Although carbon is the most important constituent of all living matter (making up, on average, approximately 45% of dry mass), cellulose and lignin, the two dominant molecules of terrestrial life, contain no nitrogen. Only plant seeds and lean animal tissues have high protein content (mostly between 10 and 25%; soybeans up to 40%), and although nitrogen is an indispensable ingredient of all enzymes, its total reservoir in living matter is less than 2% of carbon's huge stores. Unlike the cycles of water or carbon that include large flows entirely or largely unconnected to biota, every major link in nitrogen's biospheric cycle is mediated by bacteria. Nitrogen fixationÃ¢â‚¬â€a severing of the element's highly stable and chemically inert molecule (N2) that makes up 78% of the atmosphere and its incorporation into reactive ammoniaÃ¢â‚¬â€launches this great cascade of biogenic transformations.
Read this topic