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 (FB), FA supplemented with 754 mg m(-3) (650 mu l l(-1)) CO2, FA supplemented with 100 mu g m(-3) 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 area (SLA) and N concentration in the needles. The activity of peroxidase (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 needles, 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 are discussed. The expected stimulation of biomass production by elevated CO2 may be reduced in the presence of atmospheric NH3.