3.5.4 Nutrient cycles (A-level only

Question 1

Outline the role of fungi and bacteria (saprobionts) in nutrient recycling.

Answer 1

• In natural ecosystems (hasn’t been changed by human activity), nutrients recycled through food webs.
• Some microorganisms are saprobionts (a type of decomposer) - feed on remains of dead plants and animals and on their waste products (faeces / urine), breaking them down —> allows important chemical elements in the remains to be recycled.
• Saprobionts excrete enzymes and digest their food externally and then absorb the nutrients they need - extracellular digestion. Organic molecules —> inorganic ions.

Question 2

Comment on the symbiotic relationships that some fungi form with plant roots.

Answer 2

= These relationships are called Mycorrhizae.

• Fungi made up of long, thin strands called hyphae, which connect to the plant’s roots.
• The hyphae greatly increase the surface area of the plant’s root system, helping the plant to absorb usually scarce ions from the soil - phosphates.
• Hyphae also increase water uptake by plant.
• In turn, the fungi obtain organic compounds, such as glucose from the plant.

Question 3

Define saprobiotic nutrition.

Answer 3

= The process of obtaining nutrients from dead organic matter using extracellular digestion.

Question 4

Comment on the necessity of the Nitrogen Cycle.

Answer 4

• Plants and animals need nitrogen for proteins and nucleic acids.
• They need bacteria to convert nitrogen in air to N-containing compounds.

Question 5

Draw out the Nitrogen Cycle.

Answer 5

Atmospheric N —> N-compounds in plants (nitrogen fixation).
—> N compounds in animals (feeding).
—> to NH3 (ammonification by saprobionts).
—> NH4+ -> Nitrites -> Nitrates (nitrification).
—> Atmospheric nitrogen (denitrification)
OR
—> nitrogen compounds in plants again.

Question 6

Outline the process of Nitrogen Fixation (NC).

Answer 6

1. N2 gas in atmosphere converted to N-containing compounds by bacteria.
2. Bacteria found inside root nodules of leguminous plants.
3. Bacteria there form a mutualistic relationship with the plants - they provide the plant with nitrogen compounds and the the plant provides them with carbohydrates.

Question 7

Outline the process of Ammonification (NC).

Answer 7

1. Nitrogen compounds from dead organisms are turned into NH3 by saprobionts, which goes on to form NH4+ ions.
2. Animal urine and faeces also contain N compounds —> also turned into NH3 by saprobionts, which goes on to form NH4+ ions.

Question 8

Outline the process of Nitrification (NC).

Answer 8

1. NH4+ ions in the soil are changed into N compounds that can be used by plants (-> nitrites -> nitrates).
2. Nitrifying bacteria called Nitrosomonas change NH4+ ions into nitrites.
3. Other nitrifying bacteria called Nitrobacter then turn nitrites into nitrates.

Question 9

Outline the process of Denitrification (NC).

Answer 9

1. Nitrates in soil converted into N2 gas by denitrifying bacteria - they use nitrates in the soil to carry out respiration and produce N2 gas.
2. Happens under anaerobic conditions, such as those present in waterlogged soils.

Question 10

Suggest other ways of getting Nitrogen into an ecosystem (NC).

Answer 10

1. Lightning - fixes atmospheric nitrogen.

2. Artificial fertilisers - produced from atmospheric nitrogen on an industrial scale in the Haber process.

Question 11

Draw out the phosphorus cycle (PC).

Answer 11

soil  seas / lakes / rivers. 
-> algae and other primary producers. 
-> fish (feeding) -> birds (feeding).
-> guano (excretion).
-> soil -> plants -> animals (feeding). 
-> decaying organisms -> soil.
AND
-> faeces and urine -> soil.

Question 12

Outline the processes of the phosphorus cycle (PC).

