Is yield enhancement by CO2 enrichment greater in genotypes with a higher capacity for nitrogen fixation?

Yield responsiveness to elevated CO2 concentration [CO2] was previously found to be greater when nitrogen (N) was supplied in adequate amounts; however, it remains unclear whether genotypic differences in N2-fixing capacity affect yield responsiveness in soybean. We tested the hypothesis that yield responsiveness to elevated [CO2] in soybean is greater in a genotype with a higher capacity for N2 fixation. We used three near-isogenic genotypes with contrasting nodulation capacities: super-nodulating, normally nodulating and non-nodulating genotypes. Plants were subjected to two levels of [CO2] (ambient or elevated: ambient + 200 μmol mol−1) and two temperature regimes (low or high: low + ca. 4–5 °C) using temperature gradient chambers. The super-nodulating genotype exhibited a higher N content in leaves, regardless of [CO2] and temperature. Photosynthetic rates were enhanced by CO2 enrichment at earlier growth stages, but not at later growth stages, regardless of genotype. This photosynthetic acclimation was reflected in biomass production in all the genotypes examined. Yield responsiveness to elevated [CO2] was greater in the nodulating genotypes than in the non-nodulating genotype, but the genotypic differences were obscured between the normally nodulating and super-nodulating genotype, thus our hypothesis was not fully verified.