In the nitrogen cycle, nitrogen can return to the lithosphere directly from the atmosphere by

a small amount of nitrogen is fixed by lightning, but most of the nitrogen harvested from the atmosphere is removed by nitrogen-fixing bacteria and cyanobacteria (formerly called blue-green algae). The nitrogen cycle transforms diatomic nitrogen gas into ammonium, nitrate, and nitrite compounds.

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the nitrogen cycle provides nitrogen to the ecosystem from the atmosphere, ground and oceans. nitrogen is an important component of complex molecules such as amino acids and nucleotides, which lead to the creation of proteins and dna, the building blocks of all life. plants absorb nitrogen in the form of ammonium, nitrate ion and, on rare occasions, as amino acids. animals receive nitrogen necessary for biological processes from feeding on living or dead organic matter.