If a Azotobacter is free living nitrogen fixer it will also require ATP through cellular respiration. I am confused here: if Nitrogenase is highly sensitive to oxygen molecule how will Azotobacter be able to fix N2 and carry out cellular respiration at the same time?
Nitrogen fixation is highly sensitive to the presence of oxygen, so Azotobacter developed a special defensive mechanism against oxygen, namely a significant intensification of metabolism that reduces the concentration of oxygen in the cells.
From that cite "Respiratory Protection nitrogenase complex in Azotobacter vinelandii"(google translated)
The second strategy, dubbed "breath protection ", is based on a significant increase in the rate of the rate of respiration, resulting in the concentration of oxygen in the cytoplasm decreases to a level at which nitrogenase no longer experiences damaging action О2 .
Also, a special nitrogenase-protective protein protects nitrogenase and is involved in protecting the cells from oxygen. Mutants not producing this protein are killed by oxygen during nitrogen fixation in the absence of a nitrogen source in the medium.