NITROGEN UPTAKE IN NEEDLES OF SCOTS PINE (PINUS-SYLVESTRIS L) WHEN EXPOSED TO GASEOUS AMMONIA AND AMMONIUM FERTILIZER IN THE SOIL

Young saplings of Pinus sylvestris L. were exposed to gaseous NH3 at 5 3 or 105 mug m-3 for one year in open-top chambers. Saplings received N-15-labelled (NH3)2SO4 via the soil. To examine the importance of fol iar N uptake, changes in the concentration of total and labelled N in the needles were followed. Increase in needle biomass and N concentrat ion were found in trees exposed to NH3, confirming that atmospheric NH 3 acted as a N fertilizer. NH3 had a greater and quicker effect than ( NH4)2SO4: compared with the growth in ambient air, the N concentration in the needles exposed to NH3 had increased by 49% in four months, wh ile the increase after highest N-fertilization (200 kg N ha-1 y-1) was only 8%. The small contribution of NH4+ fertilization to the total N concentration was not due to a deficient N uptake: the N-15 concentrat ion in the needles increased significantly with time. On the other han d, NH3 uptake in shoots may have a negative effect on the NH4+ root up take. The relation between plant N and atmospheric NH3 concentration w as non-linear and possible reasons for this observation are discussed. Fumigation with NH3 significantly decreased the ratios of K/N and P/N , showing that fumigation disrupted the nutrient balance.