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1)by N2 fixing bacteria(Nitrogen gas (N2) diffuses into the soil from the atmosphere, and speciesof bacteria convert this nitrogen to ammonia ions, which can be used by plants. Legumes are often grown by farmers because they have nodules on their roots that contain nitrogen-fixing bacteria.)
2)Through lightning: Lightning converts atmospheric nitrogen into ammonia and nitrate(NO 3-) that enter soil with rainfall.
3)Industrially: People have learned how to convert nitrogen gas to ammonia (NH3) and nitrogen-rich fertilisers to supplement the amount of nitrogen fixed naturally.
4)Nitrification: Nitrifying bacteria in the soil convert ammonia into nitrite (NO2-) and then into nitrate (NO3-). This process is called nitrification. 
5)Denitrification completes the nitrogen cycle by converting nitrate (NO3-) back to gaseous nitrogen (N2).
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Nostoc bacteria fixes nitrogen in soil