Overview
  Biogeochemical Cycles
  Spheres of B. Cycles
  Nitrogen Properties
  Simple Nitrogen Cycle
    Glossary
  Human Influences
  Spheres of the N Cycle
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Natural inputs and outputs

Natural inputs to and outputs of nitrogen (N) from a sphere area a direct result of natural processes. Fixation of N2 by lightning and subsequent washout of the resulting nitrate is clearly a natural N input to the hydrosphere, geosphere and bioshpere. In other cases it may be difficult to distinguish between natural and anthropogenic inputs. For example, fixation of N2 by cyanobacteria (blue-green algae) in a lake may be considered a natural input to the biosphere. However, if excessive growth of cyanobacteria (an algal bloom) is caused by pollution of the lake, then fixation of N2 in the lake may be only partly natural.

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