Environmental Engineering Reference
In-Depth Information
Symbioses with N fixers have powerful effects on the ecology of the primary producers
involved in these relationships. These organisms can send up to 50% of the energy that
they fix from the sun to their N-fixing symbionts, reducing the amount of energy available
for growth, chemical defense, and other functions. As a result, legumes and Alnus plants
tend to live in specialized temporal or spatial niches where their ability to fix N gives
them a competitive advantage and/or their slow growth is not a hindrance, for example,
in the early phases of succession, or on substrates very low in N such as recent lava flows
or sand dunes. Cyanobacteria tend to thrive in water bodies with low N:P ratios where
their ability to fix N gives them a selective advantage.
Nitrogen Mineralization and Immobilization
Pathways:
NH
4
Immobilization: NH
4
or NO
3
!
Mineralization: Organic N
!
Organic N
In addition to N fixation, the other main natural source of soluble reactive N in ecosys-
tems is N mineralization. In this transformation, N contained in organic matter is con-
verted to an inorganic form as ammonium (NH
4
1
). The mineralization process is
sometimes referred to as ammonification.
At the molecular level, N mineralization is a by-product of the microbial degradation of
N-containing compounds such as proteins. Microorganisms degrading these compounds
convert the carbon components into biomass or carbon dioxide and either convert the N
components into their own biomass (e.g., proteins) or release them to the environment. In
the case of proteins, this results in the release of an amino group (NH
2
), which is rapidly
converted to NH
4
1
in the environment (although NH
3
can be released as a gas under high
pH conditions). Mineralization thus depends on microbial needs for N. If the microbes are
degrading compounds with lots of N, such as proteins, their needs for N are readily satis-
fied and excess N is released to the environment (mineralized). If the microbes are degrad-
ing compounds low in N (e.g., oak leaves or sawdust), they become N limited and need to
take up (immobilize) inorganic N (either NH
4
1
or nitrate (NO
3
2
)) from some other environ-
mental source (see Chapter 4). It is energetically preferable for microbes to take up NH
4
1
because NO
3
2
has to be reduced to NH
4
1
(which requires energy) before it can be used to
construct proteins or other compounds in microbial cells.
It is useful to distinguish between “gross” and “net” mineralization and immobilization.
Given that these are molecular-scale processes, we can observe only the net balance by
measuring net changes in inorganic N pools over time. For example, if gross mineraliza-
tion is 10 mg N m
2
2
d
2
1
and gross immobilization is 11 mg N m
2
2
d
2
1
, conventional meth-
ods detect only a net immobilization of 1 mg N m
2
2
d
2
1
. However, using stable isotope
methods (see Chapter 1), gross processes can be inferred. The two processes of gross pro-
duction and consumption of inorganic N have been shown to exist in a dynamic balance
such that gross fluxes are often 10 or 100 times the net fluxes.
Many microorganisms, including all heterotrophic (nutrition obtained by digesting
organic compounds) microbes, carry out mineralization and immobilization. Thus, these
