Nutrient cycles

Question 1

Name the 5 general stages in the phosphorus cycle.

Answer 1

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

Question 2

Why is the phophorus cycle a slow process?

Answer 2

• It has no gas phase, so there is no atmospheric cycle.

- Most phosphorous is stored as PO₄³⁻ in rocks

Question 3

What happens during weathering and runoff?

Answer 3

Phosphate compunds from sefimentary rocks leach into surface water and soil.

Question 4

Explain the significance of phosphorus to living organisms.

Answer 4

Plants convert inorganic phosphate into bilogical 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 mainstages of the nitrogen cycle.

Answer 7

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

Question 8

Why can’t organims use nitrogen directly from the atmosphere?

Answer 8

N₂ is very stable due to strong covalent triple bonds.

Question 9

What happens during atmospheric fixation of nitrogen?

Answer 9

1. High energy of lightning breaks N₂ into N.
2. N reacts within 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 ammonia.

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 ammonia.

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 ammonia.

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 ammonia.

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 ammonia.

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 ammonia.

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 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 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 denfitrication.

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 uptakes 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 anf fungus increases SA 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 propotions of certain ions.

Question 17

Name 2 categories of fertiliser and state the purple of using fertiliser.

Answer 17

• Organic: decaying orhanic matter & animal waste.
• Inorganic: minerals from rocks, usually containing nitrogen, phosphorus, potassium
• To increase fross 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 enviromental issues caused by the use of fertilisers.

Answer 19

1. Leaching: nitrates dissolves 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 decreases 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.