THE EFFECTS OF EXCESS NITROGEN DEPOSITION ON YOUNG NORWAY SPRUCE TREES .2. THE VEGETATION

The effects of wet-deposited nitrogen on soil acidification and the he alth of Norway spruce were investigated in a pot experiment using an o pen-air spray/drip system. Nitrogen was applied as ammonium ((NH4)2SO4 ) or nitrate (HNO3/NaNO3) in simulated rain to either the soil or the foliage. Symptoms of forest decline as observed in the field were not reproduced, and there was no evidence of direct toxicity. Treatments d id, however, have significant effects on tree nutrition. Both NH4+ and NO3- treatments applied to the foliage lowered foliar K concentration s, NH4+ to a greater extent. Soil-applied NH4+ reduced foliar Mg conce ntrations and increased foliar Al and Fe. Soil-applied NO3- significan tly reduced foliar P concentrations, and at high doses prevented the a lleviation of P deficiency by fertiliser. These effects could be impor tant in some field situations. Ammonium deposition is predicted to be more damaging than nitrate deposition, although the latter may, be cri tical for forests where P status is marginal, such as in parts of the British uplands.