<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 + 4), nitrite (NO − 2), nitrate (NO − 3), 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 process releases nitrogen gas into the atmosphere