Uptake and translocation of nitrogen was studied in laboratory microcosms consisting of Alnus glutinosa (L.) Gaertn., Frankia sp., Paxillus involutus (Fr.) Fr. and Pinus contorta Dougl. ex Loud. P. involutus was shown to form a fully functional ectomycorrhizal association with alder as well as pine, and the seedlings thus became interconnected by a common mycelium.
When microcosms were exposed to 15N2 gas, interplant translocation of 15N was observed in two out of three experiments. 15N2 was fixed by Frankia and translocated to all other parts of the system. In the two experiments in which interplant translocation occurred, between 5 and 15% of the 15N recovered was found in the pine seedlings.
Within seven days, fixed N2 was incorporated into amino acids in the Frankia nodules, translocated to both the A. glutinosa and P. contorta seedlings and incorporated into macromolecules. In alder seedlings, citrulline and ornithine were the free amino acids that had both the highest 15N enrichment levels and concentrations. In pine, glutamine and citrulline had the highest 15N concentrations, and glutamine had the highest level of 15N enrichment. 15N enrichment levels were greatest in the nodules, at between 5.5 and 29% in the different amino acids and 12% in the macromolecular fraction. Enrichment levels decreased with increasing distance from the nodules.
The uptake and translocation of 15N applied as 15NH4Cl to the mycelium was also studied. 15N was incorporated into amino acids in the mycelium and translocated further in this form. Generally, free amino acids had high 15N enrichment levels in the mycelium, decreasing along the translocation pathway. Citrulline and glutamine were the amino acids with highest 15N concentrations in all parts of the system. 15N was also found in the macromolecular fraction.