Nitrogen Cycle




BIO326 : Ecosystem : Nutrients : Nitrogen Cycle

Exercise: Nitrogen Cycle

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DUE: 28-Apr-2000. Work sent after this date will receive a grade of zero.

Nitrogen is one of the most important nutrients in ecosystems, judging by the fact that it is almost always identified as the element that limits productivity.

1. Once an organism dies, what is the next step in the nitrogen cycle for the organic nitrogen contained in the organism's body? Does this step take energy or release energy?

2. Conditions in the soil during nitrogen transformations have a strong influence on the fate of the nitrogen. What possible fates are possible for the ammonium ion and what soil conditions are relevant to each fate?

3. Because of the importance of nitrogen to the ecosystem, one would expect that there would be tight cycling within the ecosystem which retains its nitrogen in various organic forms. Yet under some conditions, denitrification occurs. What are the conditions that determine whether cycling within the system or denitrification will be followed? What microorgansisms facilitate each pathway? What influence does each pathway have on the ozone layer?

4. Estimates of the compartment sizes and rates of exchange (numbers next to arrows) in the nitrogen cycle provide an opportunity to assess ecosystem functioning. Adding the four indicated inputs to land plants (industrial fixation, lightning fixation, biological fixation, and sea spray), what is the total input from external sources? What is the total export from land plants (human activities, river flow, dust, NOx, and denitrification)? If this system were in steady state, what would be the missing flow and its magnitude (include units with your answer)?

5. Biological systems conserve limited nutrients in various ways, including internal cycling. What percentage of the nitrogen in land plants is replaced each year by internal cycling? What percentage is replaced by external inputs? Which of these two sources of nitrogen seems more important to ecosystem function and to what extent?

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