Ecology and Ecosystems: Question 4
Syllabus 19.3
A market gardener grows French beans, a type of legume, in one plot. Swellings called root nodules on the bean roots contain nitrogen-fixing bacteria. After harvesting the beans, the gardener digs the whole bean plant, roots, stems and leaves, back into the soil as green manure instead of removing it.
The following season, maize is grown in the same plot without adding any artificial fertiliser, and it grows well. At the end of the maize season, the old maize stalks are collected and burned on a bonfire, and the ash is spread back onto the soil.
(a) Name the process by which the bacteria in the bean's root nodules convert nitrogen gas from the air into a form the plant can use. [1]
(b) Explain how nitrogen locked up in the proteins of the buried bean plant eventually becomes available to the following season's maize plants as nitrate ions. Refer to the roles of decomposers and nitrifying bacteria. [4]
(c) Other soil bacteria convert some nitrate ions in the soil back into nitrogen gas, reducing the amount available to crops. Name this process. [1]
(d) Explain what happens to the carbon that was stored in the maize stalks when they are burned, naming the process involved. [2]
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Worked solution
Part (a): Nitrogen fixation
Bacteria living in the root nodules of the French bean (a legume) convert nitrogen gas from the air into nitrogen-containing compounds the plant can use. This process is called nitrogen fixation.
Part (b): From buried protein to nitrate ions
When the whole bean plant is dug into the soil, the nitrogen it contains is locked up in proteins within its dead tissues. Two groups of microorganisms act in sequence to release this nitrogen in a form the maize plants can absorb:
- Decomposers (bacteria and fungi) break down the proteins in the decaying bean plant, releasing ammonium ions into the soil.
- Nitrifying bacteria then convert these ammonium ions into nitrite ions, and then into nitrate ions, in the process of nitrification.
Once nitrate ions are present in the soil, the maize plants can absorb them through their roots and use them to build their own proteins, which is why the maize grows well even without artificial fertiliser.
Part (c): Denitrification
Some soil bacteria convert nitrate ions in the soil back into nitrogen gas, which then escapes into the atmosphere. This process is called denitrification, and it reduces the amount of nitrate available for the maize to absorb.
Part (d): Combustion and the carbon cycle
Burning the maize stalks is an example of combustion. During combustion, the carbon that had been stored in the organic compounds of the maize stalks combines with oxygen from the air and is released back into the atmosphere as carbon dioxide gas.
Final answers
- (a) Nitrogen fixation
- (b) Decomposers convert protein into ammonium ions; nitrifying bacteria then convert these into nitrite ions and nitrate ions (nitrification), which the maize roots absorb.
- (c) Denitrification
- (d) Combustion: the carbon in the maize stalks combines with oxygen and is released as carbon dioxide.