5.4 Nutrient cycles

Question 1

What is the role of saprobionts in recycling chemical elements?

Answer 1

• Feed on remains of dead plants / animals and their waste products e.g. faeces, urea, and break down the organic molecules
• By secreting enzymes for extracellular digestion
  Saprobionts absorb soluble needed nutrients

Question 2

What is the role of mycorrhizae?

Answer 2

• Symbiotic relationship between fungi and roots of plants = mycorrhizae
• Fungi act as an extension of the plant roots (made of thin strands called hyphae): increase surface area of root system increase rate of absorption of water / nutrients
• Mutualistic relationship – plant also provides fungi with carbohydrates

Question 3

What are the main stages of the nitrogen cycle?

Answer 3

Ammonification
Nitrification
Nitrogen fixation
Denitrification

Question 4

What is ammonification?

Answer 4

Nitrogen-containing compounds e.g. proteins from dead organisms / animal waste broken down.

Converted to ammonia which goes on to form ammonium ions (NH4+) in the soil by saprobionts.
- By secreting enzymes for extracellular digestion.

Question 4

What is nitrification?

Answer 4

Ammonium ions in the soil converts into nitrites then nitrates (nitrogen-containing compounds)
A two-stage oxidation reaction
- By nitrifying bacteria
Bacteria need oxygen to carry out conversions 10
Nitrates (nutrients) can be absorbed by plant root hair cells by active transport

Question 5

Why do farmers aerate their soil?

Answer 5

Farmers aerate their soil increase O2 allowing number of nitrifying bacteria to increase and denitrifying bacteria to decrease maximise nitrogen availability

Question 6

What is denitrification?

Answer 6

Nitrates in the soil converts into nitrogen gas
By denitrifying bacteria (anaerobically respire)
- When low oxygen conc. in soil i.e. waterlogged
- Because more anaerobic denitrifying bacteria (and less aerobic nitrifying and nitrogen fixing bacteria)
(Reduces availability of nitrogen compounds for plants)

Question 7

What is nitrogen fixation?

Answer 7

Nitrogen gas (N2) converted (reduction) to nitrogen containing compounds e.g. ammonia
By nitrogen-fixing bacteria
- Can be ‘free living’ in the soil
- Or ‘mutualistic’ (live in nodules on roots of plants e.g. legumes; acquire carbohydrates from plant while the plant acquires amino acids from bacteria)

Question 8

Why is the nitrogen cycle important?

Answer 8

• Nitrogen gas (N2) is unreactive and not easily converted into other compounds
• Most plants can only take up nitrogen (by active transport in roots) in the form of nitrate
• Used by plants / animals to make proteins / nucleic acids (assimilated) growth

Question 9

What are the stages of the phosphorous cycle?

Answer 9

1. The physical processes of the climate (weathering) release phosphate ions from rocks
2. Phosphate ions pass to the environment
3. Producers absorb phosphate ions from the environment
4. When producers and consumers die, decomposition as the result of the feeding activities of fungi and bacteria releases phosphate ions into the environment
5. Therefore, phosphate ions are recycled and available for:
   - Absorption by new generations of producers
   - Transfer from producers to consumers

Question 10

Why do you need fertilisers?

Answer 10

Replaces nutrients (nitrates and phosphates) lost from an ecosystem’s nutrient cycle when…
* Crops are harvested
* Livestock (animals) removed

Nutrients removed from soil and incorporated into their biomass can’t be released back into the soil through decomposition by saprobionts

Question 11

How do fertilisers improve the efficiency of energy transfer?

Answer 11

• Nutrient could no longer be a limiting factor
• Increase productivity of agricultural land

Question 12

What are artificial fertilisers?

Answer 12

• Inorganic
• Contain pure chemicals e.g. ammonium nitrate as powders / pellets
• Inorganic substances more water soluble so larger quantities washed away, impacting the environment

Question 13

What are natural fertilisers?

Answer 13

• Organic
  Eg/ manure, compost, sewage
• Cheaper/free but exact nutrients cannot controlled

Question 14

What are the environmental issues of fertilisers?

Answer 14

Leaching of nutrients
- Rain / irrigation systems wash water-soluble compounds out of soil into waterways e.g. rivers
- Worse when more fertiliser added to field than used (excess)

Leaching leads to eutrophication

Leaching less likely with natural fertilisers - organic molecules less soluble in water

Can also reduce species diversity