Answer 12

1. Phosphate ions in rocks released into soil by weathering.
2. Phosphate ions absorbed through roots - mycorrhizae greatly increase the rate at which phosphorus can be assimilated.
3. Phosphate ions transferred through the food chain as animals eat the plants and are in turn eaten by other animals.
4. Phosphate ions lost from animals in waste products.
5. When plants and animals die, saprobionts involved in breaking down the organic compounds, releasing phosphate ions into soil for assimilation by plants. These microorganisms release phosphate ions from urine and faeces.
6. Weathering of rocks also releases phosphate ions into seas / lakes / rivers —> taken up by aquatic producers such as algae and passed along food chain to birds.
7. Guano (sea bird waste) contains a high [phosphate ion] —> guano returns a significant amount of phosphate ions to soils (particularly in coastal areas) => guano is therefore often used as a natural fertiliser.

Question 13

How are nutrients lost when crops are harvested?

Answer 13

1. Crops take in minerals from the soil as they grow and use them to build their tissues.
2. When crops are harvested - they can no longer die and decompose in the field in which they were grown —> mineral ions they contain not returned to soil by decomposers in N / P Cycles.
3. Phosphates and Nitrates are also lost from the system when animals or animal products are removed from the land —> when animals are taken for slaughter, the nutrients they took up when eating are not replaced through their remains or waste products.

Question 14

What is the role of fertilisers? Outline the differences between artifical and natural fertilisers.

Answer 14

= Added to replace lost nutrients —> more energy from the ecosystem can be used for growth, increasing the efficiency of energy transfer.
Artificial fertilisers are inorganic —> pure chemicals (ammonium nitrate, for example) as powders or pellets.
—> Inorganic ions present in chemical fertilisers are soluble so excess minerals more likely to be leached into waterways.
Natural fertilisers are organic matter —> include manure, composted vegetables, crop residues and sewage sludge.
—> Nitrogen and Phosphorus are still contained in organic molecules that need to be decomposed by microorganisms before they can be absorbed by plants —> release into soil is more controlled —> leaching is less likely.

Question 15

What are the environmental issues related to fertiliser use?

Answer 15

1. Excess fertiliser in soils can lead to leaching into the waterways.
2. This can lead to eutrophication.
3. Leaching is more likely if fertiliser applied just before heavy rainfall.
4. Leaching of phosphates is less likely as phosphates are less water-soluble than nitrates.
5. Using fertilisers also changes the balance of nutrients in the soil —> an excess of a particular nutrient can cause crops and other plants to die.

Question 16

Define leaching.

Answer 16

= When water-soluble compounds in soil are washed away by rain or irrigation systems —> often washed in nearby ponds and rivers.
=> Can lead to eutrophication

Question 17

Define Eutrophication.

Answer 17

= The process by which nutrient concentrations increase in bodies of water.

Question 18

Outline the process of Eutrophication.

Answer 18

1. Naturally low [nitrate] in lakes and rivers - limiting factor.
2. As [nitrate] increases as a result of leaching, no longer a limiting factor for growth of plants and algae —> populations grow.
3. Algae grow mostly at the surface, the upper layers of water become densely populated with algae => algal bloom forms.
4. Preventing light from reaching lower depths.
5. Light then becomes limiting factor for growth of plants algae at lower depths and so they eventually die.
6. Lack of dead plants and algae no longer a limiting factor for saprobiontic bacteria so their populations grow.
7. Saprobiontic bacteria require oxygen for their respiration, creating an increased demand for oxygen.
8. [O2] decreases and nitrates released from decaying organisms.
9. Oxygen then becomes the limiting factor for populations of aerobic organisms such as fish —> ultimately die as oxygen used up altogether.
10. Without aerobic organisms, there is less competition for anaerobic organisms whose populations now increase.
11. Anaerobic organisms further decompose dead material, releasing more nitrates and some toxic wastes such as hydrogen sulphide, making the water putrid.
    NB => organic manures, animal slurry, human sewage, ploughing old grassland and natural leaching can all contribute to eutrophication, but the leaching of artificial fertilisers is the main cause.