Geoscience Reference
In-Depth Information
Atmospheric reaction
NO(g)
Nitric oxide
N
2
O(g)
Nitrous oxide
N
2
(g)
Molecular nitrogen
Denitrification
(anaerobic)
Nitrogen fixation
(aerobic)
2
-
NO
Nitrite ion
Ammonification
NO
3
-
2
-
+
NO
Nitrite ion
NH
3
(g)
,
NH
Ammonia, ammonium ion
Organic compounds
containing N
4
Nitrate ion
Nitrification
(aerobic)
Figure 2.10.
Diagram showing bacterial and chemical processes affecting the nitrogen cycle.
N
2
(g) is slowly removed from the air by
nitro-
gen fixation
.During this process, nitrogen-fixing bac-
teria, such as
Rhizobium
,
Azotobacter
, and
Beijer-
inckia
, convert N
2
(g) to the ammonium ion [NH
4
+
]
dissolved in water, some of which evaporates back
to the air as ammonia gas [NH
3
(g)]. Nitrogen-fixing
bacteria are prokaryotic and may be aerobic or anaer-
obic. Most today reside on the roots of leguminous
plants.
Another source of ammonium in soils is
ammonifi-
cation
,aprocess by which bacteria decompose organic
compounds to ammonium. Today, an anthropogenic
source of ammonium is fertilizer.
Ammonium is converted to nitrate in soils during
atwo-step process called
nitrification
.This process
occurs only in aerobic environments. In the first step,
nitrosofying (nitrite-forming) bacteria produce nitrite
from ammonium. In the second step, nitrifying (nitrate-
forming) bacteria produce nitrate from nitrite. Once
nitrate is formed, the nitrogen cycle continues through
the denitrification process. Nitrifying bacteria exist in
most waters of moderate pH and soils.
N
2
(g) has few chemical sinks. Because its chemi-
cal loss was slow and because its removal by nitrogen
fixation was slower than is its production by denitrifi-
cation, N
2
(g)'s concentration accumulated over time.
Table
2.6
summarizes
the
sources
and
sinks
of
N
2
(g).
2.3.7. Summary of Atmospheric Evolution
Figure 2.11 summarizes the temporal evolution of
N
2
(g), O
2
(g), CO
2
(g), and H
2
(g) in the Earth's sec-
ond atmosphere. The atmosphere of the early Earth may
have been dominated by carbon dioxide. Nitrogen grad-
ually increased due to denitrification. Oxygen increased
following the GOE 2.45 b.y.a. It reached 1 percent of
its present level 1.85 b.y.a., but it did not approach the
present level until after the evolution of green plants
around 400 m.y.a.
100
N
2
(g)
80
Table 2.6.
Sources and sinks of atmospheric
molecular nitrogen
60
40
O
2
(g)
CO
2
(g)
20
H
2
(g)
Sources
Sinks
0
Denitrification by bacteria
Nitrogen fixation by bacteria
4
3
2
1
0
Billions of years ago
Atmospheric chemical
reaction
High-temperature combustion
Atmospheric reaction
and photolysis of
N
2
O(g)
Figure 2.11.
Estimated change in composition during
the history of the Earth's second atmosphere.
Modified from Cattermole and Moore (1985).
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