This is where nitrogen gas from the atmosphere is turned into ammonia by nitrogen-fixing bacteria (called Rhizobium). The ammonia can then be utilised by plants.
Rhizobium are found inside root nodules of leguminous plants e.g peas.
The form a symbiotic relationship with the plants, they provide the plant with nitrogen compounds and the plant provides them with carbohydrates.
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Ammonification
This is where nitrogen compounds from dead organisms (detritus) are turned into ammonium compounds by decomposers.
Animal waste (urine and faeces) also contain nitrogen compounds. These are also turned into ammonium compounds by decomposers.
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Nitrification
This is where ammonium compounds in the soil are changed into nitrogen compounds that can be used by plants.
First, nitrifying bacteria (Nitrosomonas) change ammonium compounds into nitrites.
Then, other nitrifying bacteria (Nitrobacter) change nitrites to nitrates.
Ammonium Compounds ---> Nitrites ---> Nitrates
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Denitrification
This is where nitrates in the soil are converted back into atmospheric nitrogen by denitrifying bacteria - they use nitrates in the soil to carry out respiration and produce nitrogen gas.
This happens under anaerobic conditions (NO OXYGEN) e.g in waterlogged soils.
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