Effects of elevated carbon dioxide concentration on growth and N2 fixation of young Robinia pseudoacacia

Effects of elevated CO2 concentration ([CO2]) on carbon (C) and nitrogen (N) uptake and N source partitioning (N-2 fixation versus mineral soil N uptake) of 1-year-old Robinia pseudoacacia were determined in a dual C-13 and N-15 continuous labeling experiment. Seedlings were grown for M weeks in ambient (350 ppm) or elevated [CO2] (700 ppm) with (NH4)-N-15 (NO3)-N-15 as the only mineral nitrogen source. Elevated [CO2] increased the fraction of new C in total C, but it did not alter C partitioning among plant compartments. Elevated [CO,] also increased the fraction of new N in total N and this was coupled with a shift in N source partitioning toward N-2 fixation. Soil N uptake was unaffected by elevated [CO2], whereas N-2 fixation was markedly increased by the elevated [CO2] treatment, mainly because of increased specific fixation (mg N mg(-1) nodule). As a result of increased N-2 fixation, the C/N ratio of tree biomass tended to decrease in the elevated [CO2] treatment. Partitioning of N uptake among plant compartments was unaffected by elevated [CO2]. Total dry mass of root nodules doubled in response to elevated [CO2], but this effect was not significant because of the great variability of root nodule formation. Our results show that, in the N-2-fixing R. pseudoacacia, increased C uptake in response to increased [CO2] is matched by increased N-2 fixation, indicating that enhanced growth in elevated [CO2] might not be restricted by N limitations.