3.5.4 Nutrient cycles

Question 1

Name the general stages in the 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 stage 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 the significance of phosphorus to living organisms

Answer 4

Plants convert inorganic phosphate into biological molecules eg 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

Name the 4 main stages of the nitrogen cycle

Answer 7

Nitrogen fixation
Ammonification
Nitrification
Denitrification

Question 8

Why cant 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

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

Outline the role of bacteria in ammonification

Answer 11

Saprobionts feed in and decompose organic waste containing nitrogen eg urea proteins
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- + 2H2O + 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 and use them to make biological compounds eg
Amino acids, NAD/NAPP, nucleic acids

Question 15

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

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

Answer 16

Nitrogen fixing crops eg 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 the 2 categories of fertiliser and state the purpose of using fertiliser

Answer 17

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

Question 18

At a certain point using more fertiliser no loner 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 environmental issues caused by the use of fertilisers

Answer 19

Leaching - nitrates dissolve in rainwater and runoff into water sources
Eutrophication - water sources become putrid as a result of algal bloomW

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 decrease as population of aerobic sapriobionts 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