A lesser amount of nitrogen is fixed from the atmosphere through reactions occurring in the gaseous emissions from volcanoes and from lightning discharges. This nitrogen is washed out of the atmosphere by rain and is added to the soil and water where it, too, can be used by organisms. Does this make sense to you yet? If not, continue reading on, it should be clearer soon.
Human activities have a significant effect on nitrogen cycling. Production and use of nitrogen fertilizer, combustion of fossil fuels, and planting crops that fix nitrogen have unbalanced the previously stable relationship between fixation and denitrification. Gaseous industrial pollutants—abbreviated as NO x compounds—foul the air in many cities and wash out in sufficient amounts to constitute “acid rain” in some parts of the industrialized world.
The nitrogen cycle can be understood most easily by looking at its separate parts: nitrogen fixation, ammonification, nitrification, assimilation, and denitrification.