Nutrient cycles

Question 1

Name 4 main stages of the nitrogen cycle

Answer 1

1. nitrogen fixation
2. ammonification
3. nitrification
4. denitrification

Question 2

Outline the role of bacteria in nitrogen fixation

Answer 2

mutualistic nitrogen-fixing bacteria in nodules of legumes and free living bacteria in soil
- use the enzyme in nitrogenase to reduce gaseous nitrogen into ammonia

Question 3

Outline the role of bacteria in ammonification

Answer 3

1. saprobionts feed on and decompose organic waste containing nitrogen
2. NH3 released
3. NH3 dissolves in water in soil to form NH4+

Question 4

Outline the role of bacteria in denitrification

Answer 4

Anaerobic denitrifying bacteria convert soil nitrates back into gaseous nitrogen

Question 5

Outline the role of mycorrhizae

Answer 5

Mutualistic relationship between plant and fungus increases surface area of root system = increases uptake of water and mineral ions

Question 6

Outline 2 main environmental issues caused by the use of fertilisers

Answer 6

1. leaching: nitrates dissolve in rainwater and ‘runoff’ into water sources
2. eutrophication: water source becomes putrid as a result of algal bloom

Question 7

What happens during eutrophication?

Answer 7

1. aquatic plants grow exponentially since nitrate level is no longer a limiting factor
2. algal bloom on water surface prevents light from reaching the bottom and plants die
3. oxygen levels decrease as population of aerobic saprobionts increases to decay dead matter, so fish die
4. anaerobic organisms reproduce exponentially and produce toxic waste which makes water putrid