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Crop production - nitrogen fixation - Answers

1. Draw and label a graph showing the amounts of nitrogen fixed for each of the species presented in the table.
   
   
2. From the table and graph that you have drawn, determine which species fixed the greatest amount of nitrogen?
   Lupins resulted in the greatest amount of nitrogen fixed.
   
   
3. Compare the amount of nitrogen fixed for non-irrigated Faba beans with partially irrigated Faba beans.
   Partially irrigated faba beans fixed 36kg nitrogen/ha more than dryland faba beans.
   
   
4. Can you suggest any reason for the answer to question 3?
   The extra water provided by irrigating the faba beans would result in a higher rate of photosynthesis than the dryland faba beans. This would mean that both dry matter and root growth would be increased in the partially irrigated faba beans resulting in increased levels of nitrogen fixation.
   
   
5. Describe the process that occurs in legumes resulting in the fixation of nitrogen.
   Symbiotic nitrogen fixation is brought about by species of Rhizobium bacteria. The rhizobia infect or invade the roots of legumes and produce nodules containing millions of bacterial cells. An exchange of nutrients occurs between the rhizobia and the legume. The rhizobia provide the legume with nitrogen compounds that are in a form that can be used readily by the legume, while the rhizobia obtains nutrients, in particular carbon compounds from the legume.
   
   Rhizobia are able to convert the nitrogen from the air into nitrogenous compounds that can be converted into amino acids by plants. This is known as nitrogen fixation.
   
   
6. Describe why legumes are a useful plant to be used in crop rotations.
   Legumes are a useful plant to be used in crop rotations for two reasons. Firstly, because of nitrogen fixation legumes plants contain higher levels of nitrogen than non-leguminous plants. Therefore if the crop residues are ploughed back into the soil after harvest of the seeds, nitrogen is added to the soil, being made available for future crops in the rotation. Secondly, if legumes grow well, then excess nitrogen is released into the soil. These nitrogenous compounds are then made available for future crop growth.

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