Nitrogen-fixing plant species growing in elevated atmospheric carbon dioxide concentration ([CO2]) should be able to maintain a high nutrient supply and thus grow better than other species. This could in turn engender changes in internal storage of nitrogen (N) and remobilisation during periods of growth. In order to investigate this one-year-old-seedlings of Alnus glutinosa (L.) Gaertn and Pinus sylvestris (L.) were exposed to ambient [CO2] (350 µmol mol−1) and elevated [CO2] (700 µmol mol−1) in open top chambers (OTCs). This constituted a main comparison between a nitrogen-fixing tree and a nonfixer, but also between an evergreen and a deciduous species. The trees were supplied with a full nutrient solution and in July 1994, the trees were given a pulse of 15N-labelled fertiliser. The allocation of labelled N to different tissues (root, leaves, shoots) was followed from September 1994 to June 1995. While N allocation in P. sylvestris (Scots pine) showed no response to elevated [CO2], A. glutinosa (common alder) responded in several ways. During the main nutrient uptake period of June–August, trees grown in elevated [CO2] had a higher percentage of N derived from labelled fertiliser than trees grown in ambient [CO2]. Remobilisation of labelled N for spring growth was significantly higher in A. glutinosa grown in elevated [CO2] (9.09% contribution in ambient vs. 29.93% in elevated [CO2] leaves). Exposure to elevated [CO2] increased N allocation to shoots in the winter of 1994–1995 (12.66 mg in ambient vs. 43.42 mg in elevated 1993 shoots; 4.81 mg in ambient vs. 40.00 mg in elevated 1994 shoots). Subsequently significantly more labelled N was found in new leaves in April 1995. These significant increases in movement of labelled N between tissues could not be explained by associated increases in tissue biomass, and there was a significant shift in C-biomass allocation away from the leaves towards the shoots (all above-ground material except leaves) in A. glutinosa. This experiment provides the first evidence that not only are shifts in C allocation affected by elevated [CO2], but also internal N resource utilisation in an N2-fixing tree.
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