Nitrogen cycle.
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   Biological  importance:  Nitrogen  is  a  component of amino acids and
   nucleic acids.
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   Forms   available  to  life:  Producers  can  use  inorganic  ammonium
   (NH[4]^+)  or  nitrate  (NO[3]^-).  Consumers  can  only  use  organic
   nitrogen.
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   Reservoirs:  The  main  reservoir  is  the  atmosphere,  which  is 80%
   nitrogen gas (N[2]).
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   Key processes: Nitrogen enters ecosystems through nitrogen fixation by
   Rhizobium bacteria.
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   Ammonifying bacteria decompose organic nitrogen to NH[4]^+.
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   Nitrifying  bacteria  convert  NH[4]^+  to NO[3]^-, while denitrifying
   bacteria use NO[3]^- in their metabolism and releases N[2].
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