1. THE NITROGEN CYCLE

2. Nitrates are essential for plant growth
Root
uptake
Nitrate
NO3
-
Plant
protein
© 2008 Paul Billiet ODWS

3. Nitrates are recycled via microbes
Nitrification
Nitrification
Ammonium NH4
+
Ammonification
Nitrite NO2
-
Soil organic nitrogen
Animal
protein
Root
uptake
Nitrate
NO3
-
Plant
protein
© 2008 Paul Billiet ODWS

4. Nitrogen enters the soil through the
decomposition of protein in dead organic
matter
This process liberates a lot of energy which
can be used by the saprotrophic microbes
Ammonification

5. Atmospheric nitrogen fixation
Electrical storms
Lightning provides sufficient energy
to split the nitrogen atoms of nitrogen
gas,
Forming oxides of nitrogen NOx and
NO2

6. This also happens inside the internal
combustion engines of cars
The exhaust emissions of cars contribute a lot
to atmospheric pollution in the form of NOx
These compounds form photochemical smogs
They are green house gases
They dissolve in rain to contribute to acid rain
in the form of nitric acid
The rain falling on soil and running into rivers
They contribute to the eutrophication of
water bodies
Atmospheric Pollution

7. Conclusion
Adding nitrate fertiliser clearly helps the
growth of both plants
The presence of microbes permits the peas to
grow much better than the oats
The peas grow better in the presence of the
microbes than they do with nitrate fertiliser
added
The difference is due to the present of
mutualistic nitrogen fixing bacteria which live in
the pea roots.

8. These organisms possess the nif gene complex which
make the proteins, such as nitrogenase enzyme, used in
nitrogen fixation
Nitrogenase is a metalloprotein, protein subunits being
combined with an iron, sulphur and molybdenum
complex
The reaction involves splitting nitrogen gas molecules
and adding hydrogen to make ammonia.
This is extremely energy expensive requiring 16 ATP
molecules for each nitrogen molecule fixed
The microbes that can fix nitrogen need a good supply
of energy
Only prokaryotes show nitrogen fixation

9. The nitrogen fixers
Cyanobacteria are nitrogen fixers that also fix
carbon (these are photosynthetic)
Rhizobium bacteria are mutualistic with certain
plant species e.g. Legumes
They grow in root nodules
Azotobacter are bacteria associated with the
rooting zone (the rhizosphere) of plants in
grasslands

10. Nitrate NO3
-
Atmospheric
fixation
Out
gassin
g
Plant
protein
Atmospheric Nitrogen
Ammonium
NH4
+
Soil organic
nitrogen
The human impact
Biological
fixation
Industrial
fixation
© 2008 Paul Billiet ODWS

11. Eutrophication
Nutrient enrichment of water bodies
Nitrates and ammonia are very soluble in
water
They are easily washed (leached) from free
draining soils
These soils tend to be deficient in nitrogen
When fertiliser is added to these soils it too
will be washed out into water bodies
There algae benefit from the extra nitrogen
This leads to a serious form of water
pollution

12. Fertilisers washed into river or lake
New limiting factor imposes itselfSewage or other
organic waste
Eutrophication

13. Increased Biochemical
Oxygen Demand (BOD)
Hot water
from industry
(Thermal
pollution)
Pollution
from oil or
detergents
Reduction in dissolved O2
Making things worse!