THE NITROGEN CYCLE
   Biological  importance:  Nitrogen  is  a  component of amino acids and
   nucleic acids.

   Forms   available  to  life:  Producers  can  use  inorganic  ammonium
   (NH[4]^+) or nitrate (NO[3]^-). Animals can only use organic nitrogen.

   Reservoirs:  The  main  reservoir  is  the  atmosphere,  which  is 80%
   nitrogen  gas (N[2]). The other reservoirs are bound in soils, aquatic
   sediments, dissolved in water, and in living biomass.

   Key  processes:  Nitrogen  enters ecosystems by nitrogen fixation, the
   conversion  of  N[2]  to NH[4]^+ or NO[3]^-. Ammonification decomposes
   organic  nitrogen  to  NH[4]^+. Nitrifying bacteria convert NH[4]^+ to
   NO[3]^-,  while  denitrifying  bacteria use NO[3]^- instead of O[2] in
   their metabolism and releases N[2]. 1