What is the most common way that nitrogen fixation occurs?

B nitrogen - fixing bacteria

"This complex process is carried out by nitrogen-fixing bacteria present in the soil"
http://library.kcc.hawaii.edu/external/c...

- this is biological fixation and it is 2 to 1 to the non-biological which includes lightning, fires etc
Critters
The answer is BA relatively small amount of ammonia is produced by lightning. Some ammonia also is produced industrially by the Haber-Bosch process, using an iron-based catalyst, very high pressures and fairly high temperature. But the major conversion of N2 into ammonia, and thence into proteins, is achieved by microorganisms in the process called nitrogen fixation (or dinitrogen fixation).

The total biological nitrogen fixation is estimated to be twice as much as the total nitrogen fixation by non-biological processes.