1. 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

2. How Nitrogen is Cycled There are large stores of N 2 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

3. Nitrogen (N 2 ) Fixation Process that converts N 2 into nitrate NO 3 - OR ammonium NH 4 + compounds. These 2 compounds: NO 3 - and NH 4 + 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

4. 1. N 2 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 N 2 to react with O 2 in the atmosphere where it is converted into nitrate (NO 3 - ) Newly formed nitrate enters aquatic and terrestrial ecosystems by rain!

5. 2. N 2 Fixation in SOIL & WATER Occurs when N 2 is converted into ammonium (NH 4 + ) by nitrogen fixing bacteria during decomposition. Example: Rhizobium, a N 2 fixing bacteria lives in the root 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 N 2 ? 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 (NH 4 + ). IN WATER: occurs in water via a bacteria found in aquatic ecosystems called cyanobacteria. This bacteria also converts N 2 into ammonium (NH 4 + ).

6. 3. Nitrification and Uptake Not all plants live in association with nitrogen-fixing bacteria they must get N 2 in another form. Nitrification : process where NH 4 + is converted into NO 3 - 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 NH 4 + is converted into nitrite NO 2 - 2. Nitrite (NO 2 - ) is converted into nitrate (NO 3 - ) 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 the nitrate (NO 3 -) that they need by eating plants!

7. How does N 2 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 N 2 to the atmosphere by converting NO 3 - back into N 2 gas N 2 is also returned to the atmosphere via volcanic ash and nitrogen oxide gases from industry and automobiles.

8. 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.

9. Human Factors and The N 2 Cycle 1. Fossil Fuel Burning – via automobiles and industry releasing NO and NO 2 gases 2. Clearing Forests and Grasslands – releases trapped nitrogen into the atmosphere and returns to the ground as acid rain. 3. Chemical Fertilizers – results in an excess amount of NH 4 and NO 3 which can escape back into the atmosphere as NH 3 (ammonia) or leached away by rain. Can cause eutrophication: increased amounts of dissolved nitrogen causes plant growth and decay. Causing excessive algae growth in aquatic ecosystems which deprives aquatic organisms of oxygen.