Nutrient cycles

Question 1

Name the general stages in the phosphorus cycle.

Answer 1

1. Weathering
2. Runoff
3. Assimilation
4. Decomposition
5. Uplift

Question 2

Why is the phosphorus cycle a slow process?

Answer 2

● Phosphorus has no gas phase, so there is no atmospheric cycle.
● Most phosphorus is stored as PO43- in rocks.

Question 3

What happens during weathering and runoff?

Answer 3

Phosphate compounds from sedimentary rocks leach into surface water and soil.

Question 4

Explain the significance of phosphorus to living

organisms.

Answer 4

Plants convert inorganic phosphate into biological molecules e.g. DNA, ATP, NADP…
Phosphorus is passed to consumers via feeding.

Question 5

What happens during uplift?

Answer 5

Sedimentary layers from oceans (formed by the bodies of aquatic organisms) are brought up to land over many years.

Question 6

How does mining affect the phosphorus cycle?

Answer 6

Speeds up uplift.

Question 7

Name the 4 main stages of the nitrogen cycle.

Answer 7

1. Nitrogen fixation
2. Ammonification
3. Nitrification
4. Denitrification

Question 8

Why can’t organisms use nitrogen directly from the

atmosphere?

Answer 8

N2 is very stable due to strong covalent triple bond.

Question 9

What happens during atmospheric fixation of

nitrogen?

Answer 9

1. High energy of lightning breaks N2 into N.
2. N reacts with oxygen to form NO2-.
3. NO2- dissolves in water to form NO3-.

Question 10

Outline the role of bacteria in nitrogen fixation.

Answer 10

Mutualistic nitrogen-fixing bacteria in nodules of legumes & free-living bacteria in soil.
Use the enzyme nitrogenase to reduce gaseous nitrogen into ammonia.

Question 11

Outline the role of bacteria in ammonification.

Answer 11

1. Saprobionts feed on and decompose organic waste containing nitrogen (e.g. urea, proteins, nucleic acids…).
2. NH3 released.
3. NH3 dissolves in water in soil to form NH4+.

Question 12

Outline the role of bacteria in nitrification.

Answer 12

2-step process carried out by saprobionts in aerobic conditions:
2NH4+ + 3O2 → 2NO2- + 2H2O + 4H+
2NO2- + O2 → 2NO3

Question 13

Outline the role of bacteria in denitrification.

Answer 13

Anaerobic denitrifying bacteria convert soil nitrates back into gaseous nitrogen.

Question 14

Explain the significance of nitrogen to living

organisms.

Answer 14

Plant roots uptake nitrates via active transport & use them to make biological compounds e.g:
● amino acids
● NAD/ NADP
● nucleic acids

Question 15

Outline the role of mycorrhizae.

Answer 15

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

Question 16

Give 3 benefits of planting a different crop on the

same field each year.

Answer 16

● Nitrogen-fixing crops e.g. legumes make soil more fertile by increasing soil nitrate content.
● Different crops have different pathogens.
● Different crops use different proportions of certain ions.

Question 17

Name the 2 categories of fertiliser and state the

purpose of using fertiliser.

Answer 17

● Organic: decaying organic matter & animal waste.
● Inorganic: minerals from rocks, usually containing nitrogen, phosphorus, potassium.
● To increase gross productivity for higher yield.

Question 18

At a certain point, using more fertiliser no longer

increases crop yield. Why?

Answer 18

A factor unrelated to the concentration of mineral ions limits the rate of
photosynthesis, so rate of growth cannot increase any further.

Question 19

Outline 2 main environmental issues caused by the use of fertilisers.

Answer 19

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

Question 20

What happens during eutrophication?

Answer 20

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.

Question 21

How can the risk of eutrophication be reduced?

Answer 21

● Sewage treatment marshes on farms.
● Pumping nutrient-enriched sediment out of water.
● Using phosphate-free detergent.