Environmental Engineering Reference
In-Depth Information
peptidases that liberate N from proteins. This enzymatic flexibility extends to other nutri-
ents, for example, production of phosphatase increases when phosphorus is limiting to
microbial growth or activity.
Nitrification
Pathway:
NH
4
!
NO
2
!
NO
3
Nitrification refers to two transformations that take place within the soluble inorganic
N pool: the conversion of NH
4
1
to nitrite (NO
2
2
), followed by the conversion of NO
2
2
to
NO
3
2
. The transformations are carried out by two different groups of chemoautotrophic
(get their energy from inorganic compounds and carbon to make their cells from carbon
dioxide) microbes that derive energy from these oxidations. Nitrite is highly reactive (both
chemically and biologically) and is usually present for very short periods and thus is in
vanishingly small quantities in the environment, making NO
3
2
the dominant product of
the nitrification process.
Nitrification has great ecological importance. Due to its negative charge, NO
3
2
is much
more mobile than NH
4
1
. This is because soil and sediment particles generally carry a nega-
tive charge to which positively charged ions such as NH
4
1
are attracted. Therefore, rates of
nitrification are key controllers of hydrologic losses of N. Production of NO
2
2
and NO
3
2
by
nitrifiers feeds the denitrification process (discussed later) and thus fosters gaseous losses
of N, cycling of N between the atmosphere and the biosphere, and the production of trace
gases that affect the chemistry and physics of the atmosphere. If nitrification did not exist,
N limitation of terrestrial production and N-related water and air pollution would be rare.
Like N fixation, the ecology of nitrification is strongly influenced by energetic con-
straints. The substrates used by nitrifiers (NH
4
1
,NO
2
2
) are not rich in energy relative to
the substrates used by heterotrophic microbes (see the Appendix), and as a result, nitrifiers
grow slowly and compete poorly with heterotrophs and plants for oxygen, NH
4
1
, and
other resources. The competition for NH
4
1
with plants and immobilizing heterotrophs is a
particularly strong regulator of nitrification activity. Thus, we expect to find high nitrifica-
tion activity in situations with high levels of NH
4
1
, for example, crop fields fertilized with
NH
4
1
, heavily manured crop fields, N-rich sediments, or ecosystems with some distur-
bance that reduces plant uptake, such as clear-cutting of a forest.
The microbiology and physiology of nitrification are more complex than they first
appeared. For many decades, it was assumed that NH
4
1
oxidation was carried out domi-
nantly by one genus of bacteria, Nitrosomonas, and that NO
2
2
oxidation was carried out by
another single genus, Nitrobacter. Recent molecular analysis has shown that there are sev-
eral other genera of NH
4
1
- and NO
2
2
-oxidizing bacteria, as well as newly discovered
NH
4
1
-oxidizing Archaea. Other recent studies have identified several heterotrophic nitrifi-
cation pathways whereby organic N is converted directly to NO
3
2
.
Even more interesting than the discovery of multiple nitrifying genera and pathways is
the developing understanding of nitrifier physiology. It was originally thought that these
organisms could use only their primary substrates (NH
4
1
or NO
2
2
), but it turns out that
the ammonia monoxygenase enzyme is capable of oxidizing methane as well as a series of
