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Topic 1.6

THE PHOSPHORUS CYCLE

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Phosphorus Cycle Overview

  • Movement of P atoms & molecules b/w sources and Sinks/Reservoirs
    • Rocks & Sediments containing P-minerals = major reservoirs
  • P-Cycle is VERY slow compared to C/H20/N cycle
    • Takes a long time for P minerals to be weathered out of rocks and carried into soil & water
    • There is no gas phase of the P-cycle (does not enter the atmosphere)
    • Because it cycles so slowly, it is a limiting nutrient for plant growth in ecosystems.
  • Like Nitrogen, phosphorus is a needed element for growth of plants and animals
    • Need for DNA, ATP generation, bone and teeth enamel
  • Commercially added through fertilizers

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Phosphorus Sources

Major natural source of P is weathering of rocks that contain P minerals

  • Wind & rain break down rock & phosphate (PO4-3) is released and dissolved into water; rain water carries phosphate into nearby soil & bodies of water
  • Weathering is so slow that P is often a limiting nutrient in aquatic & terrestrial ecosystems

Synthetic (human) Sources of P=mining phosphate minerals & adding to products like synthetic fertilizer & detergents/cleaners

  • Synthetic fertilizers containing phosphate are added to lawns and agricultural fields
    • Runoff carries excess P into nearby bodies of water
    • Phosphate from detergent & cleaners enter bodies of water via wastewater from homes.

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Adding Human Component

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Phosphate Sinks/Reservoirs

  • N, P is absorbed by plant roots & accumulate into tissues; animals assimilate P by eating plants or other animals
  • Animal waste, plant matter & other biomass is broken down by bacteria/soil decomposers that return phosphate to the soil
  • Assimilation & excretion/decomposition form a mini-loop with P cycle just like assimilation & ammonification in N cycle and photosynthesis & respiration in C cycle.
  • Sedimentation & Geologic Uplift
    • Phosphate doesn’t dissolve very well into water. Falls (precipitates) out of the water and falls as sediment at the bottom (sedimentation)
    • P sediments can be compressed into sedimentary rock over a long period of time by pressure of overlying water
    • These deep seminary rock is moved as tectonic plates move
      • Uplift is the process by which rock layers are forced up to form mountains exposing sedimentary rocks contain P, weather occurs and the cycle starts again.

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Eutrophication (Over use of N & P)

  • Occurs from over fertilization and human/animal waste runoff
  • Both N and P are limiting nutrients.
    • Excess inputs of P and N lead to eutrophication, an excess of nutrients results in massive algae blooms
    • Algae blooms cover the surface of the water blocking light below
    • Eventually algae die, bacteria break down the algae and up all the O2 in the water
    • Lower O2 levels (dissolved oxygen) in water kills aquatic animals, especially fish
    • Bacteria use up more O2 decomposing dead marine life
    • Creates Postive feedback loop

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Saharan Dust Feeds Amazon’s Plants

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Other Resources