Nitrogen Stress Effects on Growth and Seed Yield of Nonnodulated Soybean Exposed to Elevated Carbon Dioxide

Limitations in nutrient availability apparently can restrict plant response to CO 2 enrichment; however, the alterations in physiological processes associated with such restrictions have not been defined. This experiment was conducted to investigate certain physiological responses of N‐limited soybean [ Glycine max (L.) Merr. cv. Lee] plants growing in a CO 2 enriched environment and to examine their role in determining growth and yield. The nonnodulating soybean plants were grown to maturity in controlled environment chambers at 350 or 700 μL L −1 CO 2 and at 0.05, 1.0, 2.5, 5.0, or 10.0 m M KNO − 3 supplied in nutrient solution. Substantial increases in whole‐plant growth and seed yield occurred in both CO 2 treatments with increasing nitrate levels; the increases were greater, however, at high CO 2 . At all NO − 3 levels except the lowest, exposure to high CO 2 resulted in increased total leaf area, mean net assimilation rate, NO − 3 uptake, and N utilization efficiency. Increased NO − 3 uptake was associated with larger root systems, as uptake per unit of root mass was lower than controls. Carbon dioxide enrichment had little effect on dry matter partitioning among plant parts or harvest index. Alterations in partitioning were related to differences in NO − 3 supply. The results suggest that atmospheric CO 2 enrichment can stimulate seed yield of soybean even when the availability of N in the rhizosphere is limited.