Elevated CO₂ Increases Nitrogen Fixation at the Reproductive Phase Contributing to Various Yield Responses of Soybean...

Nitrogen deficiency limits crop performance under elevated CO₂ (eCO₂), depending on the ability of plant N uptake. However, the dynamics and redistribution of N₂ fixation, and fertilizer and soil N use in legumes under eCO₂ have been little studied. Such an investigation is essential to improve the adaptability of legumes to climate change. We took advantage of genotype-specific responses of soybean to increased CO₂ to test which N-uptake phenotypes are most strongly related to enhanced yield. Eight soybean cultivars were grown in open-top chambers with either 390 ppm (aCO₂) or 550 ppm CO₂ (eCO₂). The plants were supplied with 100 mg N kg^-1 soil as ¹⁵N-labeled calcium nitrate, and harvested at the initial seed-filling (R5) and full-mature (R8) stages. Increased yield in response to eCO₂ correlated highly (r = 0.95) with an increase in symbiotically fixed N during the R5 to R8 stage. In contrast, eCO₂ only led to small increases in the uptake of fertilizer-derived and soil-derived N during R5 to R8, and these increases did not correlate with enhanced yield. Elevated CO₂ also decreased the proportion of seed N redistributed from shoot to seeds, and this decrease strongly correlated with increased yield. Moreover, the total N uptake was associated with increases in fixed-N per nodule in response to eCO₂, but not with changes in nodule biomass, nodule density, or root length.