NITROGEN-DIOXIDE - A GASEOUS FERTILIZER OF POPLAR TREES

Cuttings of a poplar clone (Populus x euramericana 'Dorskamp') were ex posed to filtered air and to filtered air with 80 to 135 nl 1(-1) NO2 added in climate chambers during 12 weeks. Three different levels of n itrogen fertilization were used, the lowest causing symptoms of nitrog en deficiency. NO2 fumigation caused no visible injury to the plants. Fumigated plants showed elevated activity of nitrate reductase and hig her leaf nitrogen concentrations relative to the control, indicating n itrogen assimilation from NO2. Fumigation enlarged the foliar area and , at the lowest nitrogen supply from the substrate, elevated the net C O2 assimilation rate. At the highest level of soil nitrogen supply, fu migation enlarged the width of xylem and bark tissue in the main stem. Fumigation had a stimulating effect on total biomass production durin g the exposure period. Thus, NO2 acted as a nitrogen fertilizer, regar dless of the nitrogen supply from the substrate. The results are discu ssed with regard to hypotheses concerning the impact of nitrogen oxide s on forest ecosystems.