The Nitrogen Cycle
• All life requires nitrogen-compounds, e.g., proteins and nucleic acids. • Air, which is 79% nitrogen gas (N2), is the major reservoir of nitrogen. • But most organisms cannot use nitrogen in this form. • Plants must secure their nitrogen in "fixed" form, i.e., incorporated in compounds such as: o nitrate ions (NO3'')
o ammonia (NH3)
o urea (NH2)2CO
• Animals secure their nitrogen (and all other) compounds from plants (or animals that have fed on plants). Four processes participate in the cycling of nitrogen through the biosphere: • nitrogen fixation
Microorganisms play major roles in all four of these.
The nitrogen molecule (N2) is quite inert. To break it apart so that its atoms can combine with other atoms requires the input of substantial amounts of energy. Three processes are responsible for most of the nitrogen fixation in the biosphere: • atmospheric fixation by lightning
• biological fixation by certain microbes — alone or in a symbiotic relationship with some plants and animals • industrial fixation
The enormous energy of lightning breaks nitrogen molecules and enables their atoms to combine with oxygen in the air forming nitrogen oxides. These dissolve in rain, forming nitrates, that are carried to the earth. Atmospheric nitrogen fixation probably contributes some 5– 8% of the total nitrogen fixed.
Under great pressure, at a temperature of 600°C, and with the use of a catalyst, atmospheric nitrogen and hydrogen (usually derived from natural gas or petroleum) can be combined to form ammonia (NH3). Ammonia can be used directly as fertilizer, but most of its is further processed to urea and ammonium nitrate (NH4NO3).
The ability to fix nitrogen is found only in certain bacteria and archaea. • Some live in a...
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