What They Saw
They got nodules from different kinds of plants, infected with either wild bacteria or commercial strains, and measured hydrogen production in air or argon, and acetylene reduction.
In an in vitro nitrogenase system, activity depended on the presence of ATP, reducing equivalents, and the enzyme. The ratio of hydrogen in argon to hydrogen in air here was about 3.
There was a pretty wide variety of values they saw, but most seemed to cluster around a measure of 50% of the hydrogen produced in argon was produced in air; so the presence of nitrogen reduced the hydrogen production by half. It's not clear how uptake hydrogenases might have affected these numbers though. They got similar results with whole plants. This means that each nitrogen gets 6 electrons to become 2 ammonia, and another 6 electrons go to hydrogen; that seems like a lot.
They also noticed that adding enough acetylene stopped the hydrogen production, so it seemed like hydrogen is not a necessary byproduct of that reaction.
What This Means
Correlation between amounts of acetylene reduced and nitrogen fixed are not necessarily accurate, because of the hydrogen produced as a byproduct. But the hydrogen production seems to be a big waste of efficiency; reducing this could help increase yield, maybe by choosing good nodulating bacteria. They wonder whether the hydrogen could be captured from plants somehow and used for energy.