(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 Biosphere Hydrosphere Atmosphere Geosphere
Crust Soils and Sediments Mantle and Core
Metric Tons2.8 x 10112.3 x 10133.86 x 10151.636 x 10170.13
- 1.4 x 10160.35 - 4.0 x 10151.6 x 1017
% of Total 0.0002 0.014 2.3 97.7 0.78-8.4 0.21-2.4 95.6
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
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.
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