Nutrient cycles

Question 1

Describe ammonification

Answer 1

1. Saprobiotic bacteria secrete enzymes onto dead organisms, faeces, and urine
2. Hydrolyse nitrogen compounds (e.g. proteins) by extracellular digestion and absorb them
3. NH4+ released into soil

Question 2

Describe nitrification

Answer 2

1. Nitrifying bacteria oxidise NH4+ to nitrites and nitrites to NO3-
2. Plants absorb NO3- via active transport and use them to make proteins etc.

Question 3

Describe denitrification

Answer 3

Denitrifying bacteria reduce NO3- to N2 in anaerobic conditions e.g. waterlogged soil

Question 4

Describe nitrogen fixation

Answer 4

N2 reduced to NH4+ by nitrogen fixing bacteria in the soil and root nodules of legumes

Question 5

Describe the phosphorus cycle (first 4 steps)

Answer 5

1. Phosphate ions in rocks released through erosion and dissolve into lakes and soil
2. Plants absorb phosphates into roots via active transport
3. Mycorrhizae increase surface area for absorption as phosphate concentration is low
4. Plants use phosphates to make ATP, DNA etc.

Question 6

Describe the phosphorus cycle (last 3 steps)

Answer 6

5. Phosphates transferred to consumers via consumption, digestion, and absorption and assimilate into DNA, phospholipids etc.
6. When animals and plants die, decomposed by saprobiotic bacteria/fungi which digest molecules via extracellular digestion and return phosphates to soil (also digest faeces + urine, guano excreted which is made up of phosphates)
7. Phosphate in soil returns to rocks when sedimentary rocks formed via deposition

Question 7

When and why are fertilisers added to soil?

Answer 7

Add NO3- and phosphates to soil to replace those lost after harvesting crops or removing livestock

Question 8

How does harvesting crops + removing livestock cause NO3-/phosphates to be lost?

Answer 8

Harvesting crops: crops don’t decompose to replace NO3-/phosphates they absorbed

Removing livestock: livestock no longer excrete/egest to replace NO3-/phosphates they consumed from plants eaten

Question 9

What are natural and artificial fertilisers?

Answer 9

Natural: compost/manure
Artificial: chemical compounds added to soil

Question 10

Why do natural fertilisers take longer to provide plants with NO3-?

Answer 10

Need saprobiotic bacteria to decompose proteins into NH4+ first, whilst artificial fertilisers add NO3- and NH4+ straight to the soil

Question 11

Why does the addition of high concentrations of fertiliser decrease productivity of crops?

Answer 11

Lowers water potential of soil so plant root cells have higher water potential than soil so water moves out of plant root cells via osmosis

Question 12

Describe leaching

Answer 12

Removal of NO3-/phosphates from soil by rainwater into rivers and lakes

Question 13

Describe eutrophication and how it can lead to the death of fish (5 steps)

Answer 13

1. NO3-/phosphates enter river from the leaching of fertilisers
2. NO3-/phosphates increase growth of algae on the surface of river (algal bloom)
3. Reduces light available to plants in water so these plants stop photosynthesising and die
4. Saprobiotic bacteria reproduce rapidy and decompose dead plant matter, using O2 for aerobic respiration
5. Reduces O2 available for other organisms in river, such as fish, so they can’t respire and die