The nitrogen cycle is the bio and geochemical cycle by which nitrogen is converted into multiple chemical forms as it circulates among 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 nitrogen fixation and 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 or use of artificial nitrogen fertilizers and release of nitrogen in wastewater have dramatically altered the global nitrogen cycle. Human modification of the global nitrogen cycle can negatively affect the natural environment systems and also human health. Organic nitrogen can be present in the form of humus of a living organism or in the intermediate products of organic matter decomposition. The goal of processes in the nitrogen cycle is to transform nitrogen from one form to another. Many of these processes are carried out by microbes either in their effort to harvest energy or to accumulate nitrogen in a form needed for their growth. The nitrogenous wastes in animal urine are broken down by nitrifying bacteria in the soil to be used by plants. The conversion of nitrogen gas into nitrates and nitrites through atmospheric industrial and biological processes is called nitrogen fixation. Atmospheric nitrogen must be processed or fixed into a usable form to be taken up by plants. Most of the fixation is done by free living or symbiotic bacteria known as diazotrophs. These bacteria have the an 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 a complex enzyme found in a wide variety of bacteria. An example of free living bacteria is Azotobacter. Symbiotic nitrogen fixing bacteria such as Rhizobium usually live in the root nodules of legumes. Here they form a mutual 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 thirty percent of the total fixed nitrogen is produced industrially using the Haber Bosch processes which uses high temperatures and pressures to convert nitrogen gas and a hydrogen source into ammonia. Plants can