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SUMMARY The transition of Dutch heathlands dominated by Calluna vulgaris to grass heaths occurs rapidly and is primarily ascribed to nitrogenous air pollution. C. vulgaris and Deschampsia flexuosa were fumigated with 15NH3 and the adsorption of ammonia to the leaf surface as well as the uptake and translocation of ammonia nitrogen was determined after 4 and 9 weeks. There appeared to be little or no NH3‐nitrogen adsorbed to the leaf surface of either species. D. flexuosa took up more NH3 per unit shoot weight, especially during the first 4 weeks of fumigation. The proportion of nitrogen in the shoots of C. vulgaris taken up as NH3 was approximately 6% and 15% after 4 and 9 weeks exposure to NH3, respectively, while corresponding proportions for D. flexuosa were approximately 17% and 28%. The nitrogen fraction derived from the foliar uptake of NH3 was a factor 1·5 higher in the roots of D. flexuosa than in the woody parts and roots of C. vulgaris after 9 weeks fumigation. Regular spraying with tap water did not influence the concentration of nitrogen or that of cations in either species. The uptake and allocation of NH3‐nitrogen in the two species is discussed in relation to growth and consequences for competition in the field.

To determine the fate of atmospheric ammonium in forest soils, one calcareous and two acid forest soils were incubated with ~SN ammonium. In the calcareous soil about 65% of the applied ~5N-ammonium was recovered as nitrate after 98 days of incubation, whereas in the acid soils less than 10% of the 15N-ammonium was converted to nitrate. In all soils a large proportion of the ~5N was incorporated in organic nitrogen compounds. This incorporation limits the use of 15N tracers for the elucidation of the fate of atmospheric ammonium in soils.

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