5.4 - Nutrient cycles

Question 1

describe the first stage of the nitrogen cycle

Answer 1

ammonification: saprobionts break down organic matter e.g proteins, DNA, RNA, urea into ammonia using enzymes in two stages

Question 2

describe the second stage of the nitrogen cycle

Answer 2

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

Question 3

describe the third stage of the nitrogen cycle

Answer 3

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

Question 4

describe the fourth stage of the nitrogen cycle

Answer 4

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

Question 5

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

where is phosphate released from?

Answer 7

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

Question 8

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

Answer 8

increase surface area and act as a sponge holding water and minerals
so plants can better resist droughts and take up inorganic ions more easily

Question 9

what do natural and artificial fertilisers do?

Answer 9

replace nitrates and phosphates lost by harvesting plants and removing livestock

Question 10

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

Answer 10

pros of both:
- increase crop yields

cons of both:
- reduce biodiversity
- leaching
- eutrophication

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

Question 11

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

Answer 11

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 nitrate ions, which blocks light, plants photosynthesise less and die, saprobionts aerobically respire using up oxygen so less oxygen for other organisms to respire
eventually leads to ecosystem death

Question 12

Describe the phosphorus cycle

Answer 12

• plants absorb phosphate ions via AT
• animals digest plant material, absorb and incorporate phosphate ions into their bio molecules e.g DNA
• excretion: phosphate ions into oceans, rivers, soil
• excretion and decomposition: decay of bones, shells and excretion of some birds (guano), eventually erode, phosphate ions-> oceans, rivers, soil OR deposition to form phosphate ions in rocks
• oceans etc. to rocks: sedimentation
• phosphate 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)