Even though organisms use nitrogen less than they do carbon or water, it could also be said that a lack of nitrogen would eradicate life on Earth. As with carbon, most of the nitrogen in the world is found in the atmosphere. One major difference between carbon and nitrogen gases, however, is that most organisms on Earth are completely unable to use atmospheric nitrogen directly. In order for nitrogen to enter a community, it must first be converted into a usable form by special nitrogen "fixing" bacteria in the soil. These bacteria often live close to plants who can take the "fixed" nitrogen and incorporate it into their proteins. Nitrogen, like carbon, moves through the community as organisms eat and are eaten. After organisms die and decompose, their nitrogen is released back into the atmosphere in the same way it was extracted: by bacteria.
The bacteria that fix nitrogen do it in a symbiosis with the plants: they actually live inside special parts of the roots, called nodules, where they perform nitrogen fixation. See a movie of it here.