NITROGEN-METABOLISM OF DOUGLAS-FIR AND SCOTS PINE AS AFFECTED BY OPTIMAL NUTRITION AND WATER-SUPPLY UNDER CONDITIONS OF RELATIVELY HIGH ATMOSPHERIC NITROGEN DEPOSITION

Nitrogen metabolism of the needles of 40-year-old Douglas fir and Scot s pine trees, growing in two forest stands on cation-poor and acidic s andy soil with a relatively high atmospheric nitrogen deposition was s tudied. The composition of the free amino acid (FAA) pool, the concent rations of total nitrogen and soluble protein and the activities of gl utamine synthetase (GS) and glutamate dehydrogenase (GDH) were determi ned in the needles. An excessive nitrogen supply by a high atmospheric nitrogen deposition in both forest stands was indicated by the high c oncentrations of total nitrogen and the amino acids arginine, glutamic acid, glutamine and aspartic acid in control trees. In addition the e ffect of optimal nutrition and water supply (fertigation) on the needl e nitrogen metabolism was evaluated. The total concentration of the FA A pool in needles of both tree species was lower in the fertigated tha n in the non-fertigated (control) trees, except for 1-year-old needles of Scots pine, in which the concentration after fertigation did not d iffer from the control. The lower total FAA concentration in the ferti gated trees could be attributed to arginine, the concentration of whic h was on average 60% lower than in the control. Neither the concentrat ion of soluble protein nor the activity of GS were influenced by ferti gation. The activity of GDH in fertigated trees only differed signific antly from the control in October. Scots pine needles had higher conce ntrations of protein (50%) and higher activities of GS (44%) and GDH ( 25%) than Douglas fir needles. Possible explanations for the lower vit ality of Douglas fir compared to Scot pine are given.