With inert gases (argon, helium) or with a partial vacuum, the concentration of nitrogen had to get down below 0.15 atm before the fixation decreased much. It gets to 50% around 0.01 atm. But when there was hydrogen, the rate decreased more quickly; the more, the faster. So hydrogen seems to inhibit the nitrogenase, but only at very high concentrations (over 20% of the atmosphere). This inhibition is competitive and reversible.