The Nitrogen Cycle

Question 1

What are the four main stages to the nitrogen cycle?

Answer 1

Ammonification
Nitrification
Nitrogen fixation
Denitrification

Question 2

What is ammonification?

Answer 2

The production of ammonia from organic ammonium containing compounds.
Such compounds include urea, proteins, nucleic acids and vitamins.
Saprobiotic microorganisms, mainly fungi and bacteria, feed on these materials, releasing ammonia, which then forms ammonium ions in the soil.

Question 3

What is nitrogen fixation?

Answer 3

This is a process by which nitrogen gas is converted into nitrogen containing compounds.
Industrially
Lightning
Carried out by microorganisms.

Question 4

What are free living nitrogen fixing bacteria?

Answer 4

These bacteria reduce gaseous nitrogen to ammonia, which they then use to manufacture amino acids.
Nitrogen rich compounds are released from them when they die and decay.

Question 5

What are mutualistic nitrogen fixing bacteria?

Answer 5

These bacteria live in nodules on the roots of plants such as peas and beans. They obtain carbohydrates from the plant and the plant acquires amino acids from the bacteria.

Question 6

What is denitrification?

Answer 6

When soils become waterlogged- therefore short of oxygen the type of microorganism present changes- more anaerobic denitrifying bacteria.
These convert soil nitrates into gaseous nitrogen.
This reduces the availability of nitrogen containing compounds for plants.

Question 7

4 Steps to the nitrogen cycle

Answer 7

1. Nitrogen gas is converted to nitrate compounds by nitrogen-fixing bacteria in soil or root nodules. Lightning also converts nitrogen gas to nitrate compounds. The Haber process converts nitrogen gas into ammonia used in fertilizers. Ammonia is converted to nitrates by nitrifying bacteria in the soil.
2. Plants absorb nitrates from the soil and use these to build up proteins. The plant may be eaten by an animal, and its biomass used to produce animal protein.
3. (Deamination) Urea and egested material is broken down by decomposers. This results in nitrogen being returned to the soil as ammonia.
4. Decomposers also break down the bodies of dead organisms resulting in nitrogen being returned to the soil as ammonia.

Question 8

The processes which naturally form part of the nitrogen cycle can make nitrogen contained in urine and faeces available to crop plants. Describe how these processes occur.

Answer 8

Organic compounds of nitrogen.
Converted to ammonia by saprophytes.
Nitrites to nitrates by nitrifying bacteria.
Uptake by roots.

Question 9

Explain the advantage in this system of growing leguminous plants such as groundnuts or beans.

Answer 9

Nitrogen fixing bacteria
in root nodules convert nitrogen to ammonium and converted to nitrates; less need for fertiliser;

Question 10

Give one organic compound in which the nitrogen will be found in the heather plants.

Answer 10

Proteins/nucleic acids/amino acids/ATP/nucleotide

Question 11

Give one ion in which the nitrogen will be found in the soil.

Answer 11

Ammonium/nitrate/nitrite

Question 12

Explain how bacteria make nitrogen present in the leaf litter available to the heather plants.

Answer 12

Organic material/proteins to ammonia by saprobiotic bacteria.
Ammonia to nitrite.
Nitrite to nitrate by nitrifying bacteria.

Question 13

By what process are ammonium ions converted into nitrite ions?

Answer 13

Nitrification

Question 14

By what process are nitrate ions converted into nitrogen?

Answer 14

Denitrification

Question 15

Leguminous crop plants have nitrogen-fixing bacteria in nodules on their roots.
On soils with a low concentration of nitrate ions, leguminous crops often grow better than other types of crop. Explain why.

Answer 15

Nitrogen to ammonia

Produce protein/amino acids

Question 16

Describe the role of microorganisms in producing nitrates from the remains of dead organisms.

Answer 16

Saprobiotic bacteria break down remains  into
ammonia
Ammonia into nitrite and then
into nitrate
By nitrifying bacteria (nitrification)

Question 17

Nitrate from fertiliser applied to crops may enter ponds and lakes. Explain how nitrate may cause the death of fish in fresh water.

Answer 17

1. Growth of algae/surface
plants/algal bloom blocks light;
2. Reduced/no photosynthesis so
(submerged) plants die;
3. Saprobiotic
(microorganisms/bacteria);
4. Aerobically respire / use oxygen
in respiration;
5. Less oxygen for fish to respire /
aerobic organisms die;

Question 18

Describe how the action of microorganisms in the soil produces a source of nitrates for crop plants.

Answer 18

1. Protein into ammonia.
2. By saprophytes
3. Ammonia into
   nitrite.
4. Nitrite into nitrate
5. By nitrifying bacteria
6. Nitrogen to ammonia
7. By nitrogen-fixing bacteria in soil

Question 19

What process involves nitrification?

Answer 19

Ammonium ions converted into nitrite ions.