5.4 Nutrient Cycles

Question 1

General stages of phosphorus cycle

Answer 1

• Weathering
• Runoff
• Assimilation
• Decomposition
• 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 PO4^3- 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 significance of phosphorus to living organisms

Answer 4

• Plants convert inorganic phosphate into biological molecules e.g. DNA, ATP, NADP, etc
• 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 5

How does mining affect the phosphorus cycle?

Answer 5

Speeds up uplift

Question 6

Name 4 main stages of the nitrogen cycle

Answer 6

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

Question 7

Why can organisms use nitrogen directly from the atmosphere?

Answer 7

N2 is very stable due to strong covalent triple bond

Question 8

What happens during atmospheric fixation of nitrogen?

Answer 8

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

Question 9

Outline the role of bacteria in nitrogen fixation

Answer 9

• 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 10

Outline the role of bacteria in ammonification

Answer 10

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 11

Outline the role of bacteria in nitrification

Answer 11

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

Question 12

Outline the role of bacteria in denitrification

Answer 12

Anaerobic denitrifying bacteria convert soil nitrates back into gaseous nitrogen

Question 13

Explain the importance of nitrogen to living organisms

Answer 13

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

Question 14

Outline the role of mycorrhizae

Answer 14

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

Question 15

Give 3 benefits of planting a different crop on the same field each year

Answer 15

• 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 16

Name the 2 categories of fertiliser and state the purpose of using fertiliser

Answer 16

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

Question 17

At a certain point, using more fertiliser no longer increase crop yield. Why?

Answer 17

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

Question 18

Outline two main environmental issues caused by the use of fertilisers

Answer 18

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

Question 19

What happens during eutrophication?

Answer 19

1. Aquatic plats 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 20

How can the risk of eutrophication be reduced?

Answer 20

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