Organisation at the Ecosystem Level 69
such as clover, beans or peas. This bacterium fixes N 2 which will be used
by the leguminous plants, while it takes from these plants the necessary
energy and substances (symbiosis). Additionally, non-symbiotic bacteria
exist which are able to fix N 2 , such as the aerobic nitrobacterium and the
anaerobic clostridium. Some algae that belong to the blue-green algae group
(cyanophyta) can also realize N 2 fixation.
Artificial nitrogen fixation is a 20th century invention and is mostly
realised through industrial production of artificial fertilizers, which is of
the same order of magnitude with biological fixation. In 1968, industrial
N 2 fixation was equal to 30 × 10 6 tons. Significant artificial N 2 fixation is
also caused as a side effect of internal combustion engines functioning or
of some industries which emit nitrogen oxides (NO x ) in the atmosphere.
These oxides represent, on one hand, a serious air pollutant (Chapter 9),
and on the other hand, they are oxidised to nitrates and end up either in
waters or the soil.
Nitrogen incorporated into living organisms returns to the environment
in an inorganic form, ammonia (NH 3 ), or the product of its ionisation
(NH 4 ) + , as a result of dead organisms’ decomposition and their waste. This
phenomenon is defined as ammonisation.
The chemoautotrophic bacteria nitrosomonas and nitrobacterium are
interesting for pollution control technology. They are strictly aerobic
bacteria and obtain the necessary energy by oxidising ammonia to nitrites
(NO 2 ) – and nitrate (NO 3 ) – , according to the following chemical reactions:
NH
O
NO H H O energy
NO
O
NO energ
3
2
2
2
2
2
3
3 2
1 2
+
→
+
+
+
+
→
+
−
+
−
−
y y
The phenomenon of biological conversion of ammonia to nitrates is
defined as nitrification. Nitrates are, in general, more conducive to biological processes but also constitute a pollutant of aquatic bodies. Nitrification
is a water deoxygenation factor, due to aerobic oxidation. There are also
some other bacteria categories, which can convert nitrates to nitrites and
nitrites to ammonium nitrate.
Denitrification is the process of nitrate reduction to N 2 and N 2 O, during
anaerobic oxidation of organic matter by microorganisms. N 2 O is reduced
to N 2 by further bacterial action or by photochemical reactions within the
atmosphere. Thus, N 2 returns to the atmosphere, closing the cycle. Without
denitrification, all the nitrogen in the atmosphere would have been bound,
during Earth’s long history, in the land and oceans in the form of nitrates.
The main reservoirs of the nitrogen cycle and the estimated storages is the
sea (dissolved, 20,000 × 10 9 tons), or the atmosphere (3,800,000 × 10 9 tons),
organisms (13 × 10 9 tons), dead organic matter (1660 × 10 9 tons), and
depositions (4,000,000 × 10 9 tons).
such as clover, beans or peas. This bacterium fixes N 2 which will be used
by the leguminous plants, while it takes from these plants the necessary
energy and substances (symbiosis). Additionally, non-symbiotic bacteria
exist which are able to fix N 2 , such as the aerobic nitrobacterium and the
anaerobic clostridium. Some algae that belong to the blue-green algae group
(cyanophyta) can also realize N 2 fixation.
Artificial nitrogen fixation is a 20th century invention and is mostly
realised through industrial production of artificial fertilizers, which is of
the same order of magnitude with biological fixation. In 1968, industrial
N 2 fixation was equal to 30 × 10 6 tons. Significant artificial N 2 fixation is
also caused as a side effect of internal combustion engines functioning or
of some industries which emit nitrogen oxides (NO x ) in the atmosphere.
These oxides represent, on one hand, a serious air pollutant (Chapter 9),
and on the other hand, they are oxidised to nitrates and end up either in
waters or the soil.
Nitrogen incorporated into living organisms returns to the environment
in an inorganic form, ammonia (NH 3 ), or the product of its ionisation
(NH 4 ) + , as a result of dead organisms’ decomposition and their waste. This
phenomenon is defined as ammonisation.
The chemoautotrophic bacteria nitrosomonas and nitrobacterium are
interesting for pollution control technology. They are strictly aerobic
bacteria and obtain the necessary energy by oxidising ammonia to nitrites
(NO 2 ) – and nitrate (NO 3 ) – , according to the following chemical reactions:
NH
O
NO H H O energy
NO
O
NO energ
3
2
2
2
2
2
3
3 2
1 2
+
→
+
+
+
+
→
+
−
+
−
−
y y
The phenomenon of biological conversion of ammonia to nitrates is
defined as nitrification. Nitrates are, in general, more conducive to biological processes but also constitute a pollutant of aquatic bodies. Nitrification
is a water deoxygenation factor, due to aerobic oxidation. There are also
some other bacteria categories, which can convert nitrates to nitrites and
nitrites to ammonium nitrate.
Denitrification is the process of nitrate reduction to N 2 and N 2 O, during
anaerobic oxidation of organic matter by microorganisms. N 2 O is reduced
to N 2 by further bacterial action or by photochemical reactions within the
atmosphere. Thus, N 2 returns to the atmosphere, closing the cycle. Without
denitrification, all the nitrogen in the atmosphere would have been bound,
during Earth’s long history, in the land and oceans in the form of nitrates.
The main reservoirs of the nitrogen cycle and the estimated storages is the
sea (dissolved, 20,000 × 10 9 tons), or the atmosphere (3,800,000 × 10 9 tons),
organisms (13 × 10 9 tons), dead organic matter (1660 × 10 9 tons), and
depositions (4,000,000 × 10 9 tons).