Nitrogen Cycle

A few organisms can convert nitrogen gas into compounds useful to other organisms in a process known as "nitrogen fixation". These organisms can be free living or in a relationship with certain "plants". 

Most plants obtain their nitrogen by absorbing "nitrate ions" from the soil through their "root hairs" by active transport. They then convert this to "protein", which is passed to animals when they eat the plants. 

On death, "decomposers" break down these orgainisms, releasing "ammonia", which can be oxidised to form nitrites by "nitrifiying" bacteria. Further oxidation by the same type of bacteria forms "nitrate ions". These ions may be converted back to atmospheric nitrogen by the activities of "denitrifying bacteria. 

This a term used to describe the production of ammonia from organic ammonia containing compounds such as urea, proteins and nucleic acids. Saprobiotic organisms feed on this material releasing ammonia which then forms ammonium ions in the soil. 

This is a reaction in which ammonium ions are oxidised into nitrate ions, the oxidation reaction also releases energy. It is carried out by free-living microorganisms in the soil called nitrifying bacteria. Ammonium ions are oxidised into nitrites and a further oxidation reaction converts nitrites into nitrates. 

As these nitrifying bacteria need oxygen farmers plough their soil to create many air spaces and raise productivity. 

This is the process by which nitrogen gas is converted into nitrogen-containing compounds.It can occur industrially and aslo when lightning passes through the atmosphere. 

Free-living nitrogen fixing bacteria: Reduces nitrogen to ammonia, which they then use to manufacture amino acids. When they die and decay nitrogen rich compounds are released. 

Mutualistic nitrogen-fixing bacteria: These bacteria live in root nodules on the roots of plants such as peas and beans. They obtain carbohydrates from the plant and the plant acquires amino acids from the bacteria. 

Denitrifying bacteria become present when soil becomes waterlogged and therefore short of oxygen. Soil nitrates are converted into gaseous nitrogen, reducing the availability of nitrogen-containing compounds for plants.