Atmospheric nitrogen cannot be absorbed directly by plants (to make proteins).
Plants absorb nitrogen from the soil in the form of ammonium, nitrites or nitrates ions. The process of converting atmospheric nitrogen into ammonium compounds is called nitrogen fixation.
Nitrogen fixation is carried out by nitrogen – fixing bacteria :
i) Nostoc sp. (cyanobacteria) & Azotobacter sp. that live freely in soil
ii) Rhizobium sp. that live in root nodule of leguminous plants.
Both use atmospheric nitrogen to make ammonium compounds –(NH3– ammonia & NH4+ - ammonium)
Nitrifying bacteria convert ammonium compounds into nitrites & nitrates.
The process is called nitrification.
i) Ammonia is oxidised into nitrites (NO2-) by Nitrobacter sp.
ii) Nitrites are oxidised into nitrates (NO3-) by Nitrosomonas sp.
Nitrates in the soil are absorbed by plants & converted into plant proteins.
When animals eat plants, the proteins are transferred into the body of the animals & become animal protein.
Dead plants, animals & waste materials are decomposed by decaying bacteria & fungi/decomposers into ammonium compounds.
The cycle is balanced by returning of nitrogen to the atmosphere by denitrifying bacteria.
The process is called denitrification.
Denitrifying bacteria break down nitrates into gaseous nitrogen & oxygen.
Oxygen – used by bacteria; nitrogen – returned to the atmosphere.