Carbon Dioxide Enhancement Effects in Container- versus Ground-Grown Soybean at Equal Planting Densities

Prior work showed that CO2 enhancement ratios (ER) were similar for plants grown in open-top chambers (OTCs) whether grown in the ground or in insulated containers aboveground. Per plant comparisons were suspect since the ground-grown plants were cultivated in rows at normal densities making it difficult to separate the effects of plant competition from the variables of interest. Soybean [Glycine max (L.) Merr. cv. Essex] was grown in the ground and in aboveground containers in OTCs in ambient and elevated CO2 at equal planting densities. The hypothesis was that at equal densities, container- and ground-grown plants would exhibit both equivalent ERs and equivalent per plant yields. Although the only differences in net photosynthetic rate (An:micromol m-2 s-1) and conductance to water vapor (gs:mol m-2 s-1) were due to CO2 and container- and ground-grown plants had similar ERs (mean = 20%), per plant yields were still less in the container-grown plants at both levels of CO2 (mean = -17%). Reproductive measures, except mass per seed, as well as total stem biomass were significantly reduced in the containers. High CO2 increased seed oil concentration and the level of fatty acid saturation. The only observed environmental difference was higher daytime root zone temperatures in containers (2-6°C). The robust ERs suggest that neither above- nor below-ground resource limitations was the cause of the yield discrepancies.