Elevated Atmospheric Carbon Dioxide and O3Differentially Alter Nitrogen Acquisition in Peanut

Elevated atmospheric CO 2 and ozone (O 3 ) may affect productivity of legumes in part by altering symbiotic N 2 fixation. To investigate this possibility, measurements of plant biomass, N levels and natural 15 N abundance (δ 15 N) were used to examine the effects of elevated CO 2 and O 3 on N acquisition in field-grown peanut (Arachis hypogaea L.) using open-top chambers. Seasonal 12-h daily average CO 2 treatment concentrations were 376, 550, and 730 μmol mol -1 . Carbon dioxide treatments were applied in reciprocal combinations with seasonal 12-h daily average O 3 concentrations of 21, 49, and 79 nmol mol -1 . At mid-vegetative growth, elevated CO 2 significantly reduced leaf N concentrations by up to 44%, but not δ 15 N values. Elevated O 3 did not significantly affect N concentrations or δ 15 Nvalues. At harvest, plant N concentrations were similar among treatments except for a 14% reduction in the highest-level CO 2- O 3 treatment. Plant N accumulation varied in proportion with treatment effects on biomass production, which was increased with elevated CO 2 when averaged over the O 3 treatments and suppressed by high-level O 3 at ambient CO 2 . Elevated CO 2 reduced plant δ 15 Nvalues in low-and mid-level O 3 treatments while mid- and high-level O 3 increased them at ambient CO 2 . The changes in δ 15 Nvalues suggested that N 2 fixation activity was stimulated with elevated CO 2 and inhibited by elevated O 3 . Elevated CO 2 ameliorated detrimental O 3 effects to varying extents depending on the concentrations of the two gases. These results indicated that interactions between CO 2 and O 3 on plant physiology can alter N acquisition processes, with impacts on peanut productivity likely dependent in part on these changes.