Global Phosphorus Cycle


This exercise guides students through creating and modifying a STELLA model of the global phosphorus cycle to test the behavior of phosphorus reservoirs in a number of scenarios. Students build a steady state system and then modify it to explore anthropogenic changes to the system, including the addition of phosphorus-rich fertilizers to the land surface and the shut down of ocean circulation due to global climate change. Other scenarios include an increase in plate tectonic rates, increased insoluble weathering due to glaciation, and changes in the planet's plant populations. The exercise includes the use of STELLA and Fortran 90 code.

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Learning Goals

The P cycle exercise has several purposes:
  • To get the students comfortable with using STELLA
  • To demonstrate the behavior of a closed system with multidirectional flows
  • To convey the important concept of steady state behavior
  • To convey the importance of the 1/2 DT rule, which says that the time step (DT) should be halved until there is no longer a change in results from model run to model run.
  • To get the students comfortable with adjusting the model to run different experiments and with interpreting the results of those experiments.

Context for Use

This activity was developed for a class introducing dynamical systems modeling to upper level undergraduate students. The author introduces a series of weekly modeling projects with this exercise.

Description and Teaching Materials

Phosphorus_cycle_exercises_for_students.pdf (Acrobat (PDF) 18kB Jun7 05)
Documented_P_Cycle.stm ( 294kB Jun7 05)
phosphorus.f90 ( 5kB Jun7 05)

Teaching Notes and Tips

Instructor Notes (Acrobat (PDF) 60kB Apr10 06)


PCycle_Exercises_Answer_key.pdf (Acrobat (PDF) 259kB Jun7 05)
assignment_assessment_1.pdf (Acrobat (PDF) 5kB Jun7 05)

References and Resources

The following are readings for the exercise.

Chameides, W.L., and Perdue, E.M., 1997, Biogeochemical Cycles: A ComputerInteractive Study of Earth System Science and Global Change, New York: Oxford University Press, p. 97-107 (Chp. 5, The Global Phosphorous Cycle).

Jahnke, R.A., 1992, The Phosphorus Cycle, in Butcher, S.S., Charlson, R.J., Orians, G.H., and Wolfe, G.V. (eds.), Global Biogeochemical Cycles, New York: Academic Press, p. 301-315.

Lerman, A., Mackenzie, F.T., and Garrels, R.M., 1975, Modeling of Geochemical Cycles: Phosphorus as an Example, Geological Society of America Memoir 142, p. 205-218.