Nutrient Cycling

The main characteristic features of nutrient cycles or biogeochemical cycles are:

All the subdivisions of the biosphere provide nutrients to the biotic components.
Materials involved in these cycles do not come from outside.
There is a specific rate of exchange of each biogenetic nutrient between biotic and abiotic components.
No amount is permanently lost from the biosphere.

There are three types of nutrient cycles that are observed in an ecosystem based on their reservoirs.

Gaseous Cycles: In these cycles, the main reservoirs of chemicals are the atmosphere and ocean. This type of cycle includes non-mineral chemicals. Nutrient cycles like the Nitrogen cycle, Carbon cycle, Hydrogen cycle and Oxygen cycle are examples of this type.
Sedimentary Cycles: In these cycles, the main reservoirs of chemicals are soil and rocks. These cycles include mineral chemicals. Phosphorus and Sulphur cycles are two examples of sedimentary cycles.
Hydrological Cycle: In this cycle, the reservoir may be in the atmosphere or in the soil. The water cycle is an example of this type.

NITROGEN FIXATION

Atmosphere is the greatest reservoir of nitrogen (approximate 78%). Atmospheric nitrogen is usually not used by the common plants. Only nitrogen fixing bacteria and blue green algae have the ability of fixing atmospheric nitrogen to convert it into organic nitrogen in their cell protoplasm.

Nitrogen fixing bacteria are :

• Clostridium : Anaerobic, free-living and soil borne
• Azatobactor : Aerobic, free-living, saprophytic and soil borne
• Rhizobium : Symbiotic, saprophytic and soil borne
• Klebsiella : Symbiotic, saprophytic and air borne
• Chlorobium : Anaerobic, autotrophic and free-living
• Rhodospirillum : Anaerobic, autotrophic and free-living
• Aerobactor : Anaerobic, heterotrophic and soil borne