Nutrient Cycles

Question 1

What are the 5 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

• P has no gas phase so there is no atmospheric cycle

- most P is stored as PO4 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 are brought up to land over many years

Question 6

How does mining affect the phosphorus cycle?

Answer 6

Speeds up uplift

Question 7

What are 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
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- + 2H20 + 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 + 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 proportions of certain ions

Question 17

Name 2 categories of fertiliser

Answer 17

• organic: decaying organic matter & animal waste

- inorganic: minerals from rocks, usually containing nitrogen, phosphorus, potassium

Question 18

What is the purpose of using fertiliser?

Answer 18

To increase gross productivity for higher yield

Question 19

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

Answer 19

• a factor unrelated to the concentration of mineral ions limits the rate of photosynthesis
• rate of growth cannot increase any further

Question 20

Outline 2 main environmental issues caused by the use of fertilisers

Answer 20

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

Question 21

What happens during eutrophication?

Answer 21

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 22

How can the risk of eutrophication be reduced?

Answer 22

• sewage treatment marshes on farms
• pumping nutrient-enriched sediment out of water
• using phosphate-free detergent