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

Nitrogen deficiency limits crop performance under elevated CO (eCO), depending on the ability of plant N uptake. However, the dynamics and redistribution of N fixation, and fertilizer and soil N use in legumes under eCO 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 CO 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 (aCO) or 550 ppm CO (eCO). The plants were supplied with 100 mg N kg soil as N-labeled calcium nitrate, and harvested at the initial seed-filling (R5) and full-mature (R8) stages. Increased yield in response to eCO correlated highly ( = 0.95) with an increase in symbiotically fixed N during the R5 to R8 stage. In contrast, eCO 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 CO 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 eCO, but not with changes in nodule biomass, nodule density, or root length.