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Forms of soil P are dependent upon... Plants and microbes ✔ Soil parent material ✔ pH ✔

Forms of soil P are dependent upon... Plants and microbes ✔ Soil parent material ✔ pH ✔ Ecosystem development and pedogenesis. N. P. N or P per unit area. Terrestrial ecosystem age. Walker and Syer’s diagram 1976. P Ca : calcium phosphates P T : total phosphorus

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Forms of soil P are dependent upon... Plants and microbes ✔ Soil parent material ✔ pH ✔

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  1. Forms of soil P are dependent upon... • Plants and microbes ✔ • Soil parent material ✔ • pH ✔ • Ecosystem development and pedogenesis

  2. N P N or P per unit area Terrestrial ecosystem age

  3. Walker and Syer’s diagram 1976 PCa: calcium phosphates PT: total phosphorus Po: organic phosphorus

  4. Chronosequence: time varies, all other state factors held constant

  5. State Factors Climate Time Ecosystem structure and function Organisms Relief (Topography) Parent material Ecosystem = (Cl, O, R, P, T)

  6. Parent Material

  7. Relief, Time

  8. Organisms

  9. Long soil age gradient on Hawaiian Islands • All sites have the same • Climate—precipitation and temperature • Organisms—Dominated by Ohia • Topography—shield surface, elevation, aspect • Parent material—basalt • All sites are different ages • Youngest—300 years old • Oldest—4.1 million years old

  10. Crews et al. 1995

  11. Questions • Do soil N and P follow the predictions generated from Walker and Syers? • Does limitation of plant productivity follow the predictions? Interesting questions that we won’t have time for in this lecture: • Does N track P? • Does litter decomp. respond to site fertility, resulting in feedbacks?

  12. Hypotheses Young Plant primary productivity Intermediate Old Control N P N&P

  13. Results How do the results compare to the model? Crews et al. 1995

  14. Crews et al. 1995

  15. Results and model agree: • Decrease in rock phosphate • Increase in dominance of occluded P fraction • Results deviate from expectations: • Shape of total phosphorus through time

  16. How do these results compare to our expectations? Vitousek & Farrington 1997

  17. Youngest site • Plenty of unweathered P • Not much N accumulated • N limits plant growth • Middle site • Lots of P and N • Co-limitation by N and P • Oldest site • Most P occluded or bound in organic forms • Lots of N—in excess • P limits plant growth • Gobi desert dust major source of P input

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