THE NITROGEN CYCLE

MINERALIZATION AND IMMOBILIZATION	NITROGEN FIXATION
NITRIFICATION	FATE OF NITRATE
DENITRIFICATION

OVERVIEW:

Nitrogen Fixation - the microbial conversion of molecular nitrogen (N₂) to ammonia (NH₃). Ammonia exists in solution as the ammonium ion (NH₄⁺). "Fixation" converts nitrogen gas to a salt that higher organisms can use. In other words, higher organisms are completely dependent on microorganisms for the nitrogen atoms in their proteins, nucleic acids, etc.

Nitrogen Assimilation - ammonia can be incorporated into organic molecules (R-NH₂) such as nucleic acids.

Deamination - conversely, organic molecules containing nitrogen are deaminated during decomposition of organic materials, producing ammonia.

Nitrification - soil bacteria collaborate to oxidize ammonia. The first oxidation product is the nitrite anion (NO₂^-) produced by bacteria primarily of the genus Nitrosomonas. Nitrite is further oxidized by bacteria primarily of the genus Nitrobacter, producing the nitrate (NO₃^-) anion.

Denitrification (or nitrate reduction) - dissimilative nitrate reduction involves the microbial reduction of nitrate, producing nitrogen gas. Plants are able to do assimilative nitrate reduction, i.e. they use nitrate as a nitrogen source by reducing it and incorporating the nitrogen atoms into organic molecules.

NITROGEN RESERVOIRS

0. Mole percent? The atmosphere is 78% nitrogen gas.
1. Total mass? There are 3.8 x 10²¹ grams of nitrogen atoms in the atmosphere.
2. Flux? Every year, 3 x 10¹⁴ grams of nitrogen are recycled biologically. This is less than a millionth of the total available nitrogen!