Theory question 107
Nutrient Cycling(For candidates in Nigeria only)
(a) Describe the carbon cycle.
(b) State the functions of: (i) nitrogen fixing bacteria; (ii) nitrifying bacteria; (iii) denitrifying bacteria; in nature.
Show the model answer
(a) Carbon dioxide in the air is taken in by green plants and made into carbohydrates by photosynthesis; animals obtain carbon compounds by eating plants or other animals; plants and animals release CO2 back to the air by respiration; when they die, decomposers (bacteria and fungi) break them down, releasing CO2; some remains form fossil fuels (coal, oil), which release CO2 when burnt; volcanic eruption also releases CO2; carbon dioxide dissolves in water and forms carbonates (limestone).
(b)(i) Nitrogen-fixing bacteria (Rhizobium, Azotobacter) convert atmospheric nitrogen into nitrogen compounds that plants can use.
(ii) Nitrifying bacteria convert ammonium compounds to nitrites (Nitrosomonas) and nitrites to nitrates (Nitrobacter), which plants absorb.
(iii) Denitrifying bacteria (e.g. Pseudomonas) convert nitrates back to free nitrogen gas, returning it to the atmosphere.
A labelled cycle diagram can support the description in (a).