In this system nitrogen fixation is determined indirectly by measuring the amount of hydrogen evolved by the nitrogen-fixing symbiotic bacterium, Rhizobium, living in nodules on the roots of plants. The concentration of hydrogen (detected with a H2 sensor) as well as the concentration of oxygen (detected with an O2 sensor) in the air passing through the root system of the plant is measured simultaneously. But before the air enters the two sensors it must have any moisture removed (because water vapor interferes with the sensors). This is accomplished by running the air through a column containing a desiccant. The air is moved through the entire system by a small air pump to which gas bags, containing different gas mixtures, may be attached. The rate of gas flow from the air pump is controlled by placing flow restrictors, providing various speeds of flow, between the pump and the next component in the pathway.
If you wish to review the components of the system or see the compete apparatus, click here.
Before you begin measuring the output of H2 from the nodules, you need to set some base lines with the various gas mixtures you will be using, such as air, N2:O2 (70:30) or an Ar:O2 (80:20) mixture. To do this the gas is pumped directly into the desiccant column and then through the two sensors, by-passing the plant. When you are ready to measure the output of the nodules the plant is placed into the gas-flow path, between the pump and the desiccant.
To assess your understanding of the two gas flow paths described above, complete the exercises that follow.
| Support and funding provided by: |
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Andrew W. Mellon Foundation |
Instrumentation manufactured by: |
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