Chemistry_Grade_10-12 (1).pdf

And at some later stage these new compounds must be

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needs to happen to the nitrogen gas to change it into a form that it can be used. And at some later stage, these new compounds must be converted back into nitrogen gas so that the amount of nitrogen in the atmosphere stays the same. This process of changing nitrogen into different forms is called the nitrogen cycle (figure 19.1). Definition: The nitrogen cycle The nitrogen cycle is a biogeochemical cycle that describes how nitrogen and nitrogen- containing compounds are changed in nature. Very broadly, the nitrogen cycle is made up of the following processes: Nitrogen fixation - The process of converting inert nitrogen gas into more useable nitrogen compounds such as ammonia. Nitrification - The conversion of ammonia into nitrites and then into nitrates, which can be absorbed and used by plants. Denitrification - The conversion of nitrates back into nitrogen gas in the atmosphere. We are going to look at each of these processes in more detail. 19.2 Nitrogen fixation Nitrogen fixation is needed to change gaseous nitrogen into forms such as ammonia that are more useful to living organisms. Some fixation occurs in lightning strikes and in industrial processes, 369
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19.2 CHAPTER 19. GLOBAL CYCLES: THE NITROGEN CYCLE - GRADE 10 Nitrites (NO ) 2 Ammonia (NH ) 3 Decomposers e.g. bacteria Lightning fixation Industrial fixation Nitrogen fixation by bacteria Animals obtain nitrates from plants Denitrification returns nitrogen to the atmosphere Plant consumption Atmosphere Soil Nitrogen in the Atmosphere Nitrification by nitrifying bacteria Nitrification Nitrates (NO ) 3 - - - Figure 19.1: A simplified diagram of the nitrogen cycle but most fixation is done by different types of bacteria living either in the soil or in parts of the plants. 1. Biological fixation Some bacteria are able to fix nitrogen. They use an enzyme called nitrogenase to combine gaseous nitrogen with hydrogen to form ammonia. The bacteria then use some of this ammonia to produce their own organic compounds, while what is left of the ammonia becomes available in the soil. Some of these bacteria are free-living, in other words they live in the soil. Others live in the root nodules of legumes (e.g. soy, peas and beans). Here they form a mutualistic relationship with the plant. The bacteria get carbohydrates (food) from the plant and, in exchange, produce ammonia which can be converted into nitrogen compounds that are essential for the survival of the plant. In nutrient-poor soils, planting lots of legumes can help to enrich the soil with nitrogen compounds. A simplified equation for biological nitrogen fixation is: N 2 + 8 H + + 8 e 2 NH 3 + H 2 Energy is used in the process, but this is not shown in the above equation. Another important source of ammonia in the soil is decomposition . When animals and plants die, the nitrogen compounds that were present in them are broken down and con- verted into ammonia. This process is carried out by decomposition bacteria and fungi in the soil.
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