Responses of soybeans and wheat to elevated CO2 in free-air and open top chamber systems

With increasing demand for agricultural products, more confidence is needed concerning impacts of rising atmospheric CO2 on crop yields. Despite debate about the merits of free-air CO2 enrichment (FACE) and open top chamber (OTC) systems, there have been no reports comparing crop yield responses to elevated CO2 in FACE and OTC systems using the same cultivar and location. In this study soybeans and winter wheat were grown for two years in FACE and OTC systems at the same time and location. An elevated CO2 treatment of ambient plus ∼200 μmol mol−1 was applied 24 h per day for one cultivar of each species in the first year, and two cultivars of each species in the second year. Leaf area index, and midday leaf gas exchange rates were measured periodically, and total above ground biomass and seed yield were determined at maturity. In soybean, seed yield was increased by elevated CO2 in both FACE and OTC in both cultivars and years. However, the ratio of seed yield at elevated CO2 to that at ambient CO2 averaged significantly higher in OTC (1.49) than in FACE (1.27). In wheat, grain yield was increased by 15-30% by elevated CO2 for both cultivars and years in the OTC, but was not increased in either cultivar or year in the FACE system. No differences in midday photosynthetic rates occurred between OTC and FACE in either species for either CO2 treatment, except one season in wheat, but stomatal conductance was more reduced by elevated CO2 in OTC than in FACE. Short-term temporal variation in CO2 concentration was larger in FACE than in OTC. It is not clear from these results which method produces plant responses equivalent to those which may occur with increased atmospheric CO2.