DIFFERENTIAL-EFFECTS OF AMMONIUM AND NITRATE ON 3 HEATHLAND SPECIES

Arnica montana and Cirsium dissectum, typical of species-rich heathlan ds and acidic grasslands, have declined rapidly in The Netherlands in recent years. Field surveys suggest that the decline is caused by soil acidification as a result of enhanced atmospheric N and S deposition. Therefore, the survival, growth and development of these species were studied in a water culture experiment, using nutrient solutions which differed both in mineral nitrogen form and in ammonium concentration. For comparison, the performance of a third, acid tolerant species, Ca lluna vulgaris, was studied. The results showed that both Arnica and C irsium performed better using nitrate than when using ammonium as a so le nitrogen source, whereas ammonium toxicity became apparent when amm onium concentrations were raised above 100 mu M. Ammonium toxicity was expressed by an increase in mortality of Arnica plants with increasin g ammonium concentrations and by a reduction of biomass in Arnica and Cirsium. Furthermore, cation concentrations in both roots and shoots d ecreased when ammonium was supplied as a nitrogen source. In contrast, Calluna showed optimal development when using ammonium as a sole nitr ogen source. In this species, only root biomass was negatively affecte d by high ammonium concentrations. The ecological implications of thes e preferences are discussed in relation to soil acidification.