IB Biology, Option C6 (HL), Nitrogen and phosphorus cycles

Outline the nitrogen cycle

The circulation of nitrogen in the environment as a result of activity of living organisms and lightning (also affected by human activity).

Describe the nitrogen cycle

1. Nitrogen fixation: nitrogen fixing microorganisms capture nitrogen + convert it to ammonia

2. Nitrification: Ammonia is converted into nitrates/nitrites (NO-2, NO-3) by nitrifying bacteria

3. Assimilation: Nitrogenous molecules are absorbed by plants which are eaten by animals

4. Ammonia taken up by autotrophic plants is used to make organic molecules

5. Ammonification: Nitrogen gas is incorporated into the body or excreted and broken down into ammonium by decomposers and bacteria

6. Organic nitrogen can also be converted to nitrogen gas by bacteria in organisms (eg. Cows and pigs produce NH3 in waste)

7. Nitrification occurs again

8. Denitrification: Nitrates are made into nitrogen gas by denitrifying prokaryotes by bacteria

9. Nitrogen fixation also occurs by lightning

What can microorganisms do when partnering with a host plant?

Fix nitrogen symbiotically

Give an example of symbiotic nitrogen fixation and outline the relationship

Rhizobium bacteria and clover / pea plants

- The pant produces sugar for the bacteria by photosynthesis

- The bacteria uses sugar for energy to fix nitrogen

- the bacteria provides fixed nitrogen for plant growth

Describe the process of symbiotic nitrogen fixation

1. Bacteria colonises host plant root system

2. Roots for nodules to host bacteria

3. Bacteria fixes nitrogen for plant

4. Plant leaves are fortified with nitrogen that is recycled

5. Increased photosynthetic abilities and nitrogen-rich seeds

What is the effect of nitrogen-poor soil on plants?

- prevents nodulation

- Chlorosis (yellowing due to lack of chlorophyll)

- Low nitrogen plant content

- Small seed yield

Define water logging

Nitrogen is lost from water logging soil by leaching and denitrification

Define leaching

Nitrogen being washed off of fertilisers and into an ecosystem

Define denitrification

Nitrogen is converted to gaseous nitrogen oxides

How does water logging affect plants?

- Covers plant roots with water

- Prevents absorption of oxygen and other nutrients via the roots

What is nitrogen needed for?

Protein production

What types of plants are found in waterlogged areas? How do they obtain nitrogen?

Carnivorous plants. They obtain nitrogen through trapping and digesting invertebrates

Do carnivorous plants photosynthesise?

Yes

Outline the Haber(-Bosch) process

The reaction of hydrogen and nitrogen at high temperatures producing fixed nitrogen

What is fixed nitrogen from the Haber-Bosch process used for?

Fertilisers

What effects does the Haber-Bosch process have on the environemtn?

Greenhouse effect contributing to global warming due to the release of gases, and eutrophication

What is the effect of farming on soil? And how is it fixed?

Farming depletes soil nitrogen content, replenished by fertilisers

What impact do fertilisers have on the environemtn?

Cause eutrophication

Define reduced species diversity:

Nitrogen-rich soils favouring specific species (eg. grasses) leading to out-competition

Outline eutrophication

Leaching causing over-enrichment of water sources' nutrients, creating excess algae growth and reducing oxygen content levels (anoxic)

Describe eutrophication

1. Fertiliser is added to soil

2. Heavy rain washes fertiliser off (leaching)

3. Nitrates and phosphates dissolve in water

4. They are taken up by plants or washed into water sources

5. Surface plants on water grow excessively and block sunlight

6. Underwater plants cannot photosynthesise and die

7. Oxygen production / concentration in water decreases due to lack of photosynthesis

8. Bacteria breaks down detritus and uses up more oxygen

9. Water is anoxic and respiring aquatic organisms suffocate

10. A sterile environment may be created

List 5 characteristics of a eutrophic body of water

- Low oxygen content (anoxia)

- Lack of aquatic life / biodiversity

- High bacteria levels

- Shallow

- Low visibility due to organic detritus

Describe the phosphorus cycle

- Occurs at a slower rate due to reliance of rock weathering

- Phosphate compounds are found in sedimentary rock

- Rocks are weathered

- Phosphorus leaches into water sources and soil

- Taken up by plants and transferred to animals (both aquatic and on earth)

- Plant and animal waste / detritus are taken up by detritivores and returned to soil

- Phosphorus compounds are also carried with surface runoff and absorbed by aquatic organisms

What other substances contain phosphorus?

Volcanic ash, aerosols, mineral dust

How do humans affect the phosphorus cycle?

Removes:

- Harvesting crops

- Cutting tropical rain forests

Adds:

- Use of agricultural fertilisers