A biogeochemical cycle is a process that allows matter to cycle between the living world and the nonliving environment. Nitrogen is an element necessary for living organisms to manufacture organic compounds containing nitrogen, including proteins. Proteins make up cell membranes, enzymes, hormones and other structural parts of the body. Atmospheric nitrogen gas or N2 must be "fixed" in order to be made usable for living things to synthesize compounds containing nitrogen. Lightning can do this naturally. However, in soil and in some root nodules of leguminous plants live nitrogen-- fixing bacteria. They convert atmospheric nitrogen into ammonia(NH3) and ammonium NH4+. Other bacteria called nitrifying bacteria convert ammonia to nitrates (NO3-). Nitrates can be absorbed by plants along with ammonia and ammonium to make organic nitrogen compounds, including plant proteins. Since plants are producers, they will eventually be consumed by animals, which then make animal proteins in the food web. Eventually, plants and animals die or produce wastes that contain nitrogen. During ammonification, these organic nitrogen compounds get decomposed by bacteria in the soil, releasing ammonia back to the cycle.