GARY HINCKS / SCIENCE PHOTO LIBRARY GARY HINCKS / SCIENCE PHOTO LIBRARY
Nitrogen cycle. Diagram illustrating the global nitrogen cycle (red arrows). Nitrogen occurs naturally in the atmosphere, but is incorporated in organic and mineral forms as its oxides or as ammonia. Some nitrogen fixation occurs through lightning strikes (top). Large amounts of fixation and release occurs through soil bacteria (bottom), including those found on the roots of plants such as clover (centre). Rabbits (right) eat the clover, returning nitrogen to the soil. Nitrogen compounds are also washed in the sea, and produced and released by marine micro-organisms (left). These and other reservoirs form the global nitrogen budget.
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