The majority of earth's atmosphere is nitrogen, n2. the percentage of nitrogen in earth's atmosphere remains constant as prescribed by the law of conservation of matter and because it is a reasonably non-reactive gas. like carbon, nitrogen is cycled through earth's spheres. however, nitrogen can be converted into forms usable by plants and animals, either by lightning or bacteria. once converted to usable forms, nitrogen is able to cycle the rest of the way through the ecosystem. but in order to be made available to life forms on earth, nitrogen must be fixed or covered into a usable form by lightning strikes or a) bacteria. b) decomposition. eliminate c) producers. d) volcanoes.