What was done
The authors grew soybeans for an entire season in growth chambers that received CO2 concentrations of 160, 220, 280, 330, 660 and 990 ppm to determine the effects of CO2 on nonstructural carbohydrate accumulation in various plant organs.

What was learned
The authors discovered a consistent increase in total nonstructural carbohydrates in all vegetative soybean components including roots, stems, petioles and especially the leaves, as the treatment CO2 concentration increased.  There was, however, no overall significant effect of treatment CO2 concentration on nonstructural carbohydrate accumulation in soybean reproductive components, including podwalls and seeds.  These observations indicate that the higher yields reported in the literature for soybeans exposed to elevated CO2 most likely result from increases in the number of pods produced per plant, and not from the production of larger individual pods or seeds.  Furthermore, the increasing concentrations of total nonstructural carbohydrates that occurred with each increment of CO2 provided the raw materials to support greater biomass production at each higher CO2 level.  Although final biomass and yield data were not reported in this paper, the authors did give biomass data obtained 66 days into the experiment.  Relative to aboveground biomass measured at ambient (330 ppm) CO2, plants grown in sub-ambient CO2 concentrations of 280, 220 and 160 ppm exhibited 12, 33 and 60% less biomass, respectively, while plants grown in supra-ambient CO2 concentrations of 660 and 990 ppm displayed 46 and 66% more biomass, respectively.

What it means
Soybean plants probably had a difficult time surviving in the past, when CO2 levels were much lower than they are today.  It is likely that soybeans began experiencing increases in total nonstructural carbohydrates and yield as the CO2 concentration of the air began to rise with the onset of the Industrial Revolution.  Because levels of nonstructural carbohydrates increase with incremental increases in the CO2 content of the air, it is likely that soybean plants will produce more total nonstructural carbohydrates that will sustain even greater yields as the CO2 content of the air continues to rise in the future.