Influence of tree internal nitrogen reserves on the response of beech (Fagus sylvatica) trees to elevated atmospheric...

We examined the influence of plant internal nitrogen (N) reserves on the response of 3-year-old beech (Fagus sylvatica L.) trees to elevated atmospheric CO2 concentration ([CO2]) in a dual 15N and 13C long-term labeling experiment. Trees were grown on sand and received either no N nutrition (-N treatment) or 4 mM N (+N treatment) for 1 year. The -N and +N pretreated trees were then placed in growth chambers and grown in 350 (ambient) or 700 ppm (elevated) of a 13CO2 atmosphere for 24 weeks. In all treatments, trees were supplied with 4 mM 15N during the experiment. Irrespective of tree N reserves, elevated [CO2] increased cumulative carbon (C) uptake by about 30% at Week 24 compared with that for trees in the ambient treatment. Elevated [CO2] also caused a shift in C allocation to belowground compartments, which was more pronounced in -N trees than in +N trees. In +N trees, belowground allocation of new C at Week 24 was 67% in ambient [CO2] compared with 70% in elevated [CO2]. The corresponding values for -N trees were 70 and 79%. The increase in C allocation in response to elevated [CO2] was most evident as an increase in belowground respiration; however, specific root respiration was unaffected by the CO2 or N treatments. Although elevated [CO2] increased root growth and belowground respiration, it had no effect on N uptake at Week 24. As a result of increased C uptake, N concentrations were decreased in trees in the elevated [CO2] treatment compared with trees in the ambient treatment in both N treatments. Partitioning of new N uptake was unaffected by elevated [CO2] in +N trees. In -N trees, however, N allocation to the stem decreased in response to elevated [CO2] and N allocation to fine roots increased, suggesting a reduction in the formation of N reserves in response to elevated [CO2]. We conclude that the response of beech trees to elevated [CO2] is affected by internal N status and that elevated [CO2] may influence the ability of the trees to form N reserves.