Nitrogen Dioxide - a Gaseous Fertilizer of Poplar Trees

Abstract

Cuttings of a poplar clone (<em>Populus x euramericana</em> 'Dorskamp') were exposed to filtered air and to filtered air with 80 to 135 nl 1<sup>-1</sup> NO<sub>2</sub> added in climate chambers during 12 weeks. Three different levels of nitrogen fertilization were used, the lowest causing symptoms of nitrogen deficiency. NO<sub>2</sub> fumigation caused no visible injury to the plants. Fumigated plants showed elevated activity of nitrate reductase and higher leaf nitrogen concentrations relative to the control, indicating nitrogen assimilation from NO<sub>2</sub>. Fumigation enlarged the foliar area and, at the lowest nitrogen supply from the substrate, elevated the net CO<sub>2</sub> assimilation rate. At the highest level of soil nitrogen supply, fumigation enlarged the width of xylem and bark tissue in the main stem. Fumigation had a stimulating effect on total biomass production during the exposure period. Thus, NO<sub>2</sub> acted as a nitrogen fertilizer, regardless of the nitrogen supply from the substrate. The results are discussed with regard to hypotheses concerning the impact of nitrogen oxides on forest ecosystems.