RG

🔬 APES Topic 1.6: The Phosphorus Cycle

  1. Why Phosphorus Matters
    Phosphorus (as phosphate, PO_4^{3-}) is a vital macronutrient, forming parts of DNA, RNA, ATP, phospholipids, and bones.
    It often limits plant growth, especially in soils and freshwater, due to its low mobility and limited bioavailability, making it a limiting nutrient in many ecosystems.

  2. Reservoirs & Sources

    • Rocks & Sediments: The major reservoir; contains phosphorus-bearing minerals that weather over time, releasing phosphate.

    • Soil: Contains weathered phosphate and organic phosphorus.

    • Living Organisms: Plants absorb phosphate; animals get it by consumption.

    • Water Bodies: Phosphate runs off into water, is taken up by aquatic life, or settles as sediment.

  3. The Main Processes / Steps of the Cycle
    The phosphorus cycle describes the movement of phosphorus atoms and phosphorus-containing molecules between various sources and sinks in the environment.

    • Weathering / Erosion: Rocks break down, releasing phosphate into soil and water.

    • Uptake by Plants: Plants absorb dissolved phosphate from soil, sometimes aided by mycorrhizal fungi.

    • Consumption / Transfer through Food Web: Animals eat plants, incorporating phosphorus into their tissues.

    • Decomposition / Mineralization: Decomposers break down organic phosphorus from dead organisms/waste, returning it to inorganic form in soil/water.

    • Runoff / Leaching: Unused soil phosphate washes into water bodies.

    • Sedimentation & Burial: Phosphate settles in aquatic sediments, becoming buried over time; eventually uplifted geologically.

  4. Key Unique Features / Contrasts
    Unlike carbon and nitrogen, the phosphorus cycle lacks a significant atmospheric component, making this cycle unique and resulting in slower movement through the environment.
    Consequently, most phosphate remains locked in rocks, sediments, or soil, with only small amounts bioavailable.
    Because there is NO atmospheric component, it is very difficult for phosphorus to make its way back to the land after reaching the ocean.

  5. Human Impacts & Consequences
    Mining of phosphate rock significantly increases reactive phosphorus in the environment.
    Agricultural runoff from fertilizers leads to excess phosphorus in waterways, causing eutrophication (algal blooms, oxygen depletion, "dead zones").
    Humans disrupt the natural balance by rapid phosphorus addition versus slow natural removal.

  6. Important Terms / Things to Know

    • Phosphate (PO_4^{3-})

    • Weathering and erosion

    • Mineralization / decomposition

    • Mycorrhizal associations

    • Runoff / leaching

    • Sedimentation & geological uplift

    • Limiting nutrient

  7. Why Phosphorus Is Often Limiting

    • Bioavailable P is scarce: slow release from rocks; soil phosphates often become unavailable.

    • No atmospheric phase: hinders long-distance and rapid global distribution, contributing to its limiting nature.

    • High biological demand: essential for growth, ATP, and DNA, making it a common growth constraint for many ecosystems.