Gaseous ammonia counteracts the response of Scots pine needles to elevated atmospheric carbon dioxide

Four-year-old saplings of Scots pine (Pinus sylvestris L.) were exposed for 8 wk in controlled-environment chambers to charcoal-filtered air (FA), FA supplemented with 754 mg m^-3 (650 μl 1^-1 )CO₂ , FA supplemented with l00 μ m¹³ NH₃ and FA+CO₂ +NH₃ . Elevated CO₂ induced a significant increase in the concentrations of NH₄ ⁺ and NO₃ in the soil solution, while exposure to NH₃ enhanced the soil NH₄ ^- concentration. Elevated CO₂ significantly increased needle biomass and area, and decreased specific leaf are (SLA) and N concentration in the needle. The activity of peroidase (POD) was decreased, while the activities of glutamine synthetase (GS) and glutamate dehydrogenase (GDH) were only slightly affected. Gaseous NH₃ enhanced the concentration of N. soluble proteins and the GS activity in the needless, while it decreased the POD and GDH activities. The effects of elevated CO₂ + NH₃ on needle biomass production, N Metabolism and POD activity were smaller than the effects of single exposures to elevated CO₂ or NH³ . Suggesting that elevated CO₂ and NH₃ counteract each other and disturb needle physiology. The possible mechanisms underlying the negative interactions of elevated CO₂ and NH₃ and discussed. The expected stimulation of biomass production by elevated CO₂ my be reduced in the presence of atmospheric NH₃ .