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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