Carbon and Nitrogen Cycles

Question 1

What are the three main ways in which CO₂ is released back into the atmosphere?

Answer 1

1. Respiration - when living plants and animals respire they relese CO₂ back into the air.
2. Decomposition - when plants and animals die, the decomposers (bacteria and fungi**) release CO₂ back into the air by respiration.
3. Combustion - some useful plant and animals products (e.g. wood and fossil fuels) are burned, which releases CO₂ into the air.

Question 2

How is CO₂ taken out of the air?

Answer 2

1. Photosynthesis - plants use the carbon in CO₂ to make carbohydrates, fats and proteins.

Question 3

What parts of the nitrogen cycle does each label refer to?

Answer 3

a) Decomposition
b) Nitrogen fixation
c) Nitrification
d) Assimilation
e) Denitrification

Question 4

What are the roles of nitrogen-fixing bacteria, decomposers, nitrifying bacteria and denitrifying bacteria?

Answer 4

• Nitrogen-fixing bacteria turn atmospheric N₂ into nitrogen compounds that can be used by plants.
• Decomposers break down proteins (in rotting plants and animals) and urea and turn them into ammonia (a nitrogen compound).
• Nitrifying bacteria turn ammonia (in decaying matter) into nitrates.
• Denitrifying bacteria turn nitrates back into N₂ gas.

Question 5

What is nitrogen fixation?

Answer 5

The process of turning N₂ from the air into nitrogen compounds in the soil which can be used by plants.