What They Saw
They purified nitrogenase from Azotobacter vinelandii and put it in a chamber with 50-51 atmospheres of pure nitrogen gas. The chamber was designed so they could start and stop the reaction by tipping the chamber to add reagents that would start or stop it, without changing the atmosphere. Then they measured pressure in the chamber and amount of hydrogen.
Even at the high pressure of nitrogen in this experiment, the amount of hydrogen produced was about one-to-one with the amount of nitrogen fixed. So it must be pretty essential to the reaction.