The Effect of Elevated [CO₂] on Uptake and Allocation of ¹³C and ¹⁵N in Beech (Fagus sylvatica L.) during Leafing

A continuous dual 13 CO 2 and 15 NH 4 15 NO 3 labelling experiment was undertaken to determine the effects of ambient (350μmol mol ‐1 ) or elevated (700μmol mol ‐1 ) atmospheric CO 2 concentrations on C and N uptake and allocation within 3‐year‐old beech ( Fagus sylvatica L.) during leafing. After six weeks of growth, total carbon uptake was increased by 63 % (calculated on total C content) under elevated CO 2 but the carbon partitioning was not altered. 56 % of the new carbon was found in the leaves. On a dry weight basis was the content of structural biomass in leaves 10 % lower and the lignin content remained unaffected under elevated as compared to ambient [CO 2 ]. Under ambient [CO 2 ] 37 %, and under elevated [CO 2 ] 51 %, of the lignin C of the leaves derived from new assimilates. For both treatments, internal N pools provided more than 90 % of the nitrogen used for leaf‐growth and the partitioning of nitrogen was not altered under elevated [CO 2 ]. The C/N ratio was unaffected by elevated [CO 2 ] at the whole plant level, but the C/N ratio of the new C and N uptake was increased by 32 % under elevated [CO 2 ].