Elevated atmospheric carbon dioxide and [O.sub.3] differentially alter nitrogen acquisition in peanut

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