Making nitrogen would be an impressive achievement for anybody, never mind a plant. Probably easiest is to make a lot of hydrocarbons with deuterium substituted for the regular hydrogen, and pack that around a source of free neutrons. The neutrons would lose energy to the deuteriums, and end up slow enough to be absorbed by the carbon nuclei, which would become nitrogen after a few steps. Also the deuterium would become tritium and then helium. Plants probably could concentrate deuterium. Where plants would get free neutrons is a poser; some plants will concentrate radionuclides from soil...
But of course there's already plenty of nitrogen in the air.