Definition:
The biogeochemical cycle by which nitrogen is converted by fixation (gaseous nitrogen is converted into ammonia, which can be taken up by plants), ammonification (organic nitrogen from decaying animal and plant matter is converted into ammonium by bacteria and fungi), nitrification (ammonium is converted into nitrate by soil bacteria), and denitrification (nitrate is reduced into gaseous nitrogen). This cycle is central to the biogeochemistry of the Earth. Oceans also have an enormous nitrogen cycle.
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