1.6 The Phosphorus Cycle

Basics

  • The phosphorus (P) cycle is the movement of phosphorus through rocks, soil, water, and living organisms.

  • No atmospheric phase: phosphorus does not become a gas, unlike nitrogen or carbon.

  • Very slow cycle; controlled mostly by geological processes.

  • Essential for life: part of DNA, RNA, ATP, and phospholipids; also important for bones and teeth in animals.

  • Limiting nutrient: often restricts plant growth, especially in aquatic systems.

Major Reservoirs

  • Rocks & sediments (largest, long-term storage)

  • Soil (short-term)

  • Water (dissolved phosphate, minor)

  • Living organisms (small, short-term)

Key Processes

  1. Weathering

    • Rocks containing phosphate minerals break down (physically or chemically).

    • Releases phosphate ions (PO₄³⁻) into soil and water.

    • Rate-limiting step; extremely slow (thousands to millions of years).

  2. Absorption / Assimilation

    • Plants absorb phosphate ions from soil via roots.

    • Phosphorus incorporated into organic molecules (ATP, DNA, RNA, phospholipids).

    • Animals get P by eating plants or other animals.

  3. Decomposition

    • When plants and animals die or excrete waste, decomposers return phosphorus to soil or water.

    • Recycles phosphorus in short-term loops.

  4. Runoff and Leaching

    • Rain or irrigation moves phosphates from soil to water bodies (lakes, rivers, oceans).

    • Excess phosphorus from fertilizers causes eutrophication.

  5. Sedimentation (Long-Term Storage)

    • Phosphorus settles as sediments at the bottom of water bodies.

    • Remains trapped for millions of years until geological processes act.

  6. Geological Uplift

    • Sediments can become rock through pressure and time.

    • Uplift exposes phosphorus-containing rocks, restarting the cycle.

Unique Features

  • No gaseous phase → moves only through lithosphere, hydrosphere, and biosphere.

  • Slowest major nutrient cycle.

  • Often limiting nutrient: growth depends on phosphorus availability.

  • Mycorrhizal fungi help plants access soil phosphorus.

Human Impacts

  • Fertilizer use: adds excess phosphorus → runoff → algal blooms → oxygen depletion → aquatic dead zones.

  • Mining phosphate rock: disrupts long-term geological storage.

  • Phosphate detergents: previously contributed to water pollution.

  • Wastewater and animal waste: high P inputs into water systems.