What process must happen to gaseous nitrogen before plants can use it to grow?
gaseous nitrogen (N2) contains a triple-bond, something plants and most other living things can't do much with. Lighting strikes and soil bacteria are the only significant natural means through which this bond can be broken and new nitrogenous compounds formed (ie. ammonia NH3).
ammonia can be used directly, but nitrifying bacteria convert ammonia into less toxic nitrates and nitrites - all of which can be taken up by plants.