Not N!
Mars atmosphere is hugely deficient in N2 compared to Earth's. There doesn't appear to be any nitrate in the rocks, so where has all the N gone?
For human purposes, it is possible, but tedious to concentrate the ~1.6% N2 from the atmosphere, probably by cryogenic processing - in the same way that liquid gases are manufactured on Earth.
You dump the excess CO2 as dry ice (well, you extract its "cold" before you vent it as CO2 gas, and use the energy saved for the next batch of chilled atmosphere)