How is nitrogen from the atmosphere, the abiotic part of the ecosystem, converted in to the biotic part of the ecosystem in organisms?

It comes from lightning and is named nitrogen fixation.

In the process of the nitrogen cycle. 
The nitrogen cycle is the biogeochemical cycle by which nitrogen is converted into various chemical forms as it circulates among the atmosphere and terrestrial and marine ecosystems. 
This process undergoes and affects the biological, geometrical and chemical aspects in the ecosystem and the abiotic and biotic community.
This cycle leads the abiotic component nitrogen to contribute to the biotic community, decomposition and primal production.

I hope this helps:)

If you remove all the lions from the savanna it will lead to a increase of gazelle and a increase of cheetahs becasue of no competition