1. The Nitrogen (N2) Cycle
Pages 78-83

2. Where Nitrogen is Stored
Large Stores
In the atmosphere where it exists as a gas (78%)
Oceans and within organic matter in soil
Small Stores
Within terrestrial ecosystems, living organisms, lakes and marshes

3. How Nitrogen is Cycled
There are large stores of N2 in the atmosphere but most organisms cannot use it in the form of a gas.
Therefore nitrogen is cycled by 3 different methods:
1. Nitrogen Fixation
2. Nitrification
3. Uptake

4. Nitrogen (N2) Fixation
Process that converts N2 into nitrate NO3- OR ammonium NH4+ compounds.
These 2 compounds: NO3- and NH4+ can only be used by plants!
Nitrogen Fixation occurs in 3 different ways:
1. In the Atmosphere via lightning
2. In Soil via Nitrogen Fixing Bacteria (aka Rhizobium)
3. In oceans, wetlands and lakes via cyanobacteria

5. 1. N2 Fixation in the ATMOSPHERE
Happens when lightening strikes! Obviously a small amount of nitrogen-containing compounds are fixed this way.
How does this happen?
Lightning provides heat energy causing N2 to react with O2 in the atmosphere where it is converted into nitrate (NO3-)
Newly formed nitrate enters aquatic and terrestrial ecosystems by rain!

6. 2,3. N2 Fixation in SOIL & WATER
Occurs when N2 is converted into ammonium (NH4+) by nitrogen fixing bacteria during decomposition.
Example: Rhizobium, a N2 fixing bacteria that lives in the roots nodules of legumes (pod-producing plants such as beans, peas, clover and alfalfa) and some other plants.
How does Rhizobium and pod-producing plants fix N2?
Through Mutualism!
IN SOIL: pod producing plants supply the Rhizobium bacteria with sugars which it needs to survive while the Rhizobium bacteria supply's its host plant with nitrogen in the form of ammonium (NH4+).
IN WATER: occurs in water via a bacteria found in aquatic ecosystems called cyanobacteria. This bacteria also converts N2 into ammonium (NH4+).

7. 3. Nitrification and Uptake
Not all plants live in association with nitrogen-fixing bacteria they must get N2 in another form.
Nitrification: process where NH4+ is converted into NO3- which involves the help of soil bacteria's called: nitrifying bacteria.
Takes place in 2 stages in soil by different types of nitrifying bacteria:
1. Ammonium NH4+ is converted into nitrite NO2-
2. Nitrite (NO2-) is converted into nitrate (NO3-) a compound that plants can readily use to synthesize plant protein and stay healthy.
Occurs in 2 steps because different types of bacteria are needed for each step.
 Animals obtain they nitrate (NO3-) that they need by eating plants!

8. How does N2 return to the atmosphere?
Nitrogen is returned to the atmosphere via a process called denitrification.
Occurs in terrestrial and aquatic ecosystems by bacteria known as; denitrifying bacteria:
Denitrifying bacteria returns N2 to the atmosphere by converting NO3- back into N2 gas
N2 is also returned to the atmosphere via volcanic ash and nitrogen oxide gases from industry and automobiles.

9. How is Excess Nitrogen Removed from Ecosystems?
Excess nitrate and ammonium that are not taken up by plants mix with rainwater and are washed away into ground water and streams.
Unused nitrogen may settle in ocean, lake or river bottoms in sediments.
Eventually these sediments turn into rock where the nitrogen will not be available.
Only after centuries of weathering will the nitrogen be re-released.

10. Human Factors and The N2 Cycle
1. Fossil Fuel Burning – via automobiles and industry releasing NO and NO2 gases
2. Clearing Forests and Grasslands – releases trapped nitrogen into the atmosphere
3. Chemical Fertilizers – results in an excess amount of NH4 and NO3 which can escape back into the atmosphere as NH3 (ammonia) or leached away by rain.
Can cause eutrophication: when excess nutrients results in increased plant production and decay. Causing excessive algae growth in aquatic ecosystems which deprives aquatic organisms of oxygen.

11. The Phosphorus Cycle Where is Phosphorous stored?
In sedimentary rock and in sediments on the ocean floor.
NOT stored in the atmosphere as a gas

12. How is Phosphorous used?
Plants – contributes to root development, stem strength and seed production Humans - large amount of phosphorous is found in bones. Works with calcium for the development of strong bones

13. How Phosphorous is Cycled
1. Chemical and Physical Weathering – process where rock is broken down releasing phosphorous back into the soil:
Chemical weathering: acid rain and acid released by lichens to break down rock releasing phosphorus into the soil.
Physical Weathering: when wind, rain and freezing releases phosphate and rock into the soil.
2. Geologic Uplift - process of mountain building where the Earth’s crust folds exposing deeply buried rocks which exposes them to weathering.
How do plants and animals take-up phosphate?
A. Plants take-up phosphate by their roots
B. Animals get phosphate by eating plants

14. Human Activities
Rock containing phosphate is mined primarily to make fertilizers and detergents (soaps).
Guano, bird/bat droppings which are rich in phosphate, used as a natural organic fertilizer
The combination of fertilizers, detergents and guano due to large scale farming inevitably, human waste all enter water-ways via run-off and leaching.
Burning forest – phosphate leaches back into the soil from the ashes
Too much additional phosphate can cause eutrophication and algae blooms and can negatively affect different species (eg can kill fish)

15. Phosphorous Cycle Review