The Nitrogen Cycle.
The nitrogen starts in a factory and comes out as nitrogen oxide. It then is in the air and ends up in the rain and lightning. The lightning creates soil nitrates once it hits the ground. The runoff from the nitrates in farming go into a water source. The water with the nitrates in it can get soaked up by the soil around it or the soil could already have the nitrates from the lightning in it. The plants obtain some of the nitrate ions from the soil and water. Animals will eat the plants. The decomposer will break down the animal and soil matter into nitrogen compounds. The nitrogen compounds end up back in the air and the cycle starts over again.