The nitrogen released into the soil by nitrogen fixers is primarily in the form of ammonia (NH3) and ammonium ions (NH4+). Although ammonia is toxic to plants, ammonium ions can be taken up safely at low concentrations. Soil bacteria called nitrifiers, which we described in Chapter 27, oxidize ammonia to nitrate ions (NO3)—another form that plants can take up—by the process of nitrification (Figure 37.8). Soil pH affects the uptake of nitrogen: Nitrate ions are taken up preferentially under more acidic conditions, ammonium ions under more basic ones.
The steps that we have followed so far are carried out by bacteria: N2 is reduced to ammonia in nitrogen fixation and ammonia is oxidized to nitrate in nitrification. The next steps are carried out by plants, which reduce the nitrate they have taken up all the way back to ammonia. All the reactions of nitrate reduction are carried out by the plant's own enzymes. The later steps, from nitrite (NO2) to ammonia, take place in the chloroplasts, but this conversion is not part of photosynthesis. The plant uses the ammonia thus formed to
^VWi 37.8 The Nitrogen Cycle Nitrogen fixation, nitrification, V "À nitrate reduction, and denitrification are the components of an J essential chemical cycle that converts atmospheric nitrogen gas into ammonium ions and nitrate ions—forms of nitrogen that can be taken up by plants—and returns N2 to the atmosphere.
Some denitrifying bacteria can oxidize ammonia back to nitrogen gas, which returns to the atmosphere.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.