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NITROGEN FIXATION

Definition

The conversion of atmospheric nitrogen (N2 ) to inorganic forms which are available to plants. Nitrogen fixation is not exclusively biological. Biological nitrogen-fixing systems include: (a) free-living bacteria and blue-green algae (several obligate aerobes, facultative and obligate anaerobes, photosynthetic bacteria and species of blue-greens of the orders nostocales, stigone metales, chroococcales, and oscillatoria; (b) plant associations and symbiotic systems (i.e. lichens: fungus plus blue-greens; azolla -water fern plus blue-greens, leaves plus azotobacteraceae). The reduction of gaseous nitrogen to ammonia or other inorganic or organic compound by microorganisms or lightning.

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