5.4 Nutrient Cycles

Question 1

Name the stages of the phosphorus cycle.

Answer 1

Weathering
Runoff
Assimilation
Decomposition
Uplift

Question 2

Why is the phosphorus cycle slow?

Answer 2

Stored as PO₄³⁻
Phosphorus has no gas phase so no atmospheric cycle

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

Converted from inorganic phosphate into biological molecules e.g. DNA, ATP, NADP

Question 5

What happens during uplift?

Answer 5

Sedimentary layers from oceans formed by 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

Nitrogen fixation
Ammonification
Nitrification
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

High energy of lightning breaks N₂ into N
N reacts with oxygen to form NO₂⁻
NO₂⁻ dissolves in water to form NO₃⁻

Question 10

Outline the role of bacteria in nitrogen fixation.

Answer 10

Nitrogen-fixing bacteria in nodules of legumes & free-living bacteria in soil

Use enzyme nitrogenase to reduce gaseous nitrogen into ammonia

Question 11

Outline the role of bacteria in ammonification.

Answer 11

Saprobionts feed on and decompose organic waste containing nitrogen (e.g. urea, proteins, nucleic acids etc)
NH₃ released
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⁻ + H₂O + 4H⁺
2NO₂⁻ + O₂ -> 2NO₃⁻

Question 12

What are saprobionts?

Answer 12

Decomposers that digest their material externally by secreting enzymes onto dead organisms and waste material

Question 13

Outline the role of bacteria in denitrification.

Answer 13

Anaerobic denitrifying bacteria convert soil nitraes 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 for 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 (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 2 categories of fertiliser.

Answer 17

Organic
Inorganic

Question 18

State the purpose of fertiliser.

Answer 18

To increase gross productivity for higher yield

Question 19

What is organic fertiliser?

Answer 19

Decaying organic matter & animal waste

Question 20

What is inorganic fertiliser?

Answer 20

Minerals from rocks, usually nitrogen, phosphorus, potassium

Question 21

Why doesn’t more fertiliser increase crop yield further at a certain point?

Answer 21

It is no longer the limiting factor
Another factor is limiting the rate of photosynthesis
Rate of growth cannot increase any further

Question 22

Outline 2 environmental issues caused by fertiliser use.

Answer 22

Leaching - nitrates dissolve in rainwater and run off into water sources
Eutrophication - algal bloom due to excessive nutrients

Question 23

What happens during eutrophication?

Answer 23

Aquatic plants grow exponentially since nitrate level no longer limiting factor
Algal bloom on surface kills other plants by blocking light
Aerobic saprobionts increases to decay dead matter and use oxygen so fish die
Anaerobic organisms reproduce and produce toxic waste

Question 24

How can the risk of eutrophication be reduced?

Answer 24

Sewage treatment on farm
Pumping nutrient-enriched sediment out of water
Using phosphate-free detergent