which are the microorganisms that are useful in nitrogen cycle ?

1. Bacteria that nitrifies ammonium compounds in the soil (called "nitrifying bacteria"). Example: Nitrosomonas.

2. Bacteria that nitrifies nitrites (NO2-) in the soil (also called "nitrifying bacteria"). Example: Nitrobacter.

3. Bacteria that denitrifies nitrates (NO3-) in the soil (called "denitrifying bacteria"). Example: Pseudomonas denitrificans.

4. Bacteria that "fixes" nitrogen (called "nitrogen-fixing bacteria"). Examples: Rhizobium (which is symbiotic) and Azotobacter (which is free-living).

5. You also have bacteria that putrefies nitrogenous waste (like urea) and the protein in dead organisms. This type of bacteria is called putrefying bacteria.