5.4 - Nutrient cycles

Question 1

Describe the 1st stage of the nitrogen cycle

Answer 1

ammonification:
- saprobionts break down organic matter e.g proteins, DNA, RNA, urea into NH3 using enzymes

Question 2

Describe the 2nd stage of the nitrogen cycle

Answer 2

nitrification:
- nitrifying bacteria convert NH3 to NO2- ions in oxidation reaction
- further oxidation: NO2- to NO3- ions
- most plants can take these up through their roots

Question 3

Describe the 3rd stage of the nitrogen cycle

Answer 3

denitrification:
- NO3- ions converted to N2 (g) by denitrifying bacteria
- can be prevented by keeping soil well drained and aerated (as occurs in anaerobic conditions)

Question 4

Describe the 4th stage of the nitrogen cycle

Answer 4

nitrogen fixation:
- N2 (g) fixed into other compounds by nitrogen fixing bacteria by reducing to NH3 which dissolves to NH4+ ions

Question 5

Describe nitrogen fixing bacteria

Answer 5

live in root nodules of leguminous plants in mutualistic relationship

Question 6

What is a mutualistic relationship?

Answer 6

when relationship is beneficial to both organisms

Question 7

How do mycorrhizae facilitate plant uptake of water and inorganic ions? (phosphorus cycle)

Answer 7

• increase SA + act as a sponge holding H2O and minerals
• so plants can better resist droughts and take up inorganic ions more easily

Question 8

What do natural and artificial fertilisers do?

Answer 8

replace nitrates and phosphates lost by harvesting plants and removing livestock

Question 9

What are the pros and cons of natural and artificial fertilisers?

Answer 9

pros of both:
- increase crop yields

cons of both:
- reduce biodiversity
- leaching
- eutrophication

cons of artificial:
- more expensive than natural as natural readily available

cons of natural:
- nutrients present not very conc. so relatively large amounts needed

Question 10

Describe the process of leaching and how it can lead to eutrophication

Answer 10

• leaching = mineral ions dissolve in rainwater and carried from soil to end up in rivers/lakes
• eutrophication = rapid growth of algae on water surface due to more NO3- ions, which blocks light
• less light so plants photosynthesise less and die
• saprobionts respire aerobically using up O2 so less for other organisms to respire
• eventually leads to ecosystem death

Question 11

Where is phosphate released from?

Answer 11

• sedimentary rocks due to weathering
• decay of bones and shells
• some birds’ excretion

Question 12

Describe the phosphorus cycle

Answer 12

• plants absorb PO4 3- ions via AT
• animals digest plant material, absorb and incorporate PO4 3- ions into their bio molecules e.g DNA
• excretion: into oceans, rivers, soil
• excretion and decomposition: decay of bones, shells and excretion of some birds (guano), eventually erode, -> oceans, rivers, soil OR deposition to form PO4 3- ions in rocks
• oceans to rocks: sedimentation
• PO4 3- ions released from sedimentary rocks by erosion into oceans etc, and fertilisers cause runoff into oceans
• phosphates (also nitrates) lost by harvesting plants and removing livestock, replaced by fertilisers
• also mycorrhizae (on other card)