Environmental Engineering Reference
In-Depth Information
NO, N
2
O, N
2
C
Organic N in
organic matter
and microbes
Fixation
Fertilizer
Deposition
F
D
B
NH
4
+
→
NO
2
-
→
NO
3
-
A
= Uptake by primary producers
B
= Production of detritus
C
= Mineralization
D
= Immobilization
E
= Nitrification (1 = NH
4
+
oxidation,
2 = NO
2
-
oxidation)
F
= Denitrification
G
= Hydrologic loss
A
E
1
E
2
Primary
producers
Simple, soluble
(inorganic) forms
G
NH
4
+
, NO
3
-
dissolved organic N
FIGURE 7.4
The N cycle. Each process indicated in the figure is discussed in the text.
N fixation refers to the conversion of dinitrogen (N
2
) into ammonia (NH
3
) or to various
forms of reactive N in the atmosphere, generally referred to as NOy. The process requires
a great deal of energy as the N
2
molecule is extremely stable. As a result, N fixation occurs
only in situations where there is ample energy. Physical/chemical fixation takes place dur-
ing lightning discharges in the atmosphere, in NH
3
manufacturing plants where large
quantities of natural gas are used to convert N
2
to NH
3
, and when fossil fuels are com-
busted, for example, in an internal combustion engine. Biological N fixation occurs in
situations where relationships between primary producers and microbes with the enzymes
necessary to convert N
2
to NH
3
allow for an energetic subsidy of the N-fixing microbes, in
cyanobacteria (blue-green algae) that are primary producers with the ability to fix N, or in
specific niches with abundant energy and low N availability, like rotting logs.
The energetics of N fixation have a powerful influence on the ecology of this process.
The N-fixation enzyme (nitrogenase) is present only in a few genera of soil bacteria (e.g.,
Rhizobia, Frankia) and in blue-green algae (cyanobacteria). Use of the enzyme can consume
more than half of the energy that the bacteria obtain from their normal consumption of
substrates. Organisms that devote half of their energy to one process tend to grow more
slowly and/or have reduced metabolic capacity compared to other organisms with which
they compete for space and substrate in the environment. As a result, “free-living” N fixa-
tion activity is usually quite low, and is restricted to environments where the ability to fix
N conveys a strong competitive advantage, for example, on substrates with high energy
but low N content such as decaying logs.
The vast majority of biological N fixation in terrestrial ecosystems occurs in the context
of symbiotic relationships between primary producers and N-fixing microbes. In terrestrial
ecosystems, examples are plants in the legume family and the genus Alnus that have
developed symbiotic relationships with Rhizobia and Frankia bacteria. The bacteria live in
specialized nodules attached to plant roots: the plant sends energy in the form of sugars to
the nodules, and the bacteria fix N in the nodules and share this N with the plant.
