Nitrogen in Soil and the

Nitrogen Cycle

What Is Nitrogen?

• Chemical Element, commonly a colorless, tasteless, odorless gas.

• 78 % by volume of the Earth’s Atmosphere.

Plants and animals need nitrogen.

• Plants and animals need nitrogen for DNA, RNA and proteins.

• Nitrogen is part of chlorophyll molecules in plants.

Relevant forms of nitrogen • Nitrogen gas: N2

• Nitrate ions: NO3-1

• Nitrite ions: NO2-1

• Ammonium ions: NH4+1

• Nitrous oxide (laughing gas) N2O

Bacteria drive the nitrogen cycle• Bacteria need to convert nitrogen gas from the air

into usable forms: nitrate and ammonium for plants!• Bacteria and fungi decompose plants and animals,

producing nitrogen compounds in the soil.• Plants absorb nitrate and ammonium through their

roots and assimilate these compounds into their structures.

• Animals eat plants and other animals to gain nitrogen.

VocabularyNitrogen Fixation: Some living organisms (rhizobium bacteria in legumes) can convert nitrogen into useful compounds such as ammonia.

Lightning also takes a part in nitrogen fixation.

Ammonification: Decomposition of detritus (dead plants, animals) by bacteria produces ammonia and ammonium compounds in soil.

Nitrification: Nitrifying bacteria convert ammonium to nitrite (toxic to plants) and nitrate.

Denitrification: Anaerobic bacteria convert nitrate, nitrite, ammonium into nitrous oxide and nitrogen in the air in swamps and wetlands. This reduces soil nitrogen.

• Nitrogen Fixation: Some living organisms (Legumes or certain bacteria) can convert nitrogen into ammonia, NH3.

• Cyanobacteria, rhizobium bacteria and other bacteria fix nitrogen from the air and convert it into ammonia.

Rhisomes = root nodules on legumes

contain rhizobmium.

bacteria.

Legumes are plants that have rhizomes.

A study of nitrogen fixation by alder trees.Alders were planted with Douglas Fir in the

1930’s after a fire. This photo was taken in 1990.

• The dark green band containing alder trees have nitrogen fixing bacteria that grow with it.

Douglas fir growing with Alder

Legumes enrich the soil:

A coffee plantation with leguminous trees in Costa Rica shades and adds soil nitrogen.

• Ammonification: You can say “decomposition”

Decomposition of nitrogen-rich organic compounds from waste materials are converted into ammonia.

Detritus: fresh to partly decomposed plant and animal matter

• Nitrification: Nitrifying bacteria convert ammonium to nitrite (toxic to plants) and nitrate

Denitrification: Anaerobic bacteria convert nitrate, nitrite, ammonium into nitrous oxide

and nitrogen in the air.

Denitrification example: This field has been very wet for ten

days. • Dentrification

has converted some of the nitrogen fertilizer into nitrogen gas.

Some plants in swamps get nitrogen from insects

The Nitrogen Cycle

Sources of Nitrogen for Plant Growth

• Nitrogen gas in the atmosphere, which must be converted into nitrate or ammonium by bacteria

• Commercial Fertilizers

• Soil Organic Matter

• Crop Residues (leaves and stalks from plants)

• Animal Manures

Commercial Fertilizers: add ammonium and nitrate to soils

Chemical Fertilizers

• Haber Process used to make synthetic fertilizers

• Advantage: Crop yields increase

• Disadvantage: Agricultural runoff pollutes lakes and rivers with fertilizers, causing algal blooms.

Runoff of fertilizers causes algal blooms

Algal blooms from runoff die and form the Dead Zone.

Compost

Questions1. What is the most abundant gas in the

atmosphere?

2. What forms of nitrogen are useful to plants?

3. What kinds of plants fix nitrogen?

4. What is a rhizome?

5. How does adding manure to the soil increase nitrogen in the soil?

6. What is nitrification?

7. Why are bacteria important for the nitrogen cycle?

More questions

1. Why are wet soils low in nitrogen?

2. Why do farmers add nitrogen to the soil?

3. What is the impact of agricultural runoff on lakes and rivers?

4. What is an algal bloom?

5. What is the dead zone?

6. Why is composting a good idea?

Bibliography

http://www.extension.umn.edu/distribution/copsystems/DC3770.html

http://academic.reed.edu/biology/Nitrogen/Nfix3.html Reed College

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