The Phosphorus Cycle (College Board AP® Environmental Science): Study Guide

What is the phosphorus cycle?

• The phosphorus cycle is the movement of phosphorus atoms and phosphorus-containing molecules through the environment

  • Phosphorus is a limiting macronutrient required in large amounts by organisms for DNA, RNA, and ATP

  • Phosphorus is essential for cell function

  • It remains a stable element but is incorporated into and released from various chemical compounds during its cycle

• It involves the transfer of phosphorus between sources (which release phosphorus) and sinks (which absorb it)

• Unlike other nutrient cycles, the phosphorus cycle does not include a gaseous phase

Examples of phosphorus sources and sinks

• Sources:

  • Weathering of rocks

  • Decomposition of organic matter

  • Agricultural runoff

• Sinks:

  • Sediments in water bodies

  • Soils

Phosphorus reservoirs

Key phosphorus reservoirs

• Rocks and sediments: phosphorus is primarily stored in the form of phosphate minerals

• Soil: contains phosphorus from weathered rocks and organic matter

• Water bodies: phosphates accumulate in sediments, especially in oceans and lakes

• Living organisms: store phosphorus in molecules such as DNA, RNA, and ATP

Steps and reservoir interactions in the phosphorus cycle

1. Weathering of rocks:

   • Rain and natural erosion break down phosphate-containing rocks

   • This releases phosphorus into the soil as phosphate ions

   • Plants absorb these phosphates through their roots

2. Plant and animal uptake:

   • Plants use phosphates to grow and produce energy

   • Animals obtain phosphorus by eating plants or other animals

3. Decomposition:

   • Plants and animals die

   • Decomposers release phosphorus back into the soil or water as they break down the organic matter in these dead organisms

4. Runoff and sedimentation:

   • Excess phosphorus in soil can be washed into rivers, lakes, and oceans through runoff

   • In water bodies, phosphorus settles into sediments

     • It can remain here for millions of years

5. Geological uplift:

   • Over time, geological processes can lift phosphorus-containing sediments to form new land

   • This can make the phosphorus available once again, restarting the cycle

Natural scarcity of phosphorus

• There is no atmospheric component in the phosphorus cycle

• This means that phosphorus does not move through the air

• This limits the movement of phosphorus between the ocean and land, making it naturally scarce in many ecosystems.

• In undisturbed ecosystems, phosphorus is often the limiting nutrient for plant growth

Examples of phosphorus scarcity

• Phosphorus is often added to agricultural fields as fertilizer

  • This is because natural soil phosphorus levels are too low to support intensive farming

• In aquatic ecosystems, a lack of phosphorus can limit the growth of algae and other organisms