<P> The nitrogen cycle is the biogeochemical cycle by which nitrogen is converted into multiple chemical forms as it circulates among the atmosphere, terrestrial, and marine ecosystems . The conversion of nitrogen can be carried out through both biological and physical processes . Important processes in the nitrogen cycle include fixation, ammonification, nitrification, and denitrification . The majority of Earth's atmosphere (78%) is nitrogen, making it the largest source of nitrogen . However, atmospheric nitrogen has limited availability for biological use, leading to a scarcity of usable nitrogen in many types of ecosystems . The nitrogen cycle is of particular interest to ecologists because nitrogen availability can affect the rate of key ecosystem processes, including primary production and decomposition . Human activities such as fossil fuel combustion, use of artificial nitrogen fertilizers, and release of nitrogen in wastewater have dramatically altered the global nitrogen cycle . </P> <P> Nitrogen is present in the environment in a wide variety of chemical forms including organic nitrogen, ammonium (NH), nitrite (NO), nitrate (NO), nitrous oxide (N O), nitric oxide (NO) or inorganic nitrogen gas (N). Organic nitrogen may be in the form of a living organism, humus or in the intermediate products of organic matter decomposition . The processes of the nitrogen cycle transform nitrogen from one form to another . Many of those processes are carried out by microbes, either in their effort to harvest energy or to accumulate nitrogen in a form needed for their growth . For example, the nitrogenous wastes in animal urine are broken down by nitrifying bacteria in the soil to be used as new . The diagram besides shows how these processes fit together to form the nitrogen cycle . </P>

What is the largest source of nitrogen on earth