THE NITROGEN CYCLE
I. Nitrogen pools and cycling times
Cycling time refers to the average time an average nitrogen
atom will remain unchanged in a given location
Pool Cycling time
Atmospheric Ammonia Days
Oceans 7 weeks
Forest Trees 16 years
Atmospheric Nitrogen 44 Million years
II. Biological uses of Nitrogen
Nucleic Acids, which are made of nitrogen containing
compound combined with a phosphate group and a sugar (N
compound-CH8O2-PO4)
Amino Acids are made a nitrogen containing amino group
(NH2) combined with a carboxyl group (COOH) and one of 20
different side groups (R). The final product is: NH2-COOH-R.
Amino acids combine to make proteins, enzymes, and
hormones, which with carbohydrates are the major building
blocks of all living organisms.
The ratio of Nitrogen to Carbon demanded by organisms
makes up part of a constant termed Redfield Ratios. These
differ between aquatic and terrestrial systems.
Habitat Optimum C:N ratio
Land 106 C : 16 N
Aquatic 106 C : 13 N
Nitrogen requirements of aquatic species are less becasue
they need less proteins to keep their body structure as they
are partially held up by water. Land species require more
proteins, as air support theur bodies much less than water.
Because of the greater amount of proteins required by land
species, Nitrogen is usually the element which will limit the
vigour of their communities.
III. The Nitrogen Cycle
Step 1: Nitrogen-fixation
Atmospheric: Happens when Nitrogen (N2) is oxidized at high
temperatures (by lightning, in internal combustion engines) to make
nitrite (NO2). This can combine with water to form nitric acid
(H2NO3), which is deposited on earth through rainfall.
Biological: Done by bacteria which can convert N2 into ammonia
(NH3) if an energy source is present. Some get this energy by
directly absorbing sunlight (blue-green algae) or by living in the
roots of plants (legumes, alder trees), who provide them with food
(Rhizobium, Azospirillium).
Step 2: Conversion to Ammonia. As amino acids and nucleic acids
require N in the form of Ammonia, if nitrate (NO3) present, it must be
converted to NH3. This is done through Nitrate reductase enzymes.
Step 3: Biological Use. Ammonia is incorporated into proteins,
nucleic acids
Step 4: When organism dies, ammonia is released back into the
biosphere through the process of Ammonification, in which water is
added to proteins to make carbon dioxide and ammonia. This
process happens during digestion, and is also done by bacterial and
fungal decomposers.
Step 5: If ammonia released into oxygen rich (anaerobic) soil, other
bacteria can convert it into nitrite or nitrate through the process of
Nitrification:
NH4+ + 2O2 = NO3- + H2O + 2H+.
This is a problem, as it gives the molecule which contains Nitrogen
a negative charge, which repels it from soil particles, causing it to
be easily leached into streams and groundwater.
Step 6: If soils remain anaerobic, another group of bacteria will
convert it back into inert, atmospheric N2 through the process of
Denitrification. In this process, bacteria use nitrate as an Oxygen
source for respiration: C6H12O6 + 4NO3- = 6CO2 + 6H2O + 2N2
This is a problem, as any nitrogen returned to atmospheric will be
stuck there for 44 million years until it is fixed again into a
biologically useful form.