There are two main ways nitrogen can become "fixed:"
Fixation by lightning: The energy from lightning causes nitrogen (N2) and water (H2O) to combine to form ammonia (NH3) and nitrates (NO3). Precipitation carries the ammonia and nitrates to the ground, where they can be assimilated by plants.
Biological fixation: About 90% of nitrogen fixation is done by bacteria. Cyanobacteria convert nitrogen into ammonia and ammonium: N2 + 3 H2 → 2 NH3. Ammonia can then be used by plants directly. Ammonia and ammonium may be further reacted in the nitrification process.
