Which bacteria are nitrogen fixing?

Hint: For plant development and output, nitrogen is a vital limiting mineral. It's a big part of chlorophyll, which is the most vital pigment for photosynthesis, and amino acids, which are the building blocks of proteins. Other essential biomolecules that include it include ATP and nucleic acids. Despite being one of the most plentiful elements (mostly in the form of nitrogen gas ($N_2$) in the Earth's atmosphere), it is also one of the least understood.

Complete answer:
Azotobacter, Bacillus, Clostridium, and Klebsiella are examples of this class of nitrogen-fixing bacteria. As previously stated, these species must generate their own energy, which is usually obtained by oxidising organic molecules supplied by other organisms or by decomposition.
This element can only be used by plants in reduced forms. Plants obtain these forms of "combined" nitrogen from a variety of sources, including: 1) the application of ammonia and/or nitrate fertiliser (from the Haber-Bosch process) or manure to the soil, 2) the release of these compounds during organic matter decomposition, 3) the conversion of atmospheric nitrogen into the compounds by natural processes such as lightning, and 4) biological nitrogen fixation.
A specialised subset of prokaryotes performs biological nitrogen fixation (BNF), which was identified by Beijerinck in 1901 (Beijerinck 1901). The enzyme nitrogenase catalyses the conversion of atmospheric nitrogen ($N_2$) to ammonia in these organisms ($NH_3$). Plants may easily digest $NH_3$ to make the nitrogenous macromolecules discussed before.
Aquatic species, such as cyanobacteria, free-living soil bacteria, such as Azotobacter, bacteria that create associative relationships with plants, such as Azospirillum, and, most critically, bacteria that form symbiosis with legumes and other plants, such as Rhizobium and Bradyrhizobium.

Note:
Rhizobia are nitrogen-fixing diazotrophic bacteria that develop themselves inside the root nodules of legumes (Fabaceae). Rhizobia need a plant host to express genes for nitrogen fixation; they can't fix nitrogen on their own. Nitrogen, the most abundant element in our atmosphere, is crucial to life. Nitrogen is found in soils and plants, in the water we drink and in the air we breathe.