There has been various scientific studies showing that plants do grow faster rate and healthy with higher CO2 levels. But it's true that a lack of nitrogen in the soil does prevent some plants from growing. Other plants use nitrogen from the atmosphere.
"Nitrogen fixation is the process by which nitrogen is taken from its relatively inert molecular form (N2) in the atmosphere and converted into nitrogen compounds useful for other chemical processes (such as, notably, ammonia, nitrate and nitrogen dioxide).

Nitrogen fixation is performed naturally by a number of different prokaryotes, including bacteria, and actinobacteria certain types of anaerobic bacteria. Many higher plants, and some animals (termites), have formed associations with these microorganisms
...
The best-known are legumes (such as clover, beans, alfalfa and peanuts,) which contain symbiotic bacteria called rhizobia within nodules in their root systems, producing nitrogen compounds that help the plant to grow and compete with other plants. When the plant dies, the nitrogen helps to fertilize the soil. "
http://en.wikipedia.org/wiki/Nitrogen_fixation

Of course the quickier, faster, and a common way in commercial farming is just added these nitrates to the soil- chemical fertilizers