(a) Fig. 1.1 shows the nitrogen cycle in an agricultural system. Fig. 1.1 (i) Name the type of bacteria found in the root nodules of legumes that convert at...
Question 1 Report
(a) Fig. 1.1 shows the nitrogen cycle in an agricultural system.
Fig. 1.1
(i) Name the type of bacteria found in the root nodules of legumes that convert atmospheric nitrogen into a form usable by plants. [1]
(ii) State the name of the process carried out by these bacteria. [1]
(iii) Name the process by which nitrates in the soil are converted back to atmospheric nitrogen. [1]
(b) Explain how growing a legume crop such as groundnuts and then ploughing it into the soil benefits the next crop planted in that field. [3]
(c) State two ways in which nitrogen can be lost from agricultural soils. [2]
[Total: 8]
Answer Details
Labelled answer diagram:
(a)(i) The bacteria found in the root nodules of legumes that convert atmospheric nitrogen into a plant-usable form are Rhizobium. [1]
(a)(ii) The process is called nitrogen fixation (biological nitrogen fixation). [1] Rhizobium bacteria use the enzyme nitrogenase to break the strong triple bond in atmospheric nitrogen gas (\(\text{N}_2\)) and convert it into ammonia (\(\text{NH}_3\)), which is then available to the host plant.
(a)(iii) The process by which nitrates in the soil are converted back to atmospheric nitrogen is denitrification. [1] This is carried out by anaerobic bacteria in waterlogged or poorly aerated soils, and it represents a loss of available nitrogen from the soil.
(b) How growing and ploughing in a legume crop benefits the next crop: [3]
- While growing, the legume fixes atmospheric nitrogen through its Rhizobium bacteria, enriching both the plant tissue and the surrounding soil with nitrogen. [1]
- When the crop is ploughed in as green manure, the nitrogen-rich plant material decomposes in the soil. [1]
- During decomposition, nitrogen compounds are released and converted to nitrates by nitrifying bacteria (mineralisation and nitrification), making this nitrogen available for uptake by the roots of the next crop planted in that field. [1]
(c) Two ways nitrogen can be lost from agricultural soils (any two): [2]
- Leaching: soluble nitrate ions are washed downward through the soil profile by heavy rainfall or excessive irrigation, moving beyond the reach of plant roots. [1]
- Removal in harvested crops: every crop harvested takes nitrogen incorporated into its proteins and other organic molecules out of the field permanently. [1]
Other acceptable answers: denitrification by anaerobic bacteria in waterlogged soils; erosion of nitrogen-rich topsoil by wind or water; volatilisation of ammonia from surface-applied manure or urea fertiliser.
Download The App On Google Playstore
Everything you need to excel in your exams
