Organisms require nitrogen to manufacture proteins, nucleic acids, and other nitrogen-containing compounds.

Plants take up most of the nitrogen they require in the form of nitrate ions from the soil, using active transport to move nitrate ions into root hair cells.

Ammonification is the conversion of organic nitrogen-containing compounds into substances like ammonia by decomposers (saprobionts).

Nitrification is the conversion of ammonia or ammonium ions into nitrites (NO₂^-) and then into nitrates (NO₃^-) by nitrifying bacteria, in a two-stage oxidation process. 

Nitrate ions can then be absorbed by plants.

Nitrogen fixation is a process by which nitrogen gas (N₂) is converted into nitrogen-containing compounds like ammonia (NH₃).

1. Living process - bacteria in soil convert nitrogen gas into ammonia, and then convert this ammonia into amino acids
2. Non-living processes - industrial processes (like the Haber process) and lightning convert nitrogen gas into ammonia

1. Free-living nitrogen-fixing bacteria - live freely in soil, releasing nitrogenous compounds into soil when they die
2. Mutualistic nitrogen-fixing bacteria - live in nodules on plant roots, providing the plant with amino acids and gaining carbohydrates from the plant

Other bacteria can convert ammonia into different organic compounds to be absorbed by plants.

Denitrification is the process in which denitrifying bacteria convert soil nitrates (NO₃^-) into gaseous nitrogen (N₂).

Waterlogged soils are problematic because they are low in oxygen. This is the ideal environment for anaerobic denitrifying bacteria, which convert soil nitrates into gaseous nitrogen, reducing the availability of nitrates for plant uptake.

1. Detritivores - speed up decay by feeding on detritus, breaking it down into smaller pieces of organic material for decomposers to work on
2. Decomposers (saprobionts) - convert nitrogen-containing molecules in dead organisms, faeces, and urine into ammonia