Compared with C02, relatively small amounts of nitrogen have been released by degassing of the earth, only about 103g/cm2 of the earth's surface. But nitrogen is chemically inert, and at least 90% of this amount is still present in the atmosphere. The nitrogen no longer present in the atmosphere is mostly present in geologic deposits; the tiny fraction (10~8) of the available atmospheric nitrogen that is "fixed'' each year is soon returned to the atmosphere when plants and animals decay. The nitrogen cycle is discussed more fully in Section 10.3.

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