What is free nitrogen? Nitrogen gas is called "free" nitrogen, meaning it is not combined with other kinds of atoms. Most organisms can use nitrogen only once it has been "fixed," or combined with other elements to form nitrogen-containing compounds.

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About 78% of the Earth's atmosphere is made up of "free" nitrogen (N2), produced by biological and chemical processes within the biosphere and not combined with other elements. ... One family of nitrogen compounds produced by nitrogen - fixing bacteria are called nitrates.

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