Higher CO2 Produces Several-fold Increases in Plant Nitrogen Fixation
One of the reasons why low soil nitrogen levels, in particular, are not an insurmountable impediment to CO2-induced growth enhancement is that plants exposed to elevated atmospheric CO2 concentrations do not need to invest as much nitrogen in their photosynthetic apparatus, as it operates so much more efficiently at higher CO2 levels. In addition, atmospheric CO2 enrichment has the ability to directly stimulate the activity of nitrogen-fixing bacteria.
The capacity of these symbiotic microorganisms to remove nitrogen from the atmosphere and make it available to vegetation appears to be limited by their host plants' rates of carbohydrate production. Consequently, anything that stimulates vegetative productivity, including atmospheric CO2 enrichment, generally stimulates bacterial nodule growth and activity. It is not surprising, therefore, that several-fold increases in the air's CO2 content have been found to produce several-fold increases in nitrogen fixation in a number of experiments.
Indeed, the evidence clearly shows that, even in the face of severe shortages of nitrogen and other nutrients, plant photosynthetic rates may still be significantly stimulated by atmospheric CO2 enrichment, setting in motion a number of additional phenomena that promote vegetative productivity even more.