The Nitrogen Cycle

• Nitrogen (N) is an essential component of DNA, RNA, and proteins, the building blocks of life.

• All organisms require nitrogen to live and grow.

• The majority (78%) of the Earth’s atmosphere is N2.

Nitrogen

Have you ever started a fishtank?

Why must you wait

to add a bunch of fish?

Like all living creatures, fish give off waste products (pee and poo). These nitrogenous waste products break down into ammonia (NH3), which is highly toxic to most fishes.

Nitrogen’s triple bond• Although the majority of the air

we breathe is N2, most of the nitrogen in the atmosphere is unavailable for use by organisms.

• This is because the strong triple bond between the N atoms in N2 molecules makes it relatively inert (like a noble gas).

How can we use N2?

• In order for plants and animals to be able to use nitrogen, N2 gas must first be converted to more a chemically available form such as ammonium (NH4

+) or nitrate (NO3

-).

WE CAN’T!

But BACTERIA & … can…

Nitrogen Fixation (N2 --> NH3 or NH4+)

HOW? Biological or Environmental or Humans

BIOLOGICAL: Bacteria (called Nitrogen-fixing bacteria)

These bacteria form symbiotic relationships with host plants.

The bacteria live in nodules found in the roots of the legume family of plants (e.g. beans, peas, and clover)

Nitrogen Fixation (N2 --> NH3 or NH4+)

HOW? Biological or Environmental or Humans

BIOLOGICAL: Bacteria (called Nitrogen-fixing bacteria)

These bacteria form symbiotic relationships with host plants.

The bacteria live in nodules found in the roots of the legume family of plants (e.g. beans, peas, and clover)

How?BIOLOGICAL

• In aquatic environments (like fishtanks),

blue-green algae (cyanobacteria) is an important free-living nitrogen fixer.

Nitrogen Fixation (N2 --> NH3 or NH4+)

Nitrogen Fixation (N2 --> NH3 or NH4+)

ENVIRONMENTAL

High-energy natural events which break the bond N2

Examples:lightningforest fireshot lava flows

Nitrogen Fixation N2 --> NH3 or NH4+

How?

HUMAN IMPACT • Burning fossil fuels, • using synthetic nitrogen

fertilizers, • and cultivation of legumes

all fix nitrogen.

Nitrogen Fixation

Atmospheric Nitrogen is broken into useable nitrogen (NHNH33 or or

NHNH44++))

Nitrogen Mineralizationalso called Ammonification

Organic N --> NH4+

• Decay of dead things, manure, etc.

• Done by decomposers (bacteria, fungi, etc.)

• During this process, a significant amount of the nitrogen contained within the dead organism is converted to ammonium (NH4

+).

Nitrification NH3 or NH4

+ --> NO2- --> NO3

-

(Nitrifying) Bacteria add oxygen to nitrogen in two steps:• STEP 1:

Bacteria take in NH3 or NH4+ & make NO2

- = nitrite

• Step 2:Bacteria take in NO2

- & make NO3

- = nitrate

Nitrification

Nitrogen Uptake

• The ammonia (NH3) produced by nitrogen-fixing bacteria is usually quickly incorporated into protein and other organic nitrogen compounds (organisms!).

• It’s either absorbed by a plant, by the bacteria itself, or by another soil organism.

• Organisms at the top of the food chain (like us!) eat and grow, uptaking nitrogen (that has already been fixed).

DenitrificationNO3

- --> N2

(Denitrifying) Bacteria do it.

Denitrification removes nitrogen from ecosystems, and converts it back to atmospheric N2.

Human Impact

• FERTILIZERS!

• Extra nitrogen fertilizer can runoff, where it contaminates surface water or infiltrates into ground water.

• In drinking water, excess nitrogen can lead to cancer in humans and respiratory distress in infants.

Human Impact• In surface waters, extra nitrogen can lead to nutrient

over-enrichment.

• This leads to – fish-kills, – harmful algal blooms, – and species shifts in

aquatic and land ecosystems.

Human ImpactSome forms of nitrogen

(like NO3- and NH4

+) can also enter the atmosphere to become:

– smog- nitric oxide (NO)

– Greenhouse gas- nitrous oxide (N2O)

– Acid Rain- (nitrogen oxides)

Nitrogen Cycle PicturesGood pictures of the nitrogen cycle have these

processes:– Nitrogen fixation (N2 bonds are broken)

– Nitrification (oxygen is added to form nitrogen oxides)

– Denitrification (N2 is put back into the air)

It is also helpful to have– Ammonification (mineralization/waste conversion by

decomposers)– Assimilation (intake by producers)

• Yellow arrows indicate human sources of nitrogen to the environment. Red arrows indicate bacterial transformations of nitrogen. Blue arrows indicate physical forces acting on nitrogen. And green arrows indicate natural, non-microbial processes affecting the form and fate of nitrogen.