THE EFFECTS OF EXCESS NITROGEN DEPOSITION ON YOUNG NORWAY SPRUCE TREES .1. THE SOIL

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 for a period of two years five months, Symptoms of forest decl ine were not reproduced. Adverse effects relating to soil acidificatio n and N saturation were observed and depended on the chemical form of N. The plant-soil system absorbed most of the soil-applied NH4+ at dos es of up to 65 kgN ha-1 year-1 but only 54% at a dose of 125 kgN ha-1 year-1. About 60% of soil-applied NO3- was absorbed in all treatments. Ammonium treatments acidified the soil, increased base cation leachin g, and mobilised acidic cations. Nitrification was not the major sourc e of acidity, however. Nitrate inputs increased soil pH. Critical load s calculated using current criteria were 60-120 and 30-60 kgN ha-1 yea r-1 for ammonium and nitrate, respectively. Ammonium is more likely to damage forest ecosystems, however, illustrating the need for care in the definition of critical loads.