 876-883
    Received: July 21, 1980

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Ammonia Exchange at the Land Surface1

  1. Edgar Lemon and
  2. Raymond Van Houtte2



Studies of NH3 exchange between plant leaves and air have mostly been made in controlled chambers where the concentrations were far above the ambient atmospheric levels usually found in nature. Under such conditions leaves act as an infinite sink for NH3. More recent controlled chamber studies, however, have shown NH3 evolving from leaves when concentrations were nearer to those of the atmosphere. In this study we measured both NH3 absorption and evolution by vegetation under field conditions using micrometeorological techniques where vertical profiles of wind and NH3 are measured. Both evolution and absorption strongly suggest that plants have a compensation point for NH3. Data are presented for fields of quackgrass (Agropyron repens L.), soybeans (Glycine max L. Merr.) and alfalfa (Medicago saliva L.). Computer simulation confirmed that vegetation exchange is concentration dependent. We speculate that plants (and soil) keep atmospheric NH3 in check, at low concentrations, with winds moving it from N rich to N poor ecosystems. Dispersed sources of NH3 from crop lands are probably increasing due to more fixed N in farming. Thus the added NH3 due to man's activity probably increases the net global biosphere uptake of both N and C.

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