ammonification as well as nitrification and denitrification. The majority of atmosphere of Earth is atmospheric 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 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 then ammonium ions for incorporation into amino or nucleic acids and chlorophyll. In plants that have a symbiotic relationship with rhizobia some nitrogen is assimilated in the form of ammonium ions directly from the nodules.