This study aims to evaluate the effects of elevated carbon dioxide (EC) on soybean growth in the water-deficient region of Huang-Huai-Hai Plain, China. A pot experiment involving two CO2 concentrations (ambient, 380.1±32.2μmolmol−1 and elevated, 740.6±45.4μmolmol−1) and two water levels (normal and drought) were conducted in enclosed top chambers. The results showed that plant height, leaf area, and shoot dry weight were increased by 25.4%, 15.8%, and 33.4% under normal water and EC conditions, respectively, at the seed-filling stage. Seed yield per plant was also improved by 25.3%. Under drought conditions, EC did not show a significant effect on plant height, leaf area, and seed yield. However, shoot dry weight was increased by 56% at the seed-filling stage. This increase was due to the higher biomass allocation toward the stems. Under normal water conditions, the photosynthetic rate (Pn) was higher (21.7–43.3%) in EC than in ambient carbon dioxide (AC) at the seed-filling stage. Under drought conditions, Pn remained high, but the transpiration rate (Tr) was reduced by EC. In addition, the water use efficiency at yield and biomass levels (WUEyield and WUEbiomass) were increased by 26.2% and 55.4% under normal water conditions in EC at the seed-filling stage. However, they were only increased by 5.9% and 13.4%, respectively, under drought conditions. These results suggested that EC improved the growth and WUE of soybean more effectively under normal water conditions than under drought stress conditions. In conclusion, EC did not mitigate drought-induced inhibition of seed yield in soybean, although it increased Pn and WUE. Soybean should be cropped under well irrigation regimes or at regions with sufficient precipitation to adapt and take full advantage of EC.
