<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> <P> Conversion of nitrogen into nitrates and nitrites through atmospheric, industrial and biological processes is called as nitrogen fixation . Atmospheric nitrogen must be processed, or "fixed", in a usable form to be taken up by plants . Between 5x10 and 10x10 g per year are fixed by lightning strikes, but most fixation is done by free - living or symbiotic bacteria known as diazotrophs . These bacteria have the nitrogenase enzyme that combines gaseous nitrogen with hydrogen to produce ammonia, which is converted by the bacteria into other organic compounds . Most biological nitrogen fixation occurs by the activity of Mo - nitrogenase, found in a wide variety of bacteria and some Archaea . Mo - nitrogenase is a complex two - component enzyme that has multiple metal - containing prosthetic groups . An example of free - living bacteria is Azotobacter . Symbiotic nitrogen - fixing bacteria such as Rhizobium usually live in the root nodules of legumes (such as peas, alfalfa, and locust trees). Here they form a mutualistic relationship with the plant, producing ammonia in exchange for carbohydrates . Because of this relationship, legumes will often increase the nitrogen content of nitrogen - poor soils . A few non-legumes can also form such symbioses . Today, about 30% of the total fixed nitrogen is produced industrially using the Haber - Bosch process, which uses high temperatures and pressures to convert nitrogen gas and a hydrogen source (natural gas or petroleum) into ammonia . </P> <P> Plants take nitrogen from the soil by absorption through their roots as amino acids, nitrate ions, nitrite ions, or ammonium ions . Most nitrogen obtained by terrestrial animals can be traced back to the eating of plants at some stage of the food chain . </P>

What is the first step in the nitrogen cycle when gaseous nitrogen is converted into ammonia