Answer
Before the evolution of land plants, soil bacteria could have worked in the nitrogen cycle since entry into a plant, or any other living thing, does not have to be involved. Nitrogen can be fixed by lightning, rain down into the soil, and then proceed through the various steps until returning through the atmosphere.
Work Step by Step
The evolution of land plants simply added a lot of nitrogen to the soil when symbioses evolved between some plants and nitrogen-fixing bacteria.
