When nitrogen is removed from a sample of tha air in the process of nitrogen fixation, what happens to the patial pressure of nitrogen the sample?

Negligible or no change. In the chemistry of nitrogen bacteria convert nitrogen from the AIR, and hydrogen from cell into cell bound ammonia

N2 + 3 H2 - > 2 NH3

But where does that H2 come from? From metabolic reactions in the cell that produce CO2 and H2O. The partial pressure from the gas CO2 molecule pretty much replaces the partial pressure of the N2, and any real pressure difference would be made up of respiration of the H20 from the cell into the air.