Nitrogen makes up 78% of the air, however neither plants nor animals can take nitrogen directly from the air because nitrogen is so unreactive.
The nitrogen cycles shows the movement of nitrogen through the environment. Nitrogen is continually cycled through the air, soil and living things.
Plants take up nitrogen compounds as nitrates from the soil. Animals then eat these plants, thus getting their nitrogen.
The process of nitrogen in the atmosphere being turned into nitrogen in the soils is called fixing. There are different ways that nitrogen is 'fixed; including nitrogen fixing bacteria in the soil which turns nitrogen from the air into nitrates. Decomposers in the soils break down animal excretion and dead organisms, returning nitrogen back to the soil as ammonia. Lightning can cause chemical reactions in the atmosphere, resulting in nitrogen reacting with oxygen to produce nitrous oxide. Burning fossil fuels also adds nitrous oxide to the atmosphere, resulting in nitric acid, but this is of course not a natural way of nitrogen fixing. Blue-green algae in the ocean also can fix nitrogen. This then provides sources of nitrogen to aquatic animals, and the nitrogen goes around a similar cycle to what happens on land.
The Haber Process makes up approximately 30% of nitrogen fixing.
Denitrifying bacteria in the soil break down nitrates and return nitrogen back to the air. This reduces the fertility of the soil.
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You will learn about "Nitrogen Cycle" in this video. Nitrogen constitutes around 78% of the air. However, atmospheric nitrogen cannot be used directly by plants and animals. Nitrogen needs to be converted to nitrogenous compounds in order to be used by plants. This process of converting nitrogen into nitrogenous compounds is called nitrogen fixation.
It takes place by a certain kind of bacteria called rhizobium bacteria which live in the root nodules of leguminous plants. They convert atmospheric nitrogen to ammonia. Nitrifying bacteria then convert ammonia to nitrites and then nitrates. Some nitrogen fixation also occurs by lightning strikes.
The plants then take these nitrates from the soil. Plants use these nitrates for the synthesis of plant proteins and other compounds. Animals then eat these plants and get the proteins. Dead plants and animal excreta contain nitrogenous wastes. Overtime, these wastes decay and ammonia is released. Nitrifying bacteria once again convert the ammonia to nitrates. Some nitrates are again used by the plants and the cycle continues. Some nitrates are converted by denitrifying bacteria into atmospheric nitrogen. Thus, Nitrogen gas goes back to the atmosphere. As a result, the percentage of nitrogen in the air remains more or less constant. This cycle is known as Nitrogen Cycle
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