Nutrient Cycles

Question 1

Nitrogen cycle - Nitrogen fixation

Answer 1

Gas in the atmosphere is turned into nitrogen-containing compounds. Biological nitrogen fixation is carried out by bacteria such as Rhizobium. They turn nitrogen into ammonia, which goes on to form ammonium ions in solution that can then be used by plants.

Rhizobium are found inside root nodules of leguminous plants (e.g. peas, beans and clover).

They form a mutualistic relationship with the plants - the plant provides them with carbohydrates.

Question 2

Nitrogen cycle - ammonification

Answer 2

When nitrogen compounds from dead organisms are turned into ammonia by saprobionts, which goes on to form ammonium ions.

Animal waste (urine and faeces) also contains nitrogen compounds. These are also turned into ammonia by saprobionts and go on to form ammonium ions.

Question 3

Nitrogen cycle - nitrification

Answer 3

When ammonium ions in the soil are changed into nitrogen compounds that can be used by plants (nitrates).

First nitrifying bacteria called Nitrosomonas change ammonium ions into nitrates.

Then other nitrifying bacteria called Nitrobacter change nitrites into nitrates.

Question 4

Nitrogen cycle - denitrification

Answer 4

When nitrates in the soil are converted into nitrogen gas by denitrifying bacteria - they used nitrates in the soil to carry out respiration and produce nitrogen gas.

This happens under anaerobic conditions, e.g. in waterlogged soils.