What source of energy ultimately drives the nitrogen cycle? Please explain?

Answer 1

Bacteria in the soil change nitrogen into forms that plants can use (nitrogen fixation) Organisms get nitrogen by eating the plants or eating organisms that eat plants. When organisms die, decomposers break down their bodies and release a form of nitrogen into the soil.

Answer 2

Nitrification, usually by bacteria and sometimes by lightning.

Are these classified as energy sources?? Depends on the meaning you are looking for, for 'energy'??