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                            Nitrogen Cycle
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                - Nitrogen (N) is an essential constituent of protein, DNA,
                RNA, and chlorophyll.
                     - N is the most abundant gas in the atmosphere, but it must
                be fixed or converted into a usable form.

             Nitrogen Fixation Methods:
                   1) High energy fixation- a small amount of atmospheric
                nitrogen is fixed by lightening. The high energy combines N and
                H2O resulting in ammonia (NH3) and nitrates (NO3). These forms
                are carried to Earth in precipitation.
                         2) Biological fixation: achieves 90% of the nitrogen
                fixation. Atmospheric nitrogen (N2) is split and combined with
                hydrogen (H) atoms to form ammonia (NH3).

            Who performs nitrogen fixation?
               - symbiotic bacteria (eg. Rhizobium spp.) living in association
                with leguminous ( plants in the pea family), and root-noduled non-
                leguminous plants (eg. Alnus spp.).
                   - free-living anaerobic bacteria
                   - blue-green algae (cyanobacteria)

               Once NH3 is in the soil it combines with H+ ions to form
                ammonium ion (NH4), or without it to form NO3. NH4+ and
                NO3 are readily absorbed by plants.

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