Environmental Engineering Reference
In-Depth Information
how the cycles interact and revisits the ability of redox to control nutrient
cycling.
NITROGEN
Nitrogen Forms
The most common form of nitrogen (N) in the biosphere is N
2
gas. The
atmosphere is composed of about 78% N
2
. Water generally contains N
2
as
a dissolved gas. Even though N
2
is less soluble in water than O
2
, the higher
atmospheric concentration of N
2
leads to dissolved concentrations similar to
those observed for O
2
. The N
2
molecule is very difficult for organisms to use
directly because it has a triple covalent chemical bond that requires a signif-
icant amount of energy to break. Thus, forms of organic and inorganic
nitrogen other than N
2
are referred to as combined nitrogen.
The two most important forms of dissolved inorganic N in natural wa-
ters are
nitrate
(NO
3
) and
ammonium
(NH
4
). Ammonium is the ionic
form found in neutral to acidic waters. Under basic conditions the ion is
converted to
ammonia
gas (NH
3
), which can move between the atmos-
phere and the water.
Nitrite
(NO
2
), an additional form of dissolved in-
organic nitrogen, occasionally is found in significant concentrations in nat-
ural water, especially when sewage is present, and can be problematic
because of its toxic nature. The sum of nitrate, nitrite, and ammonium is
often referred to as
dissolved inorganic nitrogen
. Other forms of inorganic
N that also are dissolved in water, such as N
2
gas, are not considered be-
cause they are not as biologically available. Nitrous oxide (N
2
O) is a gas
that is also found dissolved in low concentrations in many waters and in
the atmosphere. Nitrous oxide is known as laughing gas but also absorbs
heat and is an important contributor to the greenhouse effect.
Organic N can take many forms, including amino acids, nucleic acids,
proteins, and urea.
Urea
may be particularly important because it is ex-
creted by many organisms and may be a crucial form in nitrogen cycling.
Organic N in aquatic habitats can be in the form of
dissolved organic ni-
trogen
and
particulate organic nitrogen;
usually a 0.45-
m filter is used to
separate dissolved from particulate.
Nitrogen Fluxes
Nitrogen must be assimilated in some form because it is a required
component for many biological molecules. Many multicellular het-
erotrophic organisms (e.g., animals) can assimilate nitrogen only in the
form of organic molecules, such as amino acids and nucleic acids. Enzymes
that cleave proteins are excreted into the surrounding water or associated
with cells (Billen, 1991). Primary producers and bacteria also generally use
nitrate, nitrite, or ammonium. The assimilatory pathways for nitrogen ac-
quisition in such organisms require the nitrogen to be in the form of am-
monium in the cell (Fig. 13.2). Thus, the enzyme nitrate reductase takes ni-
trate to nitrite and nitrite reductase transforms nitrite to ammonium, which
can then be assimilated. These reductase enzymes require energetically
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