3.5.4 Nutrient Cycles

Question 1

(Nutrient Cycles) Name the 5 general stages in the phosphorus cycle.

Answer 1

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

Question 2

(Nutrient Cycles) Why is the phosphorus cycle a slow process?

Answer 2

• Phosphorous has no gas phase, so there is no atmospheric cycle.
• Most phosphorus is stored as PO3−4 in rocks.

Question 3

(Nutrient Cycles) What happens during weathering and runoff?

Answer 3

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

Question 4

(Nutrient Cycles) 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

(Nutrient Cycles) 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

(Nutrient Cycles) How does mining affect the phosphorus cycle?

Answer 6

Speeds up uplift.

Question 7

(Nutrient Cycles) Name the 4 main stages of the nitrogen cycle.

Answer 7

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

Question 8

(Nutrient Cycles) Why can’t organisms use nitrogen directly from the atmosphere?

Answer 8

N2 is very stable due to a strong covalent triple bond.

Question 9

(Nutrient Cycles) What happens during the 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

(Nutrient Cycles) Outline the role of bacteria in nitrogen fixation.

Answer 10

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

Question 11

(Nutrient Cycles) Outline the role of bacteria ammonification.

Answer 11

1. Saprobionts feed on the decomposing 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

(Nutrient Cycles) 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

(Nutrient Cycles) Outline the role of bacteria in denitrification.

Answer 13

Anaerobic denitrifying bacteria convert soil nitrates back into gaseous nitrogen.

Question 14

(Nutrient Cycles) Explain the significance of nitrogen to living organisms.

Answer 14

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

Question 15

(Nutrient Cycles) 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

(Nutrient Cycles) Give 3 benefits of planting a different crop on the same field each year.

Answer 16

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

Question 17

(Nutrient Cycles) Name the 2 categories of fertiliser and state the purpose of using fertiliser.

Answer 17

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

Question 18

(Nutrient Cycles) At a certain point, using more fertiliser no, longer increases crop yield. Why?

Answer 18

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

Question 19

(Nutrient Cycles) 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

(Nutrient Cycles) What happens during eutrophication?

Answer 20

1. Aquatic plants grow exponentially since nitrate level is no longer a limiting factor.
2. Algal bloom on the water surface prevents light from reaching the bottom and plants die.
3. Oxygen levels decrease as the 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

(Nutrient Cycles) 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.