Fig. 2.1 Nitrogen is needed by living organisms to make proteins. Although the air is almost four-fifths nitrogen gas, plants cannot use this gas directly a...
Question 1 Report
Fig. 2.1
Nitrogen is needed by living organisms to make proteins. Although the air is almost four-fifths nitrogen gas, plants cannot use this gas directly and must absorb nitrogen as nitrate ions from the soil. The element is recycled between the air, the soil and living organisms by several groups of bacteria. Fig. 2.1 shows part of the nitrogen cycle, including nitrogen in the air, nitrates in the soil, and the bacteria involved.
(a) State why nitrate ions are needed by plants. [1]
(b) Name the type of bacteria that convert nitrogen gas from the air into nitrogen compounds in the soil. [1]
(c) Describe how nitrogen from a dead plant is converted into nitrate ions in the soil. [4]
(d) Explain how the action of denitrifying bacteria can reduce the growth of crops. [3]
(e) Describe the role of nitrifying bacteria in the nitrogen cycle. [2]
(f) Suggest three ways a farmer could increase the amount of nitrate in the soil. [3]
Answer Details
This question is about the nitrogen cycle and the groups of bacteria that recycle nitrogen between the air, soil and living organisms.
(a) Plants need nitrate ions to make amino acids, which are used to build proteins [1] (nitrogen is an essential element of every amino acid).
(b) Nitrogen gas from the air is converted into nitrogen compounds in the soil by nitrogen-fixing bacteria [1].
(c) Conversion of a dead plant into nitrate ions (4 marks):
- Decomposers (bacteria and fungi) break down the dead plant / its proteins [1],
- producing ammonia / ammonium compounds [1];
- nitrifying bacteria convert the ammonium into nitrites [1];
- and then the nitrites into nitrates [1].
This is the sequence protein to ammonium to nitrite to nitrate.
(d) Denitrifying bacteria reduce crop growth because they convert nitrates in the soil back into nitrogen gas [1]; this reduces the amount of nitrate in the soil [1]; so plants have less nitrate to make proteins and grow less [1] [3]. Denitrification is favoured in waterlogged, poorly aerated soils.
(e) The role of nitrifying bacteria is to convert ammonium compounds into nitrites and then into nitrates [1]; this process requires oxygen / occurs in well-aerated soil [1] [2]. This makes nitrogen available in the nitrate form that plant roots can absorb.
(f) Any three ways a farmer could raise soil nitrate (1 mark each, max 3): add nitrate fertiliser [1]; add manure / compost which decomposes to release nitrates [1]; and grow legumes / use crop rotation with root-nodule (nitrogen-fixing) bacteria [1].
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