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  Spheres of the N Cycle
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Nitrogen gas (N2) comprises 78.3% of the earth's atmosphere by volume (75.5% by mass). N has two stable isotopes, 14N and 15N. The lighter isotope is 272.0 times as abundant as the heavier one; as a result the atomic weight of N is 14.0067. N2 is chemically unreactive at the temperatures and pressures of the hydrosphere, biosphere, and atmosphere. It will combine with other elements only under extreme conditions or when catalyzed by enzymes (See nitrogen fixation). N2 has a low solubility in water. The Henry's law constant for N2 is 6.6x10-4 mol/(L-atm).

The approximate global inventory of N in the four spheres is given in the table below. The bulk of the N (about 98%) exists in the geosphere, and most of the remainder is found in the atmosphere. Compared with the other spheres, the hydrosphere and biosphere contain relatively little N, but the N in the biosphere is highly reactive and is rapidly cycled. The inorganic N species ammonium (NH4+), nitrite (NO2-), and nitrate (NO3-) are highly water soluble, and are distributed in dilute aqueous solution throughout the hydrosphere. Living and dead organic matter also provide actively-cycled reservoirs of N. Soil organic matter (humus) is a substantial and relatively stable N reservoir in temperate climates.

Sizes of Global N Reservoir

Reservoir/Pool Type
           Soils and Sediments 
      Mantle and Core 
 Metric Tons
2.8 x 1011
2.3 x 1013
3.86 x 1015
1.636 x 1017
0.13 - 1.4 x 1016
0.35 - 4.0 x 1015
1.6 x 1017
% of Total

N is present in many chemical forms (compounds or "species"), both organic and inorganic, in the atmosphere, biosphere, hydrosphere, and geosphere. It occurs in the gas, liquid (dissolved in water), and solid phases. N can be associated with carbon (organic species) and with elements other than carbon (inorganic species). Important inorganic species include N2, nitric acid (HNO3), nitrate (NO3-), nitrite (NO2-), nitrous oxide (N2O), nitric oxide (NO), N dioxide (NO2), ammonium (NH4+), and ammonia (NH3). Most organic N species in the four spheres are biomolecules, such as proteins, peptides, enzymes, and genetic material (RNA and DNA). NO3- and organic-N species exist in solution and as particulates. The sum of organic and inorganic species of N in both dissolved and particulate forms is often reported as total N. It is total N for which the USEPA has established new criteria (USEPA, 2000)

The many forms which N can take is a result of its ability to gain and lose electrons to other elements. The valence range (oxidation states) of N is full: going from loss of all five of its outer-shell electrons (+5) to other elements to the gain of three electrons from other elements (-3) to completely fill all the electron orbitals of its outer shell. The table below presents several N species and their oxidation states.

Nitrogen Species and their Oxidation States

  Species   Name Oxidation State
NH3, NH4+   Ammonia, ammonium ion        -3
N2H4 Hydrazine     -2
NH2OH Hydroxylamine     -1
N2 Nitrogen       0
N2O Nitrous oxide     +1
NO  Nitric oxide     +2
HNO2, NO2- Nitrous acid, nitrite ion     +3
NO2 Nitrogen dioxide     +4
HNO3, NO3-    Nitric acid, nitrate ion     +5

N is essential to all forms of life. Many important biomolecules, including proteins and nucleic acids (RNA, DNA) contain N. Some microorganisms depend on N compounds for energy production. (See Biological Processes Involving N.) All N species are bioavailable at various rates.

N Properties Links

    N Species presents brief descriptions of several N species and their properties.

    Transformations of N Species briefly describes processes that transform N species.

Overview w B. Cycle w Spheres w N Props w N Cycle w Influences w N Spheres