Non Symbiotic Nitrogen Fixers - Nutrient Cycles

There are certain groups of free living bacteria both aerobic and anaerobic and blue green algae that fix nitrogen. Aerobic nitrogen fixing bacteria such as Azotobacter and anaerobic form Clostridium are widely distributed in soils as well as in fresh and marine waters. In fact accumulating evidence indicates that many soil and water bacteria are capable of nitrogen fixation and because they occur in abundance the total amount of nitrogen fixed is considerable. The N₂ fixed in the soil and root nodules is used by the plants to form numerous nitrogenous compounds mainly proteins which then enter the food chain. Nitrogen is returned to the soil in the form of organic compounds through manure, dead plants, and animals and micro-organism. But most of this nitrogen is insoluble and not immediately available for plant use. The organic nitrogenous compounds have to be changed to inorganic compounds to be used by plants. This is done by two processes - ammonification and nitrification.