What's the minimum, if any, concentration of atmospheric nitrogen needed by plants? Would plants be able to grow in an atmosphere with just carbon dioxide and oxygen?
Zero if you are supplying them with solid nitrates in their growth medium, plants can't use atmospheric nitrogen. Some soil bacteria can and create nitrates the plants can use but this can and often is supplied by other means.
But as Graham points out you will need another inert gas to fill in the atmosphere as high concentrations of CO2 or Oxygen are both toxic.