Phosphorus Stress Effects on Growth and Seed Yield Responses of Nonnodulated Soybean to Elevated Carbon Dioxide

The influence of P availability on plant responses to elevated atmospheric CO₂ concentrations has received limited research attention. Therefore, an experiment was conducted to examine the effect of a wide range of P availabilities on plant response to enriched atmospheric CO₂. Nonnodulating soybean [Glycine max (L.) Merr., ‘Lee’] plants were grown from germination to maturity in controlled environment chambers at 350 or 700 μL L⁻¹ CO₂ and supplied with a complete nutrient solution containing either 0.005, 0.10, 0.25, 0.50, or 1.00 mM P. Growth and seed yield were maximized at the 0.25 and 0.50 mM P concentrations at 350 and 700 μL L⁻¹ CO₂, respectively. Growth and yield were significantly increased by CO₂ enrichment at all except the lowest P concentration. The stimulation of growth at high CO₂ was consistently associated with increased leaf area, net assimilation rate, P uptake, and P utilization efficiency in the production of dry matter. When averaged over the four highest P levels, total root mass was increased 63% by CO₂ enrichment, but P uptake efficiency per unit root mass was decreased 22%. The yield enhancement of 23 to 57% at high CO₂ was associated with increases in the number and size of seed. Carbon dioxide enrichment had no significant effect on harvest index. The results indicate that CO₂ enrichment can result in stimulation of growth and yield of nonnodulated, NO⁻₃-fed soybean plants, even at concentrations of P that limit plant growth at ambient CO₂ concentrations. Cooperative investigations of Duke Univ., the USDA-ARS, and the North Carolina Agric. Res. Serv., Raleigh, NC 27695. Paper no. 11347 of the Journal Senes of the North Carolina Agric. Res. Serv. This study was supported by grant DE-FGO5-85ER60373 from the U.S. Dep. of Energy, Carbon Dioxide Res. Div.