10. The Nitrogen Cycle

Question 1

The microbial conversion of ammonium to nitrate is a two step process. Describe these steps, the microorganisms responsible and the conditions they occur in

Answer 1

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Question 2

Describe two non legume nitrogen fixing symbioses

Answer 2

• Actinorhizae
• Cyanobacteria

Question 3

Under what conditions does denitrification occur and what are the environmental impacts of this process?

Answer 3

Occurs when soil bacteria use nitrate for their respiration in the place of oxygen in the air.
- occurs most rapidly in warm, wet soils with an abundance of nitrate.
- only occurs when nitrate is present

Question 4

Detail the processes of ammonification and ammonium assimilation and the relationship between the two processes

Answer 4

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Question 5

How do the processes of nitrification and denitrification differ?

Answer 5

In nitrification nitrifying bacteria oxidise ammonia to nitrate and then it is further oxidised to nitrate, nitrate is thus made available for plants to absorb.

In denitrification the opposite of nitrification occurs, the microorganisms reduce nitrate back to nitrogen.

Question 6

Why is the process of nitrogen fixation energy expensive?

Answer 6

The 16 molecules of ATP represent the energy required for BNF reaction to take place. In biochemical terms 16 ATP represents a relatively large amount of plant energy. Thus the process of BNF is expensive to the plant in terms of energy usage