The Nitrogen Cycle is perhaps the most important life cycling system in nature. This is because of the immense need that all organisms have for nitrogen, and the fact that it is so easily trapped in non-usable forms by normal biological processes. Basically, the Nitrogen Cycle is the continuous flow of nitrogen through the biosphere by the processes of nitrogen fixation, ammonification (decay
), nitrification, and denitrification. Nitrogen is an essential constituent of all protoplasm (cytoplasm). To enter living systems, however, it must first be fixed (combined with oxygen or hydrogen) into compounds that plants can utilize, such as nitrates or ammonia. Most fixation is performed by certain bacteria living in the soil or in nodules in the roots of leguminous plants. Plants incorporate the fixed nitrogen into plant protein; then animals consume the plants and convert plant protein into animal protein. Organic nitrogen is returned to the soil as ammonia when animal remains and wastes decay. Then nitrifying bacteria oxidize the ammonia to nitrites and the nitrites to nitrates, which can, like ammonia, be taken up by plants. Still other soil microorganisms can reduce ammonia nitrates to molecular nitrogen.
One problem with an increase in nitrogen is the proliferation of harmful species. Many parasitic or destructive types of organism flourish best in conditions that are too extreme for most living things. For example, runoff from farms near the coastline produces high amounts of usable nitrogen. This excess nitrogen promotes the explosions of algae "blooms" that often appear. These eruptions choke out other life and are toxic to humans. The Pfiesteria that afflicts the Chesapeake Bay is another example of this phenomenon.