Which two distinct microbial processes are responsible for the release of fixed nitrogen as dinitrogen gas to the atmosphere?

Atmosphere consists of 78% percent of nitrogen; it is an essential element for the growth and development of plants and animals. Nitrogen is present in nucleic acids, proteins and amino acids.

Atmospheric nitrogen is triple bonded which does not dissociate easily, therefore it cannot be siphoned directly by the plants or animals. Thus, the molecular nitrogen is converted into ammonia/ammonium for the proper absorption by the plant. The conversion of dinitrogen into ammonium/ammonia then into nitrates and nitrites and after decay back to dinitrogen is called nitrogen cycle.  

The stages of nitrogen cycle are,

1. Nitrogen fixation: Dinitrogen in the soil is converted into ammonia (NH3+) by Rhizobium present in the nodes of leguminous plants.
2. Nitrification: NH₄⁺/NH₃⁺ converts into nitrites (NO₂) and then to nitrates(N0₃⁺) under aerobic conditions.
3. Assimilation: Absorption of nitrates by the plants.
4. Ammonification: After decomposition, absorbed nitrates (organic nitrogen) is converted into ammonia.
5. Denitrification: The conversion of N0₃ into N₂ which is sent to the atmosphere.