#nirtogen

The Nitrogen cycle shows how Nitrogen is recycled in an ecosystem. Living things need nitrogen to make proteins and nucleic acids (DNA & RNA) but they can't use the nitrogen in the air (even if it does make up 78% of the atmosphere, it's too unreactive for plants to use direct) - they need it in a different form. So instead, bacteria converts nitrogen in the air into nitrogen compounds. 

The nitrogen cycle shows how nitrogen is recycled, converted and hence transferred through an ecosystem between both abiotic, and biotic factors. There are four different stages of the nitrogen cycle (+ food chains where nitrogen is passed on when organisms are eaten):

NITROGEN FIXATION =  When nitrogen gas (from the atmosphere) is turned into ammonia by bacteria. The bacteria that does this is called Rhizobium. Rhizobium are found inside root nodules (growths on the roots) of leguminous plants (eg. peas, beans) and they form a mutualistic relationship with the plant - it provides nitrogen compounds for the plant and the plant provides it with carbohydrates in return.

AMMONIFICATION = When nitrogen compounds from dead organisms is converted to ammonium compounds by decomposers. Nitrogen is left in the waste (urine and faeces), and also the bodies of animals, so when they die, decomposers digest the material and turn the nitrogen compounds into ammonium compounds.

NITRIFICATION = When ammonium compounds in the soil are converted into nitrogen compounds which can be used by plants. step one, nitrifying bacteria called Nitrosomonas convert the ammonium compounds into nitrites. step two, other nitrifying bacteria called Nitrobacter change nitrites to nitrates. 

DENITRIFICATION = When nitrates in the soil are converted into nitrogen gas. This is done by denitrifying bacteria which use nitrates in the soil for respiration, and respire nitrogen.  This happens under anaerobic conditions (where there's no oxygen) eg. waterlogged soils.