What was done
Nodulated and non-nodulated lines of soybeans (Glycine max Merr.) were grown in pots within environmentally-controlled cabinets maintained at atmospheric CO2 concentrations of 360 and 700 ppm in combination with low and high soil nitrogen supply for three weeks to study the effects of these variables on the growth of these different soybean lines.

What was learned
At low nitrogen, elevated CO2 stimulated nodule numbers and nodule dry mass in nodulated soybeans by approximately 80 and 70%, respectively, while having no effect on these parameters at high nitrogen supply.  In addition, it increased total plant dry mass by approximately 40 and 80% in nodulated soybeans grown at low and high nitrogen supply, respectively, while non-nodulated plants exhibited no CO2-induced growth response at low nitrogen but an approximate 60% growth enhancement at high nitrogen supply.

What it means
As the atmospheric CO2 concentration increases, non-nodulated soybeans likely will display increases in biomass if they are growing in nitrogen-rich soils.  Nodulated soybeans, however, should display increased growth in both nitrogen-rich and nitrogen-poor soils, with their responses being about twice as large in high as in low nitrogen soils.  Thus, symbiotic nitrogen-fixation within the nodules of soybeans can allow such plants to respond positively to elevated CO2 even under nitrogen-poor conditions.