Where Nitrogen is Where Nitrogen is StoredStored

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

How Nitrogen is How Nitrogen is CycledCycled

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

Nitrogen (NNitrogen (N22) Fixation) FixationProcess 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

1. N1. N22 Fixation in the Fixation in the

ATMOSPHEREATMOSPHERE

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!

2. N2. N22 Fixation in Fixation in SOIL & SOIL &

WATERWATER

Occurs when N2 is converted into ammonium (NH4+) by nitrogen fixing

bacteria during decomposition.

Example: Rhizobium, a N2 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 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

+).

3. Nitrification and 3. Nitrification and UptakeUptake

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 the nitrate (NO3-) that they need by eating plants!

How does NHow does N22 return to the return to the

atmosphere?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.

How is Excess Nitrogen Removed from How is Excess Nitrogen Removed from

Ecosystems?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.

Human Factors and The Human Factors and The NN22 Cycle 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 and returns to the ground as acid rain.

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: increased amounts of dissolved nitrogen causes plant growth and decay. Causing excessive algae growth in aquatic ecosystems which deprives aquatic organisms of oxygen.