If we want to increase crop benefits from biological nitrogen fixation as an alternative to nitrogen fertilizers, we should focus on efficiency (N per CO2 respired) rather than rate (gN/hour). We could increase rate just by breeding plants that make more nodules than they need, but that would probably decrease yields.
John Witty and colleagues figured out how to measure nitrogen-fixation efficiency many years ago. Decreasing oxygen concentration between 20% and 15% typically has little effect on root and soil respiration, whereas diffusion-limited, nodule-interior respiration decreases, reducing ATP supply for nitrogen fixation. The nitrogen-fixation efficiency is the ratio of the fixation decrease to the respiration decrease. Only a few people have done this, however, perhaps because of the complexity and cost of the equipment needed. We’ve used a few thousand dollars worth of mass-flow controllers to mix different oxygen concentrations, another few thousand for a CO2 sensor, and $400 or so for a sensor to measure hydrogen gas, a byproduct of nitrogen fixation. In my basement lab, I’m working on a single-board system that slashes all of these costs and switches between two plants for higher throughput. We haven’t tested efficiency assays with plants yet, but hydrogen calibration using electrolysis (above) is working.