Environmental Engineering Reference
In-Depth Information
TABLE 7.1
A “cheat sheet” for the nitrogen cycle.
Pools/Chemical Forms
Importance
Gases
N
2
Dinitrogen
Dominant global pool, 79% of
atmosphere.
N
2
O
Nitrous oxide
Greenhouse gas, destroys
stratospheric ozone.
NO
Nitric oxide
Toxic, precursor of tropospheric
ozone.
NH
3
Ammonia
Plant available (soluble), can be
toxic, rapidly deposited.
NO
y
Diverse reactive forms of N in the
atmosphere produced by
combustion of fossil fuels and/or
atmospheric chemical reactions
Plant available (soluble), component
of acid rain, rapidly deposited.
Ions/Soluble Forms
NH
4
1
Ammonium
Available to plants.
NO
2
2
Nitrite
Toxic, rarely found at high levels in
nature.
NO
3
2
Nitrate
Available to plants, highly leachable.
DON
Dissolved organic nitrogen
Mixture of many different chemical
forms.
Processes
N
2
!
NH
3
Biological N fixation
See text.
N
2
!
NOy
Abiotic N fixation
See text.
NH
4
1
Organic N
!
Mineralization
See text.
NH
4
1
or NO
3
2
!
Organic N
Immobilization
See text.
NH
4
1
!
NO
2
2
!
NO
3
2
Nitrification
See text.
NO
3
2
!
NO
2
2
!
NO
!
N
2
O
!
N
2
Denitrification
See text.
NO
3
2
!
NH
3
Dissimilatory nitrate reduction to
ammonia (DNRA)
See text.
NH
4
1
1
NO
2
2
!
N
2
Anaerobic oxidation of ammonium
(anammox)
See text.
In aquatic ecosystems, specific species of cyanobacteria fix N in specialized structures
(heterocysts) that enclose nitrogenase, which is fed by sugars produced elsewhere in the
organism. Other aquatic N fixers, especially in marine systems, do not have specialized
structures, but all have some energetic subsidy to facilitate fixation.